The reportable range of an assay is the range of values that will generate a positive result for the specimens assayed by the test procedure. Clinical patient specimens, positive for the given nucleic acid analyte, are assayed to determine the values expected to be produced by clinically positive specimens. Depending on the type of assay (qualitative, quantitative, analytical), the reportable range can have multiple definitions.
For qualitative tests that are only detecting the presence of a molecular analyte, the reportable range would be that range of values that give a positive result. The reference range and analytical sensitivity have already been quantified to determine the cutoff values used to determine positive and negative results. Clinical specimens from patients that are positive for the disease condition are tested by the assay to verify that the values generated by such specimens are above the cutoff for the assay.
For quantitative assays, the reportable range will represent the linear range of quantitation for the assay. The linear range is the span of analyte concentration for which the final output value of the system is directly proportional to the analyte concentration. As with the case of analytical sensitivity, linear range must be kept in context of the whole assay, not just the amplification and detection steps. The complete assay procedure, including specimen preparation, must be performed to accurately define the linear range of results. The linearity of an assay is determined by testing different concentrations of an analyte in the appropriate specimen matrix. Known amounts of the molecular analyte can be spiked into the appropriate specimen matrix. Conversely, a specimen with an elevated amount of the analyte can be serially diluted with analyte-negative specimen matrix. After assaying the specimens, a plot of the logarithmically transformed data is performed in which the system output values are plotted against starting analyte concentrations. The range in which the plotted data conforms to a straight line defines the linear range of the assay. Guidelines for the design and data evaluation of experiments to determine linear range can be found in the current edition of NCCLS documents EP-6 (Evaluation of the Linearity of Quantitative Analytical Methods) (11) and MM-6 (Quantitative Molecular Methods Infectious Diseases) (7).
For analytical assays, the reportable range will define the alleles, mutations, or genotypes that can be detected and differentiated by the assay. The ability of the assay to detect and differentiate among these analytes must be verified and documented.
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