Non-specific interference in the measurement of plasma ammonia: importance of using a sample blank

Enzymatic assays using glutamate dehydrogenase (GLDH) to monitor the transformation of NAD(P)H to NAD(P)(+) by a spectrophotometric technique are the most common methods to measure plasma ammonia (PA) in routine laboratories worldwide. However, these assays can potentially be subject to interference by substances in plasma able to oxidize NAD(P)H at a substantial rate, thereby providing falsely high results.

To study this potential interference, we spiked a plasma pool with a liver homogenate and measured the ammonia concentration using a dry chemistry system (Vitros 250, Ortho Clinical Diagnostic, Raritan, NJ, USA), an enzymatic assay without a sample blanking step (Infinity Ammonia Liquid Stable Reagent, Thermo Fisher Scientific, Waltham, USA) and an enzymatic assay that corrects for the non-specific oxidation of NADPH (Ammonia kit, RANDOX Laboratories Ltd, Crumlin, UK).

This experiment shows that the Infinity ammonia reagent kit is subject to a clinically significant interference and explains the discrepancies previously reported between these methods in patients with acute liver failure (ALF).

When using enzymatic methods for the assessment of PA, we recommend including a sample blanking correction and this should be mandatory when monitoring patients with ALF.