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AimsAmmonia is an extremely unstable analyte and requires special attention during sampling, transport and storage. The aim of this study was to evaluate the stability of ammonia in lithium-heparin plasma during short-term (at +4°C) and long-term (at −20°C) storage.MethodsTwenty plasma samples were used for short-term stability assessment. Each sample was divided into five aliquots and stored in stoppered tubes at +4°C, for 1, 2, 3, 4 and 24 h from initial testing. Fifteen plasma samples were used for long-term stability assessment. Each sample was divided into eight aliquots and stored in stoppered tubes at −20°C for 3, 24, 48 h and 1, 2, 4, 8 and 12 weeks from initial testing. Ammonia concentration was determined on a Beckman Coulter AU2700 chemistry analyser using Randox ammonia enzymatic UV method. Bias was calculated from initial value for each time point and compared with quality specifications defined by Royal College of Pathologists of Australasia.ResultsThe average bias exceeded the total allowable error after storage of samples for 1 h at +4°C and 3 h at −20°C.ConclusionAmmonia is not stable during storage at +4°C and −20°C in lithium-heparinised plasma and should therefore be analysed immediately.