Part 1
“The UK Guidelines”
My patients have often asked me the following questions concerning the rapid 4th generation HIV Test so readily available in Singapore’s clinics. This test is also known as the “rapid HIV combo test,” the “rapid HIV Duo test,” or more correctly, the “rapid p24 antigen/HIV-1 and HIV-2 antibody combination test.”
With much anxiety, one would ask,
“Doctor, is my result conclusive at 28th days post-exposure?”
“Doctor, I’ve browsed many websites locally, and some advised that the rapid 20 minutes HIV Combo test is definitely conclusive at 28th days after my last high risk exposure. Is that true?”
“Doctor, there is a website that claims that according to the UK guidelines, the rapid HIV Combo/Duo test is conclusive on day 28. What do you think?”
I remember a limerick while I was in primary school – complete with frizzy hair and an oversized shorts – that reads:
The more you know, the more you forget,
The more you forget, the less you know,
The less you know, the less you forget,
The less you forget, the more you know.
While it’s true that the World Wide Web has some good information on various topics, sometimes the overwhelming amount of information might actually be confusing at best, and perhaps even deleterious to true knowledge at worst. As Alexander Pope has correctly observed, “A little learning is a dangerous thing.”
Firstly, what exactly is this 4th Generation HIV Test?
Unlikely previous HIV tests that only test for antibodies that our bodies produce to the invading HIV virus, 4th generation tests are also able to detect p24 antigens present on the surface of the HIV viral particle.
These 4th generation HIV tests can detect the p24 antigen as early as 14 days after a high-risk exposure (for example, a needle stick injury or unprotected sexual intercourse with a high-risk partner). Compared to older 3rd generation tests where the window period for detecting Immunoglobulin M (IgM) antibodies can be anywhere between 20 to 23 days, 4th generation tests effectively reduce the window period by about 1 week.
So do the “UK National Guidelines for HIV Testing” advise that a rapid HIV Combo test is conclusive at 28 days after exposure?
It is true that the UK Guidelines recommend the HIV Antigen/Antibody combination test (HIV Combo/Duo test) as a first-line assay. But the committee has this to say concerning rapid point of care tests (POCT) e.g. the rapid HIV Combo/Duo test:
“They have advantages of ease of use when venepuncture is not possible, e.g. outside conventional healthcare settings and where a delay in obtaining a result is a disadvantage, but these must be weighed against the disadvantages of a test which has reduced specificity and reduced sensitivity versus current fourth generation laboratory tests.” – UK National Guidelines for HIV Testing , page 9
The UK Guidelines recognize the disadvantages of rapid POCT where there is “reduced specificity and reduced sensitivity versus current fourth generation laboratory tests.” In other words, these rapid tests have somewhat lesser accuracy compared to 4th generation laboratory HIV tests.
The British Association for Sexual Health and HIV (BASHH) has this to say concerning the conclusiveness of the 4th generation HIV Antigen/Antibody Combination laboratory test, taking into account that the rapid HIV Combo/Duo test is perhaps less accurate than its laboratory equivalent:
“Patients attending for HIV testing who identify a specific risk occurring more that 4 weeks previously, should not be made to wait 3 months (12 weeks) before HIV testing. They should be offered a 4th generation laboratory HIV test and advised that a negative result at 4 weeks post exposure is very reassuring/highly likely to exclude HIV infection. An additional HIV test should be offered to all persons at 3 months (12 weeks) to definitively exclude HIV infection.” – BASHH (emphasis mine)
So, in a nutshell, according to the UK Guidelines, the rapid HIV Combo/Duo test is not conclusive at 28 days. The guidelines actually stressed the need for an additional HIV test at 3 months after exposure to “definitively exclude HIV infection.”
That said, the rapid HIV Antigen/Antibody combination test is extremely accurate, with the manufacturer claiming 100% sensitivity and 99.91% specificity. At 28 days, if a patient tests negative for HIV, there is a more than 99% chance that the result is true. But still, that is not the same as 100%. It is really not conclusive at 28 days post exposure.
Now, the UK Guidelines were published in 2008.
What about the “latest” and “most up-to-date” guidelines?
That’s for part 2 to answer. Stay tuned.
Talk to our Family Physician concerning HIV Testing and STD screening.
Furthermore, if you have exposed yourself to the HIV virus within the last 72 hours, please visit our Family Physician in Singapore for HIV Post Exposure Prophylaxis (HIV PEP).
Update:
After the date of writing of this post, BASHH published the following update on the laboratory HIV Combo Test. Do note that this update does not refer to the rapid HIV Combo test commonly offered:
“A negative result on a fourth generation test performed at 4 weeks post-exposure is highly likely to exclude HIV infection. A further test at 8 weeks post-exposure need only be considered following an event assessed as carrying a high risk of infection.” – BASHH Nov 2014
Up till now, BASHH does not consider the laboratory HIV Combo Test “conclusive” at 28 days or 4 weeks, although it is “highly likely” to exclude HIV infection.
