The Relation between the Ultrafiltrable Calcium Fraction and Blood pH and Concentrations of Total Plasma Calcium, Albumin, and Globulin

摘要

With the advent of various micropartition systems, typically as developed by Amicon, it became simple to ultrafilter serum and to obtain an apparent measure of the ultrafiltrable calcium fraction (UFCa). In studies of this type, evidence of strict monitoring of serum pH, known to affect calcium binding, was not generally reported (1)(2)(3)(4). An earlier study (5), attempted to control temperature and P co2, but in our hands the study was difficult to reproduce (unpublished). We have determined the UFCa values of 54 samples of whole blood at 37 °C at or near physiologic pH and sought an empirical relationship with more readily measured biochemical variables, which might enable us to predict UFCa from routine laboratory results. We were particularly interested in data in mild renal failure.Although any aqueous solution is subject to control by the Henderson-Hasselbach relation, the relatively low p K (6.1) means that the buffering capacity near pH 7.4 is poor. Furthermore, modest changes in the P co2 have a minor effect on the HCO3 concentration, for a marked effect on the pH. We found that the buffering capacity of whole blood allowed confidence in our procedure because the pH remained stable during handling. The Millipore? UFU 4 system allowed lateral filtration without the erythrocytes obstructing the membrane such that an ultrafiltrate of whole blood could be obtained.We used Millipore UFU 4 10KNML systems, shortened to 20 mm by cutting off part of the upper reservoir. Initially, we loaded 1 mL of plasma, but in our final work, we used 1 mL of heparinized venous blood. In pilot experiments, we took 1-mL aliquots of a pooled serum sample (five specimens) and altered the pH by adding 0.1 mol/L HCl or 0.1 mol/L NaOH. The volumes added …