I am a chemical pathologist and metabolic medicine consultant and clinical lead for adult inherited metabolic diseases at Guys and St Thomas Hospital.

Plasma ammonia measurement is a time critical test in the investigation and monitoring of patients with inherited metabolic disorders and liver failure. Clinicians rely on ammonia results to guide urgent treatment decisions; however, assay related analytical limitations can significantly affect result reliability. 

This presentation evaluates the performance of commonly used ammonia assays, highlighting how assay design influences result accuracy in hyperammonaemia patients. Single stage, rate-based assays offer rapid turnaround times, but lack a sample blank correction step and are therefore susceptible to clinically significant interference from high ALT, haemolysis, icterus, and lipaemia, which are often present during metabolic decompensation and acute liver injury. Patient comparison and interference studies demonstrated that these interferences could lead to substantial positive bias, risking misinterpretation of ammonia severity. 

In contrast, two reagent assays incorporating a blank correction step showed greater analytical resilience, particularly in patients with markedly elevated liver enzymes.  

The work illustrates that assay speed alone does not equate to clinical safety. Selection of ammonia assays must prioritise robustness to interference and consistent performance in critically unwell patients to support safe, 24/7 metabolic and hepatology services.