The truth about quality: medical usefulness and analytical reliability of laboratory tests.

BACKGROUND In this age of evidence-based medicine, nothing is more important than the quality of laboratory tests. It is commonly thought that laboratory tests provide two-thirds to three-fourths of the information used for making medical decisions. If so, test results had better tell the truth about what is happening with our patients. METHODS The age-old "truth standard" for the quality of evidence describes three dimensions that are important-a test should tell the truth, the whole truth, and nothing but the truth. This three-dimensional model can be used to characterize the clinical and analytical reliability of laboratory tests and guide the translation of outcome criteria, or quality goals, into practical specifications for method performance. RESULTS Clinical reliability, or medical usefulness, should assess the correctness of patient classifications based on stated test interpretation guidelines, taking into account the precision and accuracy of the laboratory method, and allowing for the known within-subject biologic variation and the QC needed to detect method instability. Analytical reliability should assess the correctness of a test result based on a stated error limit, taking into account the precision and accuracy of the method and allowing for the QC necessary to detect method instability. These assessments challenge the reliability of current tests for cholesterol, glucose, and glycated hemoglobin in the implementation of U.S. national clinical guidelines. CONCLUSIONS Evidence-based medicine must employ scientific methodology for translating test interpretation guidelines into practical, bench-level, operating specifications for the imprecision and inaccuracy allowable for a method and the QC necessary to detect method instability.

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