Interference by haemoglobin with the cresolphthalein complexone method for serum calcium measurement

The spectral interference caused by haemoglobin in the calcium-cresolphthalein complexone reaction was investigated. The results show that the nature of the haemoglobin derivative in the reaction mixture influences the degree of interference; in turn, the derivative formed depends on the order in which the sample and reagents are added. Methods using a single, alkaline cresolphthalein complexone reagent are most seriously affected by haemolysis due to persistence of oxyhaemoglobin. If the sample is added to an acid cresolphthalein complexone reagent then acid haematin is formed, this being converted to a different derivative on the addition of alkali, producing less pronounced interference. The addition of the sample to a buffer, followed by addition of complexone, appears to offer a means to correct for moderate haemolysis in the sample.