The official interpretation guidelines for the UK.
Discover how high and low risk samples are put into their appropriate result categories….
From the document:
Specimens from HIV-infected individuals typically give rise to strong, and often maximum, signals in most commercial screening assays whereas falsely reactive specimens infrequently do.
What makes it a false reaction? The strength of the reaction? ….Nope
However, this is not a reliable basis on which to make a diagnosis of HIV infection, and further testing is essential, employing several different tests carefully selected to minimise the possibility of each additional test being prone to the same false-positive effect as gave rise to the false reaction in the initial screening test.
So, how do they know which is a false reaction and which is a true reaction? — “Expectation“
Other pitfalls arise from the use of fourth generation tests (where, as noted above, both components need to be checked) and the application of screening tests to populations where the strong expectation is of a negative result. Reactivity on these specimens needs very careful scrutiny, unhurried by inappropriate ‘turn-around’ targets.
But – if they think you’ve got it, then, well, you’ve got it.
Clearly, waiting for six months to test is untenable, and an approach should be adopted that strikes a balance between certainty that transmission has not happened and the need to be able to reassure the patient as soon as possible.
A nucleic acid test (see below) may hasten a positive finding after exposure to HIV, but it is not a substitute for anti-HIV testing after a ‘safe’ interval.
Here the ELISA test is considered more accurate than the PCR. This is the opposite of the standard line (That PCR is more accurate than the ELISA).
But the ELISA antibody tests (what they delightfully call “anti-HIV”) are not virologic tests, they’re non-specific antibody tests, hence the call for PCR (“nucleic acid test”, which they’ll say, is more specific than ELISA (except that it’s actually not)… Clear?
We would suggest testing using a sensitive fourth (or third) generation screening test immediately after the exposure and then: at one to two months, at three to four months, and six months.
Should suspicious clinical signs and symptoms develop, ie almost anything occuring in persons in the pre-designated “risk groups” – cold, allergy, fever, etc an immediate test, including for HIV RNA, is indicated.
And here we go, just a few lines later, the PCR test is being used as the more accurate test, to confirm the first antibody test…Here’s to having it both ways
Some time and care will be needed to explain the reasons underlying the need for follow-up testing and to communicate without undue alarm an appropriate level of residual risk of infection, despite negative results, prior to completing the six-months follow up.
So, if you’re negative, but the expectation due to risk-group analysis (sexual orientation, skin color, socio-economic group is that you’re really positive…then you’re positive_.
But if you’re positive, but the expectation is that you’re really negative (because you couldn’t be positive, not You!…), then….you’re negative …
Quite a nice system they’ve worked out.
But it’s all irrelevant, because Acquired Immune Deficiency is, in the end, a clinical diagnosis world-wide; the tests are simply not required.