Investigations of blood ammonia analysis: Test matrices, storage, and stability.

An assessment of blood ammonia concentration is common medical practice in the evaluation of an individual with an unexplained mental status change or coma. The determination of a blood ammonia level is most commonly done using a glutamate dehydrogenase (GLDH)-based assay, although there are many potential sources of artifact and the literature is inconsistent regarding key preanalytic issues. Using a GLDH-based assay, we first investigated matrix effects using three anticoagulants: heparin, EDTA and oxalate. Heparin-anticoagulated plasma was substantially less precise than EDTA- and oxalate-anticoagulated plasma. Oxalate-anticoagulated plasma showed a greater baseline of apparent ammonia than either heparin- or EDTA-derived plasma, presumably due to interferants. We then evaluated the stability of EDTA-anticoagulated plasma for assessment of ammonia when stored at 4°C,-14°C or -70°C. There was a linear increase of ammonia with storage at both 4°C and -14°C. Plasma kept at -70°C for up to three weeks showed no change in measured ammonia relative to the baseline determination. This work clarifies preanalytic conditions for which a precise determination of ammonia can be accomplished using a GLDH-based assay.