BACKGROUND: For convenience, multiple instruments can be used to measure the same laboratory results within one health care system. However, the laboratory must verify the comparability of the results. In this study, we evaluated a method for verifying the comparability of patient results obtained from two instruments within one health care system, EP31-A-IR, proposed by the Clinical and Laboratory Standards Institute. METHODS: Using the range test proposed by the EP31-A-IR, we evaluated the comparability of 17 clinical chemistry test results from the HITACHII/MODULAR system (Roche Diagnostics, Switzerland) and the TOSHIBA/200FR system (Toshiba Medical Systems Co., Japan). The 0.33× biological variability, allowable total error, and standards of the Clinical Laboratory Improvement Amendments were used to determine the acceptance criteria. RESULTS: Among 16 test parameters, the differences of means between the two instruments were less than their range rejection limit in 15 tests, and so the comparability between the two instruments was considered acceptable. Creatinine was not evaluated using this protocol because its range rejection limit was not deducible from the EP31-A-IR statistics table. CONCLUSIONS: The EP31-A-IR guideline is useful for verifying the comparability of results between two instruments. However, not all parameters are covered by the guideline. With consideration of the characteristics of each test parameter, each laboratory should devise its own method for evaluating comparability.