Objective: To verify the upper reference limit and define the population-based reference interval (RI) for thyroid peroxidase antibody, according to Clinical and Laboratory Standards Institute (CLSI) guidelines. Material and Methods: We subjected serum samples from 146 subjects to thyroid-stimulating hormone (TSH) and TPO Ab analysis for verification and determination of RI. We verified an upper reference limit for thyroid peroxidase antibody (TPO Ab) and established an RI according to National

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... Academy of Clinical Biochemistry (NACB) and CLSI guidelines. Results: The manufacturer's suggested upper reference limit was not verified for use as a functional reference value. Rather, on defining the RI of TPO Ab by means of a full-scale study, we found a higher upper reference limit (13.8 vs 12.0). We calculated lower and upper reference limits of 3.3 and 13.8, with 90% confidence intervals (CIs) of 2.8 to 3.5 and 13.6 to 14.8, respectively. Conclusion: RIs provided by assay manufacturers are general data and may not always be appropriate for different locations and patient demographics. Verification of an RI is necessary before use in a given patient population.