Two methods for the direct colorimetric determination of serum calcium (BMT and OCFT) have been evaluated and compared with atomic absorption spectrophotometry (AAS). Both colorimetric methods resulted simple to use, but temperature-sensitive: values for the ratio absorbance at 10 degrees C/absorbance at 45 degrees C were 1.6 (BMT) and 1.8 (OCFT). When readings were taken at 37 degrees C good precision was achieved with both methods (between-the-series coefficients of variation were 3.5 divided by 2.2% for OFCT and 1.9 divided by 3.4% for BMT). No interference from bilirubin (up to 180 mumol/l) was observed with both methods; slight positive interference from haemoglobin (1 g/l) was observed only with OCFT method. Results from the two colorimetric methods closely agreed in the range 1.0 divided by 4.5 mmol/l: statistical analysis of comparison data gave BMT = 0.0963 + 0.9521 OCFT, r = 0.9820. From the results of direct comparison with AAS (103 serum specimens, concentration spanning from 1.0 to 4.5 mmol/l) statistical analysis gave: BMT (y)/AAS(x): y = 0.0721 + 1.0226x (r = 0.9678); OCFT(y)AAS(x): y = -0.0208 + 1.0720x (r = 0.9836): it is concluded that both methods show a slight positive bias with respect to AAS.
