When and how should we measure plasma ammonia?

Hyperammonaemia is associated with a high morbidity and mortality. It is important to diagnose as it is often treatable and perhaps, most importantly, may be genetically determined. Measurement of plasma ammonia is rarely necessary in adult medicine. In paediatrics, and particularly in the neonatal period, it is an important investigation in the diagnosis and often in the subsequent management of several inherited metabolic disorders. Patients with these disorders, particularly neonates, will deteriorate over a period of hours and investigation cannot wait. In these situations a plasma ammonia together with other investigations for metabolic disorders must be available urgently and are sometimes necessary out of normal laboratory hours. Interpretation must take into account the age and maturity of the child as well as the clinical state and results of other investigations. If treatment is initiated, frequent monitoring of plasma ammonia may be required. Plasma ammonia can be reliably and conveniently measured using a specific ion-selective electrode system or an automated enzyme method. Screening using a microdiffusion method is not a satisfactory alternative to a quantitative assay. Plasma ammonia is no longer solely the province of the specialised paediatric laboratory, but should be part of the repertoire of every laboratory supporting neonatal or paediatric units. The threshold for accepting requests should be lower than at present if we are to prevent misdiagnoses.