Assay Improves Measurement of Blood Ammonia

Increased blood ammonia is an indicator and a prognostic factor for hepatic or kidney disease and inborn errors of amino acid metabolism. Ammonia can be also increased due to therapy with drugs such as valproic acid and chemotherapeutic agents.

As both the degree of ammonia elevation and its duration are key determinants of the clinical outcome, hyperammonemia has to be recognized as early as possible. Accurate plasma ammonia measurement remains a challenge as many factors can influence the outcome.

Clinical chemists at the Academic Medical Center (AMC, Amsterdam, the Netherlands) collected patient plasma specimens as a part of routine patient care. EDTA plasma pools was made by pooling several EDTA plasma samples. These EDTA plasma samples were collected in pre-chilled K2 + EDTA Vacutainer tubes. Only EDTA plasma samples with low hemolytic (H < 5), icteric (I < 15) and lipemic index (L < 15) were used. Plasma pools were stored at − 80  °C until the analysis.

The team analyzed the samples on the Cobas c502 adapting the Roche NH3L kit. Imprecision was tested with three replicate analyses in a single run over five consecutive days for two clinically relevant levels of ammonia concentration: L1: Roche control abnormal (ammonia=200 µmol/L) and L2: Roche control normal (ammonia = 60 µmol/L). Agreement with Roche method was tested by comparing AMC NH3L adaptation with Roche rerun application. In total 63 samples across the linearity range (21 and 686 µmol/L) were compared.

The scientists reported that their AMC NH3L adaptation demonstrated acceptable within-run and total precision. Comparison studies show no differences between the Roche rerun application and AMC NH3L adaptation. The AMC NH3L adaptation solves 78% of absorbance errors and for samples with high ammonia concentration is less affected by interferences from icterus and hemolysis than the Roche rerun application. For high ammonia concentration the AMC NH3L adaptation suffered less from hemolysis, icterus and lipemia and showed no significant interferences up to I index of 1376 (unconjugated bilirubin = 1,017 μmol/L), H index of 221 (hemoglobin = 0.260 mmol/L) and L index of 71 (triglyceride = 3.3 mmol/L).

The authors concluded that the AMC NH3L adaptation is less prone to instrument error flags and for samples with high ammonia concentration, is more robust to endogenous interferences. The AMC NH3L adaptation is viable alternative to the Roche protocol for the ammonia measurement. The study was published on December 7, 2018, in the journal Practical Laboratory Medicine.

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