Plasma ammonia measurement is greatly influenced by pre-analytical conditions, which may lead to false positives. We wanted to evaluate the prevalence and clinical impact of plasma ammonia false positives in a pediatric population. Over a 28-month period, charts of patients with elevated ammonemia were retrospectively reviewed to identify false positives defined as elevated concentrations that subsequently normalized without plausible explanation for the elevations. 1880 Ammonia measurements were available in 479 patients. Elevated results that subsequently normalized were found in 86 patients. Forty-one (48%) of these patients had most likely falsely elevated ammonemia. Additional blood sampling and laboratory testing were the most frequent consequences of false positives. There is a high proportion of false positives among elevated plasma ammonia measurements. Capillary samples and delay between sampling and centrifugation are possible contributing factors. Clinical consequences of false positives were most often limited.