A new method is described for the measurement of serum calcium. Calcium in the sample displaces zinc from an EGTA complex, the zinc then forming a blue-coloured complex with a zincon indicator. The method is rapid, sensitive and linear; it may be applied directly to serum and is readily automated. No compound tested gave rise to any interference at physiological concentration, although excessive haemolysis leads to low results being obtained. To avoid interference from endogenous zinc a serum blank is required for each sample; this also avoids errors introduced by turbidity of the sample. The results obtained correlate well with atomic absorption spectroscopy (r = 0·935, y = 1·12x–0·28, n = 68).
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