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Western blot as a stand alone test? Why not? (SENSITIVITY VERSUS SPECIFICITY, 2010)
Sep 14, 2010

Doctor, I forgot to ask this in previous question. Like I have said, I have tested negative out to 11 months post exposure with standard antibody testing. I also have had PCR DNA and RNA at six months which were negative for HIV-1. I continue to have symptoms that all but convince me that I have HIV. I was pondering the thought of a Western blot test as a stand alone test, or a bDNA test. You have said before that you do not advise this, and I was wondering why? I think this thread will educate ALOT of people on the subject if you have the time to answer. Thank you.

Response from Dr. Frascino

Hi,

Western Blot testing should always be coupled with EIA screening due to a 2% rate of false-positive results. It is designed as a confirmatory test following a preliminary positive EIA (ELISA, rapid test, ICMA, etc.). The difference between the screening HIV-antibody tests such as ELISA, EIA, ICMA , rapid tests, etc., and confirmatory HIV tests, such as Western Blot or IFA, has to do with the sensitivity and specificity of the various tests. See below.

Dr. Bob

False Postitive (FALSE POSITIVE TESTS, SENSITIVITY VERSUS SPECIFICITY, 2010) Jan 9, 2010

I was tested a few months ago for HIV and other STDS. Everything was great except for my HIV test came back positive with the Elisa tests and my western blot was a negative. I came in a month later and retook the test and my elisa then came back negative. My GP told me that this ment it was a false positive and it can be caused by a bunch of different things being that the Elisa is a very sensitive test. My question to you is, would that assumption be correct. Since I came back a month later and retested with a negative elisa does that mean I can take a deep breath in and relieve all this anxiety? Thank you so much for what you do!

Response from Dr. Frascino

Hi,

The assumption is completely accurate and correct. You can relax Max!

Diagnostic HIV screening involves a two-part process. The first test (ELISA or rapid test) is extremely sensitive, so as not to miss any true HIV-positive cases. In fact, it's so sensitive it sometimes picks up some "false positive" cases. (These folks test positive, but are really negative, like you!) Once someone tests ELISA- (or rapid test-) positive, he needs a confirmatory test, such as a Western Blot, which is a more specific test. It can determine "true negatives" more accurately than an ELISA test. For someone to be diagnosed HIV positive, he needs both a positive ELISA (sensitive test) and confirmatory Western Blot (specific test). I know this gets confusing, but ELISA tests can determine more "true positives" and Western Blot tests can determine more "true negatives." A positive ELISA plus a negative Western Blot would technically be considered a negative HIV diagnostic screening! So go ahead and take that deep breath. All is well, including you!

Dr. Bob



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