How accurate are rapid HIV tests?
Study finds blood tests slightly more accurate than oral tests
Researchers from Canada, led by Nitika Pant Pai, M.D., of McGill University, conducted a systematic review and meta-analysis of studies on rapid HIV antibody testing in adults, specifically the OraQuick advance rapid HIV-1/2, from Jan. 1, 2000, to June 1, 2011.
They compared sensitivity (the percentage of true positive results) and specificity (the percentage of true negative results) of the tests.
The average sensitivity of oral samples was 98.03 percent while the average sensitivity of blood was 99.68 percent, meaning oral samples were about 2 percent more likely to miss a positive result (i.e., show a false negative). However, average specificity was almost identical for both -- 99.74 percent for oral and 99.91 percent for blood -- meaning they had roughly the same chance of having false-positive results. Overall, the chances for false positives and negatives were very low.
Another key finding was that differences in HIV prevalence between settings helped determine the overall likelihood of false positive test results. In places where more than 1 percent of the population was living with HIV, the positive predictive value (PPV), which is the percentage of people who test positive who are actually positive, was 98.65 percent for oral samples and 98.50 percent for blood.
By contrast, in places where less than 1% of the population was living with HIV, the PPV was 88.55% for oral samples compared to 97.65% for blood.
In other words, the lower the percentage of people in an area who are living with HIV, the higher the percentage of false positives. As the percentage of people who are living with HIV in an area increases, the percentage of false positives decreases.
That's because, according to Aidsmap:
"In a setting where very few people have HIV, the majority of apparent positive results will in fact be incorrect. However, as the proportion of people with HIV being tested increases, the true positives start to outnumber false positives."
In the same article, Aidsmap also reported:
The authors examined reports of false positive results in order to identify factors that could have caused them: errors in test performance and conduct of test (i.e. inaccurate specimen collection, gum swabbing more than once), and errors in the interpretation of results (interpreting weakly reactive lines). These were generally linked to inadequate training of staff or lapses in quality assurance.
However Nitika Pant Pai says that the key reason for poorer performance with oral fluid is that there are lower quantities of HIV antibodies in oral fluid than in whole blood. The quantities of antibodies in oral fluid are especially low after recent infection.
Despite higher chances of false positives in low-prevalence settings, the oral rapid HIV tests had just 2 percent less sensitivity overall than blood rapid HIV tests. And on the whole, both tests were very accurate.
The researchers say that these findings make a strong case for future over-the-counter oral HIV tests, which may appeal to those who are not comfortable getting a test in public. As Indian Express reported,
Oraquick is also being considered for potential use as an over-the-counter test in the US and in several sub-Saharan countries. This move might revolutionise HIV testing by offering a proactive option to people who, because of stigma, do not wish to attend public health centres. Hopefully, offering a confidential testing option will bring an end to the stigma associated with HIV testing, says Pai.
In a separate statement issued by McGill University, Pant Pai said, "Getting people to show up for HIV testing at public clinics has been difficult because of visibility, stigma, lack of privacy and discrimination. ... There is a huge global momentum for alternate HIV self-testing strategies that can inform people ... of their status."
While having more rapid oral tests would certainly help in low-resource settings, it also shows promise for prevention efforts in the United States. What do you think? Would having an over-the-counter option make it more convenient and encourage more people to get tested? Or would it cause as many problems as it solves?