Establishing diagnostic cut-off criteria for the COBAS AmpliPrep/COBAS TaqMan HIV-1 Qualitative test through validation against the Amplicor DNA test v1.5 for infant diagnosis using dried blood spots

Abstract

Background As antibody testing cannot confirm HIV-1 infection in children less than 18 months of age, diagnosis in these children depends on nucleic acid testing. The COBAS ® AmpliPrep/COBAS ® TaqMan ® (CAP/CTM, Roche ® Molecular Systems, Inc., Branchburg, NJ) HIV-1 Qualitative test is a total nucleic acid real-time PCR assay utilising whole EDTA blood or dried blood spots (DBS), which recently replaced the Roche ® AMPLICOR ® DNA test v1.5 (Amplicor) as the diagnostic HIV PCR assay in many South African laboratories. For the Amplicor assay, stringent diagnostic criteria were previously formulated for the local population, and a comparison reported the CAP/CTM's sensitivity at 99.7% and specificity at 100% for both sample types compared to these Amplicor criteria. Objectives To validate the assay prior to introduction in our laboratory and to define stringent diagnostic cut-off criteria. Study design Whole EDTA blood samples from patients younger than 18 months sent for routine HIV-1 diagnosis were tested by Amplicor, and positive results were confirmed from DBS. CAP/CTM assays were subsequently performed from DBS. Results The CAP/CTM had a sensitivity of 98.8% and a specificity of 97.1%, but a positive predictive value (PPV) of only 78.7% compared to the Amplicor assay. Samples positive by CAP/CTM but negative by Amplicor displayed poor amplification curves compared to concordant positive samples. Upon re-testing those with sufficient material available by CAP/CTM, all showed negative results. Conclusions The decreased PPV may either be due to false positive CAP/CTM results, or increased sensitivity compared to the Amplicor assay. Criteria were formulated for defining presumed false-positive results.