RESUMO
BACKGROUND: Limit of detection (LOD) is an important performance characteristic of clinical laboratory tests. Verification, as recommended by the CLSI EP17-A2 guideline, is done by testing a sample with a claimed LOD concentration. Claimed LOD is verified if the 95% CI for the population proportion, calculated from observed proportion of positive results, contains the expected detection rate of 95% (CLSI EP17-A2; Clin Chem 2004;50:732-40). Claimed LOD, verification sample concentration, and observed rate of positive results are subjects to systematic and random errors that can cause false failure or false acceptance of the LOD verification. The aim of this study was to assess the probability to pass or fail verification of claimed LOD with various numbers of tests as function of the ratio of test sample concentration and actual LOD for PCR-based molecular diagnostics tests and provide recommendations for study design. METHODS: A method of calculating the probability of passing the claimed LOD verification following CLSI EP17-A2 guideline recommendations, based on the Poisson-binomial probability model, have been developed for PCR-based assays. RESULTS: Calculations and graphs have shown that the probability of passing LOD verification depends on the number of tests and has local minima and maxima between 0.975 and 0.995 for the number of tests from 20 to 1000 on samples having actual LOD concentration. The probability of detecting the difference between claimed LOD and actual LOD increases with the number of tests performed. Graphs and tables with examples are included. CONCLUSIONS: Method, tables, and graphs helping in planning LOD verification study in molecular diagnostics are provided along with the recommendations on what to do in case of failure to verify the LOD claim.
RESUMO
BACKGROUND: Viral load quantification is established in the clinical management of chronic Hepatitis B virus (HBV) infection for assessing efficacies and resistance developments in anti-viral drug treatment. OBJECTIVES: The fully automated COBAS AmpliPrep/COBAS TaqMan HBV Test, v2.0 was evaluated for the linear measuring range and the inclusivity of HBV genotype determination in EDTA plasma and serum samples. STUDY DESIGN: Two kit lots of the test were used to determine the linear measuring range as well as linearity and limit of detection applying different concentration levels of specimens representing HBV genotypes A to H along with a pre-core mutant and the WHO Standard. RESULTS: The COBAS AmpliPrep/COBAS TaqMan HBV Test, v2.0 displayed a linear measuring range of seven log(10) steps from 20IU/mL (lower limit of quantification) to 2.3E+08IU/mL (upper limit of quantification) yielding similar results for EDTA plasma and serum. Inclusivity was shown by reliable quantification of HBV genotypes A to H at different concentration levels. The >or=95% hit rate LOD was 15IU/mL for genotypes C, D, F, G, the pre-core mutant and the WHO Standard and 20IU/mL for genotypes A, B, E and H matching the test's lower limit of quantification. 95% PROBIT analysis yielded concentrations of 8.9IU/mL for the WHO Standard and of 6.0-16.4IU/mL for the genotypes. CONCLUSIONS: The COBAS AmpliPrep/COBAS TaqMan HBV Test, v2.0 provides genotype inclusivity for accurate viral load monitoring in serum and EDTA plasma samples and supports clinical routine in the management of HBV infection.