Heparin interference in the measurement of gamma-glutamyltransferase activity with the Scandinavian and the IFCC recommended method.

Heparin in heparinized plasma, and when added to serum, is shown to interfere markedly in the assay of gamma-glutamyltransferase when using the method recommended by both the Scandinavian Society for Clinical Chemistry and Clinical Physiology, and the Expert Panel on Enzymes of IFCC. The effect is also revealed as an apparent sample blank reaction (donor substrate omitted). It decreases as the time of first reading is increased, and it can be reduced or nearly abolished by addition of extra NaCl (increasing the ionic strength). Evidence is presented suggesting that this photometric interference is caused by turbidity due to complex formation between heparin and various plasma proteins. Fibrinogen appears to be one of the main proteins involved. It is concluded that in general care should be taken when using heparinized plasma in enzyme assays, and that heparin should only be used when specific testing has ruled out photometric interference.