Heparinized plasma is an unacceptable specimen for ammonia determination.

The glutamate dehydrogenase (GLDH, EC 1.4.1.3) reaction is almost universally used for the determination of ammonia. Some kit manufacturers specify either heparinized or EDTA plasma, whereas others recommend EDTA only, but without saying why. The procedure in Volume 9 of Selected Methods in Clinical Chemisty (1) requires EDTA but does not discuss the consequences of substituting heparin. We have observed a phenomenon that demonstrates that heparin is sometimes an unsuitable anticoagulant for ammonia determination. The usual procedure, whether automated or manual, is to mix the sample with a buffer containing NADPH and a-ketoglutarate. An abeorbance measurement is made, a second reagent containing GLDH is added, and absorbance is measured again. The difference between the two absorbances is proportional to ammonia concentration. Some heparinized samples produce unstable initial absorbances that drift upward, indicating that a reaction involving endogenous substrates and enzymes is creating NADPH (or some other absorber). The upward drift may require >30 mm to be complete. If the GLDH is added before this time, the resulting ammonia value will be too low, perhaps even negative. This effect is not seen with samples collected with EDTA. We estimate that this has happened with about one in every 10 to 20 patients’ specimens at the Bryn Mawr Hospital. The patients involved have had no common features from which we could deduce an explanation. The reagent we use contains EDTA in its formulation, so the observed effect must be due to some EDTA-inhibited process that occurs in specimens before the test is run. Bryn Mawr Hospital Bryn Mawr, PA 19010