Microdetermination of serum calcium by parallel fast analyzer

An adaptation of a direct microdetermination of serum calcium based upon its colored reaction product with cresolphthalein complexone for use with a parallel fast analyzer is described. It is simple, reliable and clinically accurate. Twenty microliters of sample are employed in a final volume of 650 mul. Comparison studies with an atomic absorption method resulted in a correlation coefficinet (r) of 0.94. Replicate analyses at normal (8.5 mg. per dl.) and abnormal (12.1 mg. per dl) levels showed inter-day coefficients of variation of 3.2 and 2.3%, respectively. Assays of sera of male and female donors (age range: 17--60 years) showed normal ranges of 8.5--10.8 and 8.5--10.5 mg. per dl., respectively. Moderate hemolysis and jaundice and mild lipemia are associated with spuriously high results. Magnesium in serum showed no significant effect below 4.0 mEq. per l.