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

This version was published on 1 January 2010

Ann Clin Biochem 2010;47:81-83
© 2010 Association for Clinical Biochemistry


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Short Reports

Daniel Juan Herrera,
Tim Hutchin,
Donna Fullerton and
George Gray

Department of Clinical Chemistry, Birmingham Children’s Hospital, Steelhouse Lane, Birmingham B4 6NH, UK

Corresponding author: Dr George Gray. Email: George.Gray{at}

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

Methods: To study this potential interference, we spiked a plasma poolwith a liver homogenate and measured the ammonia concentrationusing a dry chemistry system (Vitros 250, Ortho Clinical Diagnostic,Raritan, NJ, USA), an enzymatic assay without a sample blankingstep (Infinity Ammonia Liquid Stable Reagent, Thermo FisherScientific, Waltham, USA) and an enzymatic assay that correctsfor the non-specific oxidation of NADPH (Ammonia kit, RANDOXLaboratories Ltd, Crumlin, UK).

Results: This experiment shows that the InfinityTM ammonia reagent kitis subject to a clinically significant interference and explainsthe discrepancies previously reported between these methodsin patients with acute liver failure (ALF).

Conclusion: When using enzymatic methods for the assessment of PA, we recommendincluding a sample blanking correction and this should be mandatorywhen monitoring patients with ALF.

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