Evaluation of the ASTRA o-cresolphthalein complexone calcium method

An automated calcium method based on o-cresolphthalein complexone chemistry and designed to be used in conjunction with the Beckman ASTRA systems (Beckman Instruments, Inc., Brea, CA) was evaluated. Day-to-day reproducibility studies yielded coefficients of variation ranging from 1.2 to 3.1%. Linearity was 5.0-15.5 mg/dL, and recovery of calcium averaged 98%. Bilirubin, creatinine, hemoglobin, and turbidity did not cause significant interference while there was a positive interference due to magnesium. The throughput is 70 samples per hour, and a single stat result can be obtained in 9 minutes from the standby mode. Very little sample (12 microL) or reagent (150 microL) is required. There was excellent correlation between the ASTRA calcium method and atomic absorption spectrophotometry (Y = 1.047x - 0.39, r = 0.9941). Analytic performance was judged acceptable according to Westgard's medical usefulness criteria of the observed total analytic errors being less than the limit of allowable error.