Automated Serum Calcium Analyses Corrected for Dialysis Effects

Abstract

Serum calcium assays by conventional continuous flow methods use aqueous standards. Less calcium diffuses across the dialyzer from aqueous standards than from the serum samples, producing falsely elevated serum values, especially when a miniature manifold and dialyzer are employed. The difference between the rates of dialysis of calcium in serum and in water has been attributed to the Donnan effect of polycationic protein at an acid pH. The differences in electrical potential across the dialyzer membrane were measured directly. Serum produced a negative difference in potential, the opposite of that required by the Donnan effect. Therefore, other factors, such as osmotic pressure, must be considered. For the continuous flow cresolphthalein complexone (CPC) method, the dialysis error can be corrected by 8% polyvinylpyrrolidone in the calcium standards, and an HC1 concentration of 0.3 N in the diluent stream and 1 N in the recipient stream. Ethylene diamine-tetraacetate (EDTA) in the base reagent subtracts a constant amount of calcium (about 3 mg. per dl.) from all the samples, allows the recorder scale to be expanded, and reduces sample carryover. A revised miniature manifold, used with a one-piece Gilford debubbler flow cell and a linear absorbance spectrophotometer, allows analysis of 100 samples per hour. Peak heights are directly proportional to concentration, thereby eliminating logarithmic conversion.