Plasma creatinine and the effect of delay in separation of samples

There is a prevailing opinion that creatinine is an analyte which may be assayed with confidence in the plasma of ageing whole-blood samples. This is true of continuous flow methods employing dialysis, but in this study, where creatinine was measured in old samples by a kinetic Jaffé method on a discrete analyser, the concentration of creatinine was falsely raised. When plasma was left in contact with cells for 48 h, there was an apparent rise in the plasma creatinine by over 100 mumol/L compared with the 2 h concentration. This method-dependent error is therefore clinically significant.