The Most Common Question About the New HIV Testing Algorithm, Answered

A primary care doctor in the Boston area recently emailed me this question:

Hi Paul,
A 28yo woman had a positive 4th gen +Ag/Ab assay, but a negative HIV-1/2 differentiation assay and negative HIV viral load. She had no signs of acute HIV, but is not using condoms with her partner, whose HIV status she doesn't know. We repeated the test yesterday and she is again Ag/Ab+, the remainder of the test is pending. If we get the same results again, would you try to get a Western blot?
[Not her real name, but I did just meet a doctor named "Morgan" for the first time, so feel compelled to comment here. If you look at this name popularity graph, I guess we have an explanation for the rarity of this name among MDs to date. Did you know Morgan was the 30th most common girl's name in the USA during the 1990s? So expect more Morgan MDs soon!]

"Morgan's" question has come up numerous times since the new algorithm kicked in, and it reflects a misunderstanding of what the newer tests can and can't do.

Remember, the big advance in moving from the 3rd to the 4th generation screening test was the addition of p24 antigen to the sensitive ELISA antibody. This shortens the window period from HIV acquisition to a positive screening test by about a week.

The second big change is that the confirmatory test is now a differentiation assay, an antibody test that tells us if the person has HIV-1 or HIV-2 -- the Western blot couldn't do that. If it's negative, an HIV RNA (viral load or other nucleic acid test, NAT) is recommended, since the screening test (with its antigen component) is more sensitive early in disease than the differentiation assay. Importantly, the Western blot -- may it R.I.P. -- offers no advantage in sensitivity over the FDA-licensed differentiation assay (in fact it's a bit worse), so won't be of help in these cases.

However, if the HIV RNA is negative, then we're dealing with a false-positive screening test, and this is exactly the scenario in the email. For relatively low-risk patients, this is a far more common explanation for the positive 4th Generation screen/negative differentiation assay pattern than true acute HIV infection, just as it was for a reactive ELISA with negative Western blot.

How much more common? In this review (pages 34-36) by the primary architect of the algorithm, Bernie Branson, we get some numbers:

The specificities of 4th generation HIV assays are >99.6% -- which means that as many as 40 per 10,000 test results may be false-positive. In most populations of persons testing for HIV, the prevalence of acute HIV infection is 2 per 10,000 persons tested or less. Thus, the frequency of false-positive immunoassay results usually far exceeds the prevalence of acute HIV infection.

For those who prefer all this stuff explained graphically, here's the key figure from the latest HIV testing guidelines; I've highlighted this case's results in bright fluorescent pink:

false positive 4th generation

To summarize:

  1. The 4th Generation screening test shortens the window period after HIV acquisition.
  2. The differentiation assay tells us if the person has antibodies to HIV-1 or HIV-2.
  3. In screening test positive/differentiation test negative cases, the next step is an HIV RNA (or other NAT).
  4. Most (but not all) will have a negative HIV RNA, and therefore don't have HIV.

In other words, false-positive screening tests will continue to happen -- even with the new algorithm.

Thank you very much for your attention, and enjoy this amazing video, which must have taken its creator many, MANY hours to complete. Wow.

And if there are any other doctors named Morgan out there, I look forward to hearing from you.

(H/T to IMS for the video, though it looks like at least 24 million people have beaten us to it.)