Measurement of Ionised Calcium in Body Fluids—A Review

Abstract

This paper reviews the techniques available to the clinical biochemist for measuring ionised calcium concentrations in biological fluids with particular reference to serum. At present ionised calcium may be measured colorimetrically, using tetramethyl murexide, or potentiometrically, using a calcium ion-selective electrode. These techniques compare favourably in terms of precision with existing methods for measuring total calcium. Advantages of measuring ionised calcium in preference to total calcium are ( a) that there is no effect of venous occlusion or change of posture on the ionised fraction of the effect on total calcium, ( b) that ionised calcium is the physiologically active form of the element, and ( c) that the ionised calcium concentration is a more reliable indicator of the calcium status of patients in certain clinical conditions. The main problem in the measurement of ionised calcium is the marked dependence of the ionised fraction on the pH of the sample. Extreme care must be taken to avoid loss of CO2 or build-up of acid during the handling of the blood sample.

Conformational Comparability of Factor IX–Fc Fusion Protein, Factor IX, and Purified Fc Fragment in the Absence and Presence of Calcium

“…Thermal studies aimed at characterizing the melting transitions for rFIX and rFIX–Fc in the presence of Ca 2+ were conducted using 2 mM CaCl 2 . This concentration of Ca 2+ was selected because it closely resembles physiological concentrations 40,41. As shown in Figure 1b, Ca 2+ does not appear to significantly change the T m (at 58°C) of the FIX peak (<0.5°C) in rFIX or rFIX–Fc.…”

Renal Excretion of Calcium

“…About 50% of the urinary calcium is normally present in the ionized form ( Fig. 6.2) and the remainder is complexed mainly with citrate, sulphate, phosphate and oxalate (Robertson 1969).…”

“…It is important before analysis that all calcium which has precipitated in vivo or after voiding is redissolved, and to ensure this the urine should be collected in a small quantity of acid. Ionized calcium is difficult to measure accurately, either with ionselective electrodes or indirectly by tetramethyl murexide dye, since the Robertson 1969) concentration range of interfering urinary ions such as sodium and hydrogen is so wide that the measurement lacks precision (Robertson 1969).…”

“…The concentration is the free ionized concentration as distinct from total concentration. To avoid the difficulty of measuring the free concentration of the ions in urine, the activity product of the calcium salt may be calculated from a knowledge of the total concentrations of calcium and oxalate and the concentrations of all their corresponding complexes Robertson 1969). The saturation of urine by a calcium salt occurs in anyone of three zones (Fig.…”

Impedance of Results Using Lithium Heparin to Plain Tubes for Ionized Calcium

“…The introduction of analyzers based on Ion-selective technology made the measurement of iCa +2 more reliable, rapid and reduced reporting time [2]. The sample for iCa +2 should be anti-coagulated with measured quantity of heparin since increased quantities of heparin may lower iCa +2 levels by binding with it, and the most important factor while measuring iCa +2 is to see that there is no change in pH of the blood [3]. Thus, handling of sample and transport of sample for analysis must be carefully done.…”