The substance concentration of ionized calcium (c Ca2+) in blood, plasma or serum preanalytically may be affected by pH changes of the sample, calcium binding by heparin, and dilution by the anticoagulant solution. pH changes in whole blood can be minimized by anaerobic sampling to avoid loss of Co2, by measuring as soon as possible, or by storing the sample in iced water to avoid lactic acid formation. cCa 2+ and pH should be determined simultaneously. Plasma or serum: If centrifuged in a closed tube, and measured immediately, the pH of the sample will be close to the original value. If a delay has occurred between centrifugation and the measurement, causing substantial loss of Co2, equilibration of the sample with a gas mixture corresponding to pCO2= 5.3 kPa prior to the measurement is recommended. Conversion of the measured values to cCa 2+ (7.4) is only valid if the pH is in the range 7.2-7.6. Ca2+ binding by heparin can be minimized by using either of the following: (1) A final concentration of sodium or lithium heparinate of 15 IU/ml blood or less (2) Calcium titrated heparin with a final concentration of less than 50 IU/ml blood. Dilution effect can be avoided by use of dry heparin in capillaries or syringes. When heparin solutions are used, errors due to dilution or calcium binding can be reduced by using syringes with a heparin solution containing free calcium ions corresponding to the mean concentration of ionized calcium in normal plasma. Conditions for blood collection, storage, and transport to avoid preanalytical errors are described in this paper.
This paper will familiarize the reader with the terms used to describe the behavior of ion-selective electrodes, particularly in relation to their use in clinical chemistry for determination of blood electrolyte cations. It serves as an introduction to a series of papers dealing with important cations in blood, namely calcium, sodium, and potassium. The detailed relationships between the ion activity determined by means of ion-selective electrode potentiometry in undiluted specimens, and the total substance concentration measured by flame atomic-emission spectrometry are described by flow chart and equations. Adoption of a convention for reporting results is recommended. The Working Group on Selective Electrodes has taken into account recent revisions of IUPAC recommendations on nomenclature and selectivity coefficient determinations for ion-selective electrodes, and benefited from the experience of a member of the WG, who was also involved in the IUPAC discussions. Nomenclature for determined quantities follows previous IUPAC/IFCC joint recommendations.
Ion-selective electrodes (ISEs) respond to ion-activity and therefore do not sense substance concentration directly. However, it is recognized that sodium and potassium in plasma will continue to be expressed for clinical purposes in terms of substance concentration (mmol/l). A convention is proposed whereby for routine clinical purposes results of ISE measurements of sodium and potassium in undiluted plasma should be reported in terms of substance concentration (mmol/l). In specimens with normal concentrations of plasma water, total CO2, lipids, protein and pH, the values will concur with the total substance concentration as determined for example by flame atomic emission spectrometry (FAES) or ISE measurements on diluted samples. In specimens with abnormal concentrations of plasma water, the results will differ. However, under these circumstances, measurements of sodium and potassium by ISE in the undiluted sample will more appropriately reflect the activity of sodium and potassium and are therefore clinically more relevant than the determination in diluted samples. Detailed recommendations are made about practical procedures to achieve this. The recommended name for this quantity is the substance concentration of ionized sodium or ionized potassium in plasma, as opposed to total sodium or total potassium determined by, e.g. FAES, or ISE measurements on diluted samples.
Schmidt and M ller-Plathe: Stability of/?O 2 , pCO 2 and pH: Influence of storage temperature and blood cell metabolism 767 Eur. J. Clin. Chem. Clin. Biochem. Vol. 30, 1992, pp. 767-773 © 1992 Summary: Influences of storage temperature and blood cell metabolism in different types of syringes were investigated. Experiments were performed on blood samples with normal and elevated leukocyte counts. After equilibration with gas mixtures at normal pO 2 (86 mm Hg/11.5 kPa) and elevated pO 2 (140 mm Hg/18.7 kPa), sequential blood gas analyses were done within one hour. Storage temperatures were 4 °C or 22 °C.In the first group of experiments we compared glass samplers with plastic syringes at different storage temperatures with regard to deviations of blood gas concentrations. The analysed samples had a normal cell count. Blood stored in glass syringes in ice water served as the reference, and it displayed virtually no changes. The deviations of pCO 2 and pH were relatively small. In plastic syringes the greatest increases for pO 2 occurred after storage at 4 °C, which can be explained by the increased solubility of oxygen and the higher O 2 affinity of haemoglobin at 4 °C. When stored at room temperature, the deviations in plastic syringes were smaller.In a second group of experiments, the influence of cell metabolism was studied. Blood gases were analysed in samples with elevated leukocyte counts (20 χ 10 9
The alterations of blood gases, pH, electrolytes and haemoglobin during 45 min storage in icewater were measured in 6 types of syringes (1 glass and 5 plastic syringes, among these 3 "blood gas samplers"). It was confirmed that pO 2 generally is not stable in plastic syringes. However, considerable differences among plastic syringes were found in this respect, the smallest increase occuring in an ordinary 2 ml syringe for injections and the greatest in one of the special blood gas samplers. Due to the "buffering effect" of deoxyhaemoglobin, the alterations of pO 2 are smaller in the hypoxaemic than in the normoxaemic range. Relevant pO 2 alterations in plastic syringes are demonstrable after 20 minutes. It is concluded that blood collected in plastic syringes must be analysed within 15 min after sampling, otherwise glass syringes should be used for blood collection. Deviations of /?CO 2 , pH and electrolytes are described in detail. In general, they are due to sampling rather than to storage, and can be effectively minimized by a small dead space of the syringe and by use of an electrolyte-balanced heparin solution. The danger of erroneous haemoglobin measurements due to unequal resuspension of the red cells after storage is pointed out.
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