**Objective:** Uncertainty is a parameter that characterize a range which includes a measurable substantiality. It also shows how much the result represents the real value. Uncertainty of measurement comprises many components. The purpose of this study was to estimate uncertainty of measurement in the glucose analysis that use glucose hexokinase method and to prove it to be useful in laboratory practice.

**Materials and Methods:** Repeatability, uncertainty of reagent and uncertainty of calibration drift for glucose were calculated. After all these standard uncertainty values were estimated, the combined uncertainty was founded. Expanded uncertainty was obtained by multiplying the combined uncertainty by a coverage factor (k). For an approximate level of confidence of 95%, k is 2.

**Results:** According to the formula of unknown test result (X), uncertainty of measurement for glucose was founded X±0.03X (%95, k=2). e.g. uncertainty was ±3.78 mg/dL for 126 mg/dL glucose concentration.

**Conclusion:** All institutions that have accreditation, must calculate the uncertainty of measurement for every parameter. The uncertainty of a measurement reflects reliability of the results. Every error source that can affect the result during preanalytical, analytical and postanalytical stages must be determined and it must be given as a uncertainty value. Some of these components may be obtained from the statistical distribution of the results of series of repeated measurements and they can be determined by means of standard deviation. The other components which can be determined by means of standard deviation are obtained from assumed probability distributions based on experience or other information. According to this study, the physicians must take care of the uncertainty of measurement during the evaluation of the test results.