Interindividual variation in relationships between plasma heparin concentration and the results of five heparin assays.

In order to determine their value for estimating the heparin concentration in plasma, we established the relationship between test result and heparin concentration in plasma from various individuals, for five assays used with heparin treatment. Only assays which can be carried out routinely in clinical laboratories were considered. The thrombin time and the whole blood recalcification time give pointless and ambiguous information respectively, concerning the heparin level. The activated partial thromboplastin time with and without heparin neutralisation give only a rough estimate. The spectrophotometric method using a chromogenic substrate gives the best information. The latter can be improved by using a non-linear (parabolic) equation for the calculation of the reference curve. Current heparin therapy, controlled with the aid of a clotting assay, may result in plasma heparin concentrations that vary widely from one patient to another.