The mechanism causing false positive reactions on serological tests is that of non-HIV antibodies cross-reacting with the test antigens. The risk is increased in lower prevalence populations. This risk of false positive HIV diagnosis on the basis of 2 RDT positive results has been shown in a number of settings with different RDTs. The compromise is that without a confirmation test, some individuals will be falsely diagnosed as HIV positive.
This strategy has allowed life-saving scale up of HIV diagnosis, as it permits testing to be decentralized outside of the laboratory. However due to issues of cost, the WHO currently recommends that confirmation testing is not performed in resource limited settings, and instead that diagnosis be made on the basis of an algorithm employing 2–3 rapid diagnostic tests (RDTs). The diagnosis of HIV is made on the basis of a reactive screening test or tests followed by a confirmation test.
Further research is needed to determine if these results can be replicated in other settings. Negative results require nucleic acid testing to rule out false negative results due to seroconversion or misclassification by the lower sensitivity dilution test. It has potential for use as a supplementary test in a confirmatory algorithm, whereby double positive RDT results undergo dilution testing, with positive results confirming HIV infection. This study provides proof of concept for a new, low cost method of confirming HIV diagnosis in resource-limited settings. Concordance between the gold standard and the 1/160 dilution strength was 99.8 %. Dilution at the level of 1/160 was able to correctly identify all these 18 false positives, but at a cost of a single false negative result (sensitivity 99.6 %, 95 % CI 97.8-100 specificity 100 %, 95 % CI: 98.5-100). The RDT diagnostic algorithm misclassified 18 samples as positive. A total of 495 samples underwent dilution testing. ResultsĢ895 samples were recruited to the study. Results were compared to the gold standard of Western Blot where Western Blot was indeterminate, PCR testing determined the final result. Dilution testing was performed using Determine™ rapid diagnostic test at 6 different dilutions. All samples positive on the initial screening RDT and every 10th negative sample underwent testing with the gold standard and dilution test.
#IMPORTANCE OF SERIAL DILUTION IN RESEARCH ARTICLES SERIES#
Participants were recruited from two testing centres in Ethiopia where a tiebreaker algorithm using 3 different RDTs in series is used to diagnose HIV. The principle of the dilution test is to alter the sensitivity of a rapid diagnostic test (RDT) by dilution of the sample, in order to screen out the cross reacting antibodies responsible for falsely positive RDT results. In this paper, we evaluate the dilution test, a novel method for confirmation testing, which is simple, rapid, and low cost. This leads to a potential risk of false positive HIV diagnosis. Current WHO testing guidelines for resource limited settings diagnose HIV on the basis of screening tests without a confirmation test due to cost constraints.
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