Aim: Uncertainty of measurement (UM) defines the distribution of quantity values attributed to a measurand. The clinician assesses how much of the reported test result reflects its true value. UM for plasma ammonia was estimated according to the International Organization for Standardizairton (IOS ISO is actually the same organization as IOS. 15189) requirements in our laboratory. Material and Method: Plasma Ammonia of UM was calculated in accordance with The Guide to The Expression of Uncertainty of Measurement (GUM) and European Analytical Chemistry (EURACHEM) principles. Laboratory reproducibility was estimated with internal quality control (IQC), while external quality assessment (EQA) results were used to estimate bias and bias uncertainty. Uncertainity of reagents and calibrators was similarly determined. Using this data, including standard uncertainty, is combined with expanded uncertainty, and was also determined for ammonia. Results: The expanded uncertainty (k = 2) of ammonia was determined as 14.72 % (95% confidence interval). Discussions: Ammonia is a sensitive test which can be easily affected by preanalytcal errors; thus, it is important that the report is comprised of all analytical uncertainity factors for ammonia. Reporting UM for ammonia answered some questions (e.g., 'How effective are the analytical sources in the analysis of ammonia?' and 'What is the true value of blood ammonia?'). This study may be a prime UM example of ammonia's function in terms of the literature.