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General Laboratory: Blood Gas: Measurement Principles Online Training

Proper sample collection and handling is important to ensure proper results in all laboratory testing but it is especially critical in blood gas analysis, where exposure to ambient air can affect oxygen and carbon dioxide concentrations in the blood sample.

Identify how to collect and handle samples for blood gas analysis properly Identify the test method principles of measured blood gas parameters pH, PCO2, pO2, total hemoglobin, and hemoglobin fractions Identify the test method principles of the measured electrolyte parameters sodium (Na%2B), potassium (K%2B), calcium (Ca%2B%2B) and chloride (Cl-) Identify the test method principles of the measured metabolite parameters glucose and lactate Welcome to the Blood Gas and Rapid Analysis Tests: Sample Collection and Measurement Principles Online Training course. Select Next to continue. Recently developed analytical systems allow complete analytical procedures for multiple parameters to be performed with small sample volumes. To guarantee good quality results and to minimize unnecessary sources of error, an analysis must be preceded by proper pre-analytical processes. This is the only way to ensure that the measured values correspond to the actual patient’s clinical status. The key to accurately measured results is the correct preparation and withdrawal of the blood and the correct handling of the samples. The suitable sample type and withdrawal site should be monitored by a clinician. To simplify sample collection, manufacturers offer ideally equipped and prepared sample collection systems. Several points need to be taken into account when handling these samples because analytical emergency procedures in the broadest sense (including oxygen status) represent particularly sensitive diagnostic procedures: the values of individual parameters are altered as a result of respiration and metabolism and the gas exchange of a blood sample with ambient air significantly affects the blood gases pO2 and pCO2. Collection, handling and transport of blood samples are key factors for the accuracy of clinical laboratory analysis and ultimately for the quality of the patient care.   Sample Collection and Patient’s Body Temperature Learn more about sample collection and patient’s body temperature. Blood gas analysis is conducted at 37°C. Interpretation errors caused by different patient temperatures can occur. However, different medical diagnostic questions require the patient’s body temperature. Therefore, the patient’s temperature should be determined at the time the sample is drawn. All state-of-the-art systems allow you to enter the patient’s body temperature to update the measured pH, pO2, pCO2 values and the oxygen saturation with respect to the patient’s actual body temperature. Sometimes, the measured values for pO2 and pCO2 change proportionally with the temperature, while pH changes reverse proportionally with respect to the body temperature. When complete, select the X in the upper-right corner to close the window and continue. The selection of the suitable site for withdrawal of blood samples should be monitored by a clinician. The puncture site is cleaned with a dermal antiseptic and must be dried completely with a sterile swab because traces of alcohol on the skin will hemolyze the blood. Different types of samples include: Arterial blood Capillary blood Venous blood Mixed venous blood   Types of Samples Learn about each type of sample collected for blood gas analysis. Select the tabs below to learn about each type of sample collected for blood gas analysis.TitleTextArterial bloodTab TitleTextDescription The complete physiology is based on arterial blood. As a result, samples collected anaerobically from an artery and heparinized represent the preferred sample material for the reliable assessment of acid-base metabolism and oxygen status. This sample material will provide evidence of diffusion, ventilation or perfusion disorders. Arterial blood can be collected by puncture of the femoral artery, brachial artery and radial artery aspiration from an indwelling arterial catheter or arterial cannula The key advantage consists in the homogeneity of arterial blood from the aorta to the peripheral circulation. The simultaneous sample withdrawal from the brachial, radial and femoral arteries at identical conditions will provide identical pH, pCO2 and pO2 values. Note: Arterial blood is largely collected with arterial catheters. In rare cases, the patient is punctured directly.   Puncture of the radial artery Aspiration from an indwelling arterial catheter  Other Facts IMPORTANT: If the patient is experiencing circulatory shock and the peripheral circulation is insufficient as a result, the content of the blood contained in the peripheral arteries and arterioles differs from the blood of the major arteries. In this case, collect the blood samples by means of arterial puncture, preferably the femoral artery. In infants younger than one year, blood can be collected by puncturing the heel (following compression). Always ensure the anaerobic withdrawal of the sample and the use of anticoagulants. This image shows lateral or medial area of the heel suitable for puncture in infants (hatched area).  Capilllary bloodTab TitleTextDescription In stable circulatory conditions, capillary blood sampling has been proven to be a practical and suitable alternative to arterial puncture, provided the following criteria are observed: Capillary blood is generally withdrawn from the earlobe or the heel of the foot (neonatology only). The selected area of skin should be warmed up prior to the puncture or the arterial circulation increased by other means to ensure the proper blood gas and pH measurement. Finalgon ointment (nicotinic acid ester) is commonly used for hyperemization. The puncture should be sufficiently deep to provide an unobstructed and rapid blood flow. The end of the capillary tube should have direct contact with the drop of blood to minimize the gas exchange of the sample with air. The risk of contamination with ambient air and the resulting falsification of the values are particularly high in this instance. This image shows capillary blood withdrawal from the hyperemized earlobe.    Other Facts Remember to hyperemize the corresponding skin area prior to the puncture, to enlarge the capillaries and increase the blood flow within the capillary vessel, e.g., with the application of Finalgon ointment.    Venous bloodTab TitleTextDescription Venous blood is not suitable for blood gas analysis because the oxygen exchange in the various regions of the body can lead to extreme differences of the values. Venous blood can be used to determine the hemoglobin, electrolytes and metabolites.    Other FactsNAMixed VenousTab TitleTextDescription Under certain conditions, mixed venous blood collected from an indwelling catheter in the pulmonary artery that was carefully cleared of infusion fluid can be used. For example, pCO2, pO2 and sO2 are relevant for the evaluation of the oxygen supply and oxygen exhaustion (heart surgery or heart catheterization).  Other FactsNA Select the X in the upper-right corner to close the window and continue. The use of a suitable blood collection system is crucial in analytical emergency procedures. The quality of the collection system used affects the accuracy of the analysis and hence the diagnostic procedure and the test results. The following blood collection devices can be used: Glass Syringes In glass syringes, the exposure to contamination by air is lower than synthetic ones as the walls are more resistant to air diffusion. Synthetic Syringes Synthetic syringes are easy to use. The gas permeability of the synthetic materials—especially CO2—constitutes a potential source of error if the time between withdrawal and evaluation is prolonged. Therefore, the sample must be analyzed immediately upon withdrawal. Synthetic syringes often contain Ca++-titrated lithium-heparin to inhibit coagulation. Capillary Tubes Glass and plastic capillary tubes of different volumes and with heparin coatings are used for capillary blood collection. They are prepared based on the conditions used to measure the individual parameters. Note: Refrain from using mixing rods because the hemolysis and consequently the falsification of the potassium values represent a source of error.   Anticoagulants Learn more about anticoagulants. IMPORTANT: Only use blood collection devices which contain calcium-titrated (balanced) lithium-heparin as an anticoagulant for whole blood samples. Other anticoagulants such as benzalkonium-heparin, EDTA, citrate, oxalate and fluoride significantly affect pH, sodium, potassium, chloride and ionized calcium results. Anticoagulants other than calcium-titrated heparin such as oxalate, citrate or EDTA cannot be used due to the induced pH shift. Sodium heparin must not be used if the sample will also be used to determine electrolytes. Due to its molecular structure, heparin binds cations, and Ca++ has the highest affinity to heparin among the measured electrolytes. In high-quality syringes or capillary tubes, this effect is negligible because heparin was pre-titrated and the free binding sites are occupied as a result. Ca++-titrated lithium heparin reduces the electrolyte binding, thus optimizing the accuracy. When using liquid heparin, ensure that sample volume is increased and the concentration of analytes reduced. Calculate heparin quantity so that the final concentration of the sample is between 50 and 100 IU/mL. When complete, select the X in the upper-right corner to close the window and continue. The observance of the following points is crucial: Mix the sample immediately after collection and before conducting the measurement. Prevent contamination of the sample with ambient air. Remember the influence of metabolic activities. Prevent hemolysis. Handling Samples Learn about handling samples for blood gas analysis. Tab TitleTextMixing Mix the sample immediately after the collection and before conducting the measurement Roll the blood collection device between your hands after collecting the sample and turn it gently to ensure the thorough mixture of the blood with the heparin. The sedimentation of erythrocytes causes the specimen to segregate, resulting in incorrect measurements for hemoglobin and hematocrit. To ensure the homogeneity of the blood sample, carefully mix the sample once more before performing the measurement. Mix the sample by rolling it between the palms of your hands.    Ambient Air Prevent the contamination of the sample with ambient air Contamination of the sample with air represents one of the most common sources of error in pre-analytical phase. Gas exchange caused by the presence of air may occur: during collection of capillary samples or by accidental aspiration of air during the sample collection. during collection of samples from an indwelling arterial catheter: please observe the dead space! due to diffusion of air through the wall of (synthetic) syringes— time-dependent The small sample CO2 concentration and the higher O2 concentration of air cause a shift in the values in the blood you wish to analyze in the respective direction of the air concentration. This is due to the equilibration tendency between the two media involving the risk of a decrease of the pCO2 in the sample and an increase of pO2 under normal conditions.     Prevent air bubbles The appearance of air bubbles should be prevented by: exercising care when collecting the sample with the careful retraction of the syringe plunger or by using self-filling syringes using precisely fitted syringes closing the sample container Should air bubbles occur, remove them prior to mixing the sample. Air bubbles can be removed by squirting out the air, e.g., into a swab, etc. (risk of infection!) or modern collection devices allow the safe removal of air bubbles. An air bubble of only 0.01 mL in the blood results in an increase of the pO2 value of more than 10%. Metabolic Activity The influence of metabolic activities The effect of metabolic activities increases proportionately with the time elapsed between the sample withdrawal and the actual measurement. Therefore, the sample should be measured without delay. Blood is a living medium: oxygen continues to be consumed even after the sample has been collected. This particularly affects the parameters pO2, glucose and lactate. Ideally, the measurements are conducted immediately. If the analysis is not being performed within ten minutes of sample withdrawal, the sample can be stored for a maximum of one hour at 0 to +4°C – in ice water, not directly placed on ice. Avoid the exposure to direct sunlight! Mix the sample once more prior to performing the measurement. Note that any storage will affect the values.  Hemolysis Prevent hemolysis Hemolysis can occur as a result of: freezing the sample strong shaking the sample forceful aspiration of the sample (application of excessive underpressure during aspiration) Hemolysis leads to falsely elevated potassium values and falsely decreased hematocrit values, depending on the system. When complete, select the X in the upper-right corner to close the window and continue.   Therapy decisions in emergency settings frequently need to be made within a few minutes. The rapid and accurate determination of vital laboratory parameters on site is indispensable for the immediate introduction of suitable procedures. Besides supplying accurate and reliable analytical results, it should be possible to use the corresponding systems regionally and they should be easy to operate by staff without special laboratory training. Analytical systems used for emergency diagnostic procedures should provide reliable results within seconds for the parameters of acid-base metabolism, oxygen status, pH value, electrolytes and metabolites in intensive care units, operating rooms, pediatric/neonatology settings and emergency admissions. Analyzers with integrated co-oximeter systems measure and display hemoglobin and the hemoglobin derivatives O2Hb, COHb, MetHb and HHb in addition to the detailed evaluation of the oxygen status. With respect to the electrolytes, pH and gas sensors, all systems should have an identical composition. The same methods should be used in all the locations (i.e. the same reference methods apply everywhere) to ensure absolute comparability of the values. Select Next to continue.   When scheduling a training session you can accept the default settings that your Training Center Web site provides, or you can specify several options for your training session. These options allow you to customize your training session for your specific needs. Once you schedule a training session, you can modify its options. You can also cancel a scheduled training session at any time. pH and pCO2 are the two measured parameters included in a blood gas analysis that help physicians assess acid-base metabolism. Measuring pH and pCO2 Learn the test method principles for measuring pH and pCO2. Select the tabs below to learn the test method principles for measuring pH and pCO2.TitleTextpHTab TitleTextMeasurement Principle The pH electrode is equipped with ISE technology. It is a half-cell; combined with a reference electrode, it forms a complete electrochemical cell. The pH electrode contains a silver/silver chloride wire covered by buffer solution (electrolyte with known pH). A glass membrane permeable for hydrogen ions separates the sample from the solution. When a sample comes into contact with the membrane of the pH electrode, a membrane potential forms due to the exchange of the hydrogen ions. The potential difference between the inner and outer solution based on this reaction is proportional to the hydrogen ion concentration. Consequently, it equals 0 if the hydrogen ion concentrations of the reference and measured solution are identical.     The inner silver/silver chloride conductor transmits the potential difference to a voltmeter where it is compared to the constant potential of the reference electrode. The measured potential reflects the hydrogen ion concentration of the specimen and is used to indicate the pH value. pCO2Tab TitleTextMeasurement Principle The pCO2 sensor is based on an electrode according to Severinghaus. This electrochemical cell consists of a measuring electrode and an inner reference electrode. The measurement electrode is a pH electrode, surrounded by buffer solution. The internal reference electrode, surrounded by chloride bicarbonate solution, supplies a constant potential. A CO2-permeable membrane separates this solution from the sample.   When the sample comes into contact with the membrane, CO2 diffuses into the internal chloride bicarbonate solution and triggers a change in hydrogen ion activity. The internal pH-electrode detects this change in potential. It leads to a measurement signal which reflects the pH change in the internal bicarbonate solution of the sensor. The change in pH corresponds to the partial CO2 pressure (= pCO2).     When complete, select the X in the upper-right corner to close the window and continue. Key measured parameters included in blood gas analysis that help physicians assess a patient’s oxygen status include: pO2 (oxygen partial pressure) tHb (total hemoglobin) Hemoglobin fractions: Oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin. Measuring pO2, tHb, and Hb Fractions Learn the test method principles for measuring pO2, tHb, and hemoglobin fractions. Select the tabs below to learn the test method principles for measuring pO2, tHb, and hemoglobin fractions.TitleTextpO2Tab TitleTextMeasurement Principle Oxygen Partial Pressure The pO2 sensor is based on an electrode according to Clark. It is a complete electrochemical cell, based on the amperometric principle of measurement. The sensor contains a platinum (Pt) cathode, a silver (Ag) anode, an electrolyte solution and a gas-permeable membrane. Constant voltage (polarization voltage) is maintained between the anode and cathode. If dissolved oxygen from the sample penetrates the membrane and enters the electrolyte solution, it is reduced at the cathode: O2 + 2 H2O + 4 e- → 4 OH- The quantity of reduced oxygen is directly proportional to the number of electrons used at the cathode. Therefore, the oxygen quantity in the electrolyte solution can be determined by measuring the current (electron flow) between the anode and cathode. Other Facts NA   tHbTab TitleTextMeasurement Principle Total Hemoglobin Cyanmethemoglobin Method Generally, hemoglobin is determined directly via photometry using the cyanmethemoglobin method. All hemoglobin derivatives (fractions) are oxidized to methemoglobin using potassium hexacyanoferrate (+3) and transformed into cyanmethemoglobin via potassium cyanide. The intensity of the resulting brownish color is measured photometrically at l = 546 nm. Other Facts Total hemoglobin can also be measured using co-oximetry or indirectly via conductivity measurement. See measurement principles for hemoglobin fractions.   Hb FractionsTab TitleTextMeasurement Principle Hemoglobin Fractions: oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin Co-Oximetry Method Different hemoglobin fractions absorb light at different wavelengths. The spectral absorption method determines the concentration by means of matrix equations. For each fraction, absorption A at a specific wavelength is equal to the product of distance covered l, concentration c and a molar absorption coefficient e. A = l x e x c In two measured substances, the measured absorption is the sum of the individual absorptions. To be able to determine the concentrations, measurements need to be conducted with two different wavelengths: A1 = l x (e1,1 c1 + e1,2 c2) A2 = l x (e2,1 c1 + e2,2 c2) Other Facts The co-oximetry process is analogous for total hemoglobin as well as all hemoglobin fractions. The total hemoglobin is the sum of all measured hemoglobin fractions and hence a measurement for the potential oxygen transport capacity. cHb = cO2Hb + cHHb + cMetHb + cCOHb Each hemoglobin fraction is determined individually via absorption measurement using the co-oximeter (spectrophotometer) at characteristic wavelengths; interferences by pigmented molecules such as bilirubin or turbidities are recognized and eliminated. When complete, select the X in the upper-right corner to close the window and continue.    Key tests ordered to quickly evaluate a patient’s metabolic function include tests that measure the metabolites glucose and lactate. Different measuring methods are available for testing blood glucose, the most frequently determined analyte, and lactate. Certain factors need to be considered depending on the method, sample type and area of application, to obtain correct and precise results.   Glucose Test Methods Lactate Test Methods Hexokinase Enzymatic Method Glucose Dehydrogenase Amperometry / Biosensors Glucose Oxidase Lactate Biosensor Amperometry / Biosensors   Glucose Biosensor   Measuring Glucose Learn about the test method principles listed above to measure glucose. Select the tabs below to learn about these test method principles to measure glucose.TitleTextHexokinaseTab TitleTextMesasurement Principle Hexokinase Method Glucose is transformed by hexokinase (HK) into Glucose-6-phosphate (Glu-6-P), which is transformed to phosphogluconolactone by Glucose-6-phosphate-dehydrogenase (Glu-6-P-DH) under reduction of the coenzyme NADP+. The increase of NADPH is proportional to the glucose quantity and is determined by means of photometry.  Other FactsThis method is deemed the reference method.DehydrogenaseTab TitleTextMesasurement Principle Glucose Dehydrogenase Method Glucose-dehydrogenase (Glu-DH) is a β-glucose-specific enzyme. The α-share is transformed into the β-form via Aldose-1-epimerase enzyme. The addition of this enzyme allows the acceleration of the speed-determining step and hence the duration of the analysis. The NADPH increase is proportional to the glucose content and is determined by means of photometry.  Other FactsBesides glucose, it also measures xylose levels.  This is not, however, a cause for concern at regular concentrations (< 2.5 mg/dL). This method cannot be used during a xylose absorption test.OxidaseTab TitleTextMesasurement Principle Glucose Oxidase Method In common urine test strips (“dry chemistry”) as well as certain older blood glucose measuring devices, the action of peroxidase (POD) reduces the resulting hydrogen peroxide. The color intensity of the concomitantly developing color indicator D2 (oxidation of the added chromogen D-H2) is proportional to the glucose content and is determined by means of photometry/reflectometry.  Other FactsNAAmperometryTab TitleTextMesasurement Principle Amperometry / Biosensors Method Amperometry involves measuring electrical currents. Biosensors consist of a biologically active component such as an enzyme and a conversion unit that converts the reaction between the biological material and the analyte into a measurable electrical signal. The biosensor allows the measurement in undiluted materials. The hydrogen peroxide generated during the first step of the reaction is anodically oxidized to oxygen by the polarization voltage. The quantity of released electrons is proportional to the glucose content.  Other FactsOne mol of oxidized hydrogen peroxide corresponds to one mol of glucose. The current coulombs is electronically converted into concentration.BiosensorTab TitleTextMesasurement Principle Glucose Biosensor Method The biosensor is equipped with four electrodes: the platinum measurement electrode applied to the glucose oxidase (GOD) enzyme separates the electrodes from the sample the Ag/AgCl reference electrode a platinum counter electrode for the stabilization of a constant potential an additional platinum measurement electrode determines the substances which may interfere with the enzymatic reaction process. The potential of the interfering substance is eliminated by the differential measurement. A microporous membrane separates the electrodes from the sample   A constant polarized voltage is applied during the measurement. Glucose is oxidized to D-Gluconate at the surface of the measurement electrode through the enzyme GOD; hydrogen peroxide is generated in the process (see reaction 1). Hydrogen peroxide oxidizes to become oxygen as a result of the polarization voltage (see reaction 2). The electrons that were released during the oxidation increase the current flow proportional to the glucose concentration of the sample. Other FactsThe glucose sensor is a specific biosensor for measuring glucose. It is a complete electrochemical cell used to determine the concentration of glucose by means of amperometry. When complete, select the X in the upper-right corner to close the window and continue.  Measuring Lactate Learn about the test method principles listed above to measure lactate. Select the tabs to learn about methods to measure lactate.TitleTextEnzymatic MethodTab TitleTextMeasurement Principle Enzymatic Method Lactate is oxidized to pyruvate by the lactate dehydrogenase enzyme in the presence of the coenzyme NAD+. Because the reaction balance is much more pronounced on the lactate side, certain reaction conditions (alkaline milieu, recovery of the formed pyruvate) need to be ensured for the quantitative oxidation. The increase of NADH is proportional to the lactate quantity and is determined by means of photometry.  Other FactsNAAmperometryTab TitleTextMeasurement Principle Amperometry / Biosensors Amperometry involves measuring electrical currents. Biosensors consist of a biologically active component such as an enzyme and a conversion unit that converts the reaction between the biological material and the analyte into a measurable electrical signal. The biosensor allows the measurement in undiluted materials. Lactate is transformed into pyruvate through lactate oxidase (LOD). The hydrogen peroxide generated in the process is oxidized to oxygen by the polarization voltage. The quantity of nascent electrons is proportional to the lactate quantity in the sample.  Other FactsOne mol of oxidized hydrogen peroxide corresponds to one mol of lactate. The current in coloumbs is electronically converted into concentration.Lactate BiosensorTab TitleTextMeasurement Principle Lactate Biosensor Method The lactate sensor is equipped with four electrodes: the platinum measurement electrode applied to the lactate oxidase enzyme an Ag/AgCl reference electrode a platinum counter electrode for the stabilization of a constant potential an additional platinum measurement electrode without enzyme determines substances which might interfere with the enzymatic reaction process. The potential of the interfering substance is eliminated by the differential measurement   A constant polarized voltage is applied during the measurement. Lactate from the sample is oxidized to pyruvate (salt of the pyruvic acid) at the surface of the measurement electrode through the lactate oxidase enzyme; hydrogen peroxide is generated in the process (see reaction 1). Hydrogen peroxide is oxidized to oxygen by the polarization voltage (see reaction 2). The electrons that were released during the oxidation increase the current flow proportional to the lactate concentration of the sample. Other FactsThe lactate sensor is a specific biosensor for measuring lactate. It is a complete electrochemical cell used to determine the concentration of lactate by means of amperometry. When complete, select the X in the upper-right corner to close the window and continue.  The serum electrolyte concentration can be determined using different methods: Flame Atomic Emission Spectrophotometry (FAES) Coulometry – Conductivity Measurement: Chloride Direct and Indirect Potentiometry [Ion Selective Electrodes (ISE)] Important: FAES, coulometry and indirect potentiometry determine the ion concentrations. If the extracellular water share decreases due to hyperlipidemia or hyperproteinemia, the essentially regular electrolyte concentration appears to be decreased as well, thus causing “pseudohyponatremia” and “pseudohypochloridemia”. Due to the large reference interval, the effect is not as pronounced with respect to potassium. The ion activity is determined by means of direct potentiometry. This measurement is not affected and allows the correct interpretation. Select Next to continue.   Flame Atomic Emission Spectrophotometry (FAES) When an alkali metal solution is held in a flame, the fluid evaporates and the salt ions are atomized. Each atom absorbs energy. As excitation energy, it directs the outer-shell electron of the alkali metal atom to a different orbital. Upon return to the original orbital, the electron releases the energy in the form of light at a wavelength that is characteristic for the atom. The intensity of the emitted light depends on the number of atoms of the corresponding element in the flame and is therefore proportional to the concentration. To conduct the measurement, the ion strength of the diluted sample (serum or plasma) is adjusted to the calibration solution and the electrolyte-free compartment of macromolecules is reduced to below 1% of the total volume. Similar to indirect potentiometry, the measured signals are converted into concentrations by comparing them with the calibration solution. The macromolecule (proteins, lipids, etc.) concentrations affect the measurement and hence the determination of the concentration. As a result, these measurements are only applicable to a certain lipid and protein concentration. When determining the electrolytes according to this method, the values for total protein and lipids should always be determined too, to allow the correct interpretation of the results. Select Next to continue. Today, coulometry is used to determine chloride in serum, urine and other body fluids. In this highly sensitive electrochemical analysis method, the electrical current is measured over time between two electrodes through which it is flowing. The consumed amount of electricity can be calculated based on this analysis. where Q = quantity of electricity (current) I = electric current in amperes t = time in seconds, during which the current is flowing According to Faraday’s Law, this amount of electricity Q is equivalent to the amount of converted chloride (N), in accordance with where a = device-specific constant. Select Next to continue. Potentiometry measures the voltage or potential generated between two electrodes in an electrochemical cell when no current is flowing. The electrochemical cell consists of two electrodes (a measuring and a reference electrode), an electrolyte solution (sample) and a measurement system, e.g. a voltmeter. The electrochemical cell can measure the concentration or activity of a substance in a solution. Together with the measuring electrode, the reference sensor of the system forms an electrochemical cell within the measurement module (E-cell). It supplies a constant potential which is dependent on the analytical activity. The system compares the constant potential of the reference sensor (E-Ref) with the measured potential of the measuring electrode (E-Meas) for the respective analyte. The reference sensor contains a silver wire coated with silver chloride (AgCl) and an ion-permeable polymer surrounded by a saturated potassium chloride (KCl) solution. As a result, the chloride concentration in the solution remains unchanged and the reference sensor maintains a constant electrical potential. The chamber of the reference sensor contains a potassium chloride (KCl) donor to ensure the saturation of the solution. The fluid potential (EFl), a small but significant voltage, develops at the transition of the fluid from the reference electrode, between the saturated potassium chloride solution on the inside and the sample solution on the outside. This potential is the result of different speeds at which the chemical components diffuse through the borders between the fluids and needs to be deducted from the measured potential. ECell = EMeas - (ERef + EFl) Direct and Indirect Potentiometry Learn more about direct and indirect potentiometry. Direct potentiometry ("direct ISE") in emergency analytical systems (without dilution) Direct potentiometry measures the ion activity. The macromolecule concentrations (proteins, lipids) do not affect this measurement. Because the extracellular water phase (plasma or serum water) is measured here, the correct interpretation of the results is also possible without knowing the lipid or protein content. The ion activity is independent hereof. Contrary to the determination of the concentration (flame photometry and indirect ISE), direct ISE records the medically relevant parameter. Indirect potentiometry ("indirect ISE") in clinical-chemical analytical systems (with dilution) Indirect potentiometry determines the concentration and only represents an estimate of the activity or free molar concentration. The water content is decreased in hyperlipidemia or hyperproteinemia. Consequence: It is possible that "pseudohyponatremia" or "-chloridemia" are simulated with regular electrolyte concentration in the serum. (The effect of hyperlipidemia or hyperproteinemia with respect to potassium is less pronounced due to the relatively large reference interval). When complete, select the X in the upper-right corner to close the window and continue. Na+, K+, Ca++, and Cl- Sensors Learn about sodium, potassium, calcium, and chloride ion selective electrodes. Select the tabs below to learn about sodium, potassium, calcium, and chloride ion selective electrodes.TitleTextNA+Tab TitleTextMeasurement Principle Sodium The sodium sensor is a half-cell forming a complete electrochemical half-cell together with the external reference sensor. The sensor is equipped with an Ag/AgCl wire, surrounded by an electrolyte solution with defined sodium ion and chloride ion concentrations. The membrane that separates the electrolyte solution from the specimen consists of a glass or PVC capillary tube and is highly-selective for sodium ions. When the sample comes into contact with the membrane, a potential develops due to the sodium ion exchange. The membrane potential is compared to the constant potential of the reference sensor. The measured potential difference is proportional to the sodium ion concentration in the sample and changes with the ion activity. K+Tab TitleTextMeasurement Principle Potassium The potassium sensor is a half-cell. Together with the external reference sensor, it forms a complete electrochemical half-cell. The sensor is equipped with an Ag/AgCl wire, surrounded by electrolyte solution with a defined concentration of potassium ions. The membrane consists of ionophoric Valinomycin in plasticized PVC. It separates the electrolyte solution from the sample. Valinomycin is a neutral ion carrier and highly-selective for potassium ions. The contact between the sample and the membrane results in a potential due to the potassium ion diffusion through the membrane. The membrane potential is compared to the constant potential of the reference sensor. The measured potential difference is proportional to the potassium ion concentration in the sample. Consequently, it changes based on the ion activity. Ca++Tab TitleTextMeasurement Principle Calcium The calcium sensor is a half-cell. Together with the external reference sensor it forms a complete electrochemical half-cell. The sensor contains an Ag/AgCl wire, surrounded by an electrolyte solution with a defined concentration of calcium ions. The membrane consists of an ionophore, embedded in a PVC membrane. It separates the electrolyte solution from the sample. The contact between the sample and the membrane results in a potential due to the calcium ion interaction with the membrane. The membrane potential is compared to the constant potential of the reference sensor. The measured potential difference is proportional to the calcium ion concentration in the sample and changes based on the ion activity. Cl-Tab TitleTextMeasurement Principle Chloride The chloride sensor is a half-cell. Together with the external reference sensor, it forms a complete electrochemical half-cell. The sensor is equipped with an Ag/AgCl wire, surrounded by electrolyte solution with a defined concentration of chloride ions. A membrane made of a PVC matrix in quarternary amine, a highly selective ion exchanger for chloride ions, separates the electrolyte solution from the sample. The contact between the sample and the membrane results in a membrane potential due to the chloride ion exchange. The potential is compared to the constant potential of the reference sensor. The measured potential difference is proportional to the chloride ion concentration in the sample and changes based on the ion activity. When complete, select the X in the upper-right corner to close the window and continue.