Comparison of serum calcium measurements obtained with the SMA 12-60 and by atomic absorption spectrophotometry.

Comparable results are obtained when serum calcium is measured by a colorimetric cresolpthalein method on the Technicon SMA 12/60 and by atomic absorption spectrophotometry (Instrumentation Laboratory Model 153 instrument), with strontium as an internal standard. Sera from 250 patients with abnormal calcium concentrations, ranging from 4.9 mg/lOU ml to 9.0 mg/100 ml, were measured by each method; the difference between the means for each method was only 0.04 mg/lOG ml. The standard deviation of the difference between paired results was ±0.16 mg/lOG ml. We were unable to discover any effect of drug or disease on the colorimetric assay, which therefore is suitable for use in large-scale surveys. On the other hand, the more precise atomic absorption spectrophotometry is suitable for use as a reference method. In addition, with an internal standard it is fast and simple, making it suitable for emergency assays of calcium 24 h a day.