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Technical Testing Question (what I meant to ask!)
Apr 9, 2009

Dear Dr Bob,

My question is two fold, but has to deal with the fact that most ELISAs, including rapid tests, detect only gp41 antibodies:

I have seen people (confirmed positive with VL) with reactive ELISAs, but negative gp41 (& also 120) bands on their Western Blots. How is this so if they don't have gp41 antibodies?

Secondly, on the flip side, is it possible for someone not to develop these antibodies (as demonstrated by those WBs) and test negative on ELISA, but have the other bands present? Basically - can they present a false negative?

Thanks for your input, Dr Bob!

Response from Dr. Frascino

Hi,

These types of basic technical questions are not the real purpose of this forum, so I'll be brief.

Your confusion has to do with the basic difference between the way a screening ELISA (EIA and rapid test) is actually performed compared with a confirmatory Western Blot test. These are two different types of test assays.

ELISA uses a mix of HIV antigens (proteins) produced in high titer in tissue culture systems or through recombinant molecular technology. These high-titer cell cultures are then lysed and the mix of soluble antigens is then coated onto the wells of a microtiter plate. The patient's serum is then added and if specific anti-HIV antibodies (many different types) are present they will bind tightly and specifically with the HIV antigens on the plate. After washing away the unbound material a goat antihuman antibody that binds to anti-HIV human antibody (which is bound to the HIV antigens on the plate) is added. The goat antihuman antibody is conjugated to an enzyme that cleaves a colorless substrate into a product that has color (usually yellow). The amount of color present is proportional to the amount of bound antibody and antigen. The amount of HIV can be determined semi-quantitatively by spectrophotometry. Basically this type of testing uses a variety of HIV antigens all mixed together.

The Western Blot test is also designed to detect the presence of anti-HIV antibodies. However, in addition to identifying the presence of such antibodies, it also allows determination of the specific antigen against which the antibodies are directed, including core (p12, p24, p55), polymerase (p31, p51, p66) and envelope (gp41, gp120, gp160).

Consequently the ELISA is the more "sensitive" test but the Western Blot is the more "specific" assay.

See, I told you this type of technical question is beyond the scope of this forum! Now can we get back to the questions concerning barely legal Russian hookers with the web feet?

Dr. Bob



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