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Here are my notes for HIV Detection:
Please comment, correct, or add anything I missed.
First do ELISA as a screening test: (very sensitive, lots of FP, so Decreased PPV)
A)if it's negative....>
-Person not infected if no risk factors for HIV.
-If reported high risk, can do PCR for HIV virus.
B)if it's positive ELISA (for p24 Ab IgG)....> confirm it with Western blot (very specific; ELISA + WB: PPV > 99%):
...(1) if the test is positive (detect IgG Ab to at least to 2 of these 3 Ags: p24, gp41, gp120 considered positive): the patient is HIV+
...(2) if the test is negative (NO reaction): the patient not infected
...(3) but! if the result of test is intermediate (any reaction that not considered + e.g. if we detect only 1 out of 3 Ab) then 2options:
......>I) Retest in 6mo by western blot: if negative: not infected
-, ......>II) use PCR as a confirmatory test: if negative: not infected.
In general if your patient is HIV negative but has risk factors, retest in 3-6mo
-ELISA is a screening test
-Western Blot is a confirmatory test
-PCR is the most expensive test
-Antibodies for HIV may not show up until weeks or months after exposure which ELISA and WB cannot detect. PCR can detect HIV at earlier stages because the viral nucleic acid is present immediately upon exposure. It is used to detect HIV in newborns with HIV + mothers.
If you want to test a neonate born to a known HIV mother what assay would you use?!
You cannot test for anti-HIV ABs because the mother passively transmits them to the baby thru the placenta. Therefore ALL babies born to HIV positive mothers will be positive for anti-HIV ABs.
So use PCR to detect the genome of the HIV!
DrVirgo and pearljam59, good points. Really well summarized.
As you see I'm resurrecting "Tales from Crypt".
ELISA/Western blot tests for HIV
A blood sample is drawn from a vein.
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
A negative test result is normal. But persons with early HIV infection (acute HIV infection or primary HIV infection) often have a negative test result.
A positive result on the ELISA screening test does not mean that the person has HIV infection. Certain conditions may lead to a false positive result, such as Lyme disease, syphilis, and lupus.
A positive ELISA test is always followed by a Western blot test. A positive Western blot confirms an HIV infection. A negative Western blot test means the ELISA test was a false positive test. The Western blot test can also be unclear, in which case more testing is done.
Negative tests do not rule out HIV infection. There is a period of time, called the window period, between HIV infection and the appearance of anti-HIV antibodies. During this period, antibodies usually cannot be measured.
If a person might have acute or primary HIV infection and is in the window period, a negative HIV ELISA and Western blot will not rule out HIV infection. More tests for HIV are needed.
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