I would like to list a few questions regarding the rapid (under 40 minutes) HIV Test:
1. My understanding is that the negative predictive value of a test is the likelihood that a negative result is a *true* negative.
Is this correct, and if so, what is the negative predictive value of rapid HIV testing (I realize there is a 3-6 month window period)?
2. Is this test more sensitive than specific? I've read that a negative result essentially rules out infection (taking into consideration the window period), but that a positive result may be a false positive. So is this test so sensitive so that false positives may occur but false negatives will not?
3. Is ELISA a highly sensitive test (rule out infection) where Western Blot is more specific (rule in infection)?
4. Is the rapid test the same as the ELISA, and if not is it as accurate?
Thanks! 1. The negative predictive value is not just a function of its sensitivity, it is also a function of the the actual rate of HIV infection in the population you are testing. The sensitivity of the SUDS test is 99.4%, but the negative predicitive value depends on whether you're testing a high prevalence or low prevalence population.
Let me illustrate:
Leaving specificty aside for a moment,
If you test 1 million people, 1% of whom actually have HIV, you would expect 990,000 negative tests. However a test with 99.4% sensititivity will miss 60 out of 10,000 that should have been positive. That means 60 out of 990,060 negative tests were wrong - there is a 0.00606% chance (roughly 1 in 16,000) a negative is a false negative.
If 100% of your million population have HIV you would expect 0 negative tests. However the test misses 0.6% of true positives, so in fact you would get 6,000 (false) negatives - and there is a 100% (1 in 1) chance that any negative is a false negative. This is why you need to know the background prevalence of a condition before you can interpret the chances that any given negative test is a true or false negative.
If the true prevalence of what your testing for is 0.3% (as is HIV in the US population) a test with 99.4% sensitivity will result in 1 in 55,000 negatives being false negatives.
Of course no test is totally specific either - there will be fewer negatives because some will be false positives.
http://en.wikipedia.org/wiki/Negative_pr...
2. Yes. All screening tests for HIV sacrifice specificity to maximise sensitivity. This is why a diagnosis should not be made on a positive screening test alone. You need a two step algorhythm for adequate specificty in most populations.
3. Not quite: it is the ELISA plus WB (the two step algorhythm) which provides the specificity, not just the WB alone.
4. No it's not the same, but they are believed to have comparable accuracy. ISBT 2006 Oral Presentation
DETECTION OF HIV-1 INFECTION IN BLOOD DONORS DURING THE
IMMUNOLOGICAL WINDOW PERIOD BY THE IMPLEMENTATION OF
NUCLEIC ACID AMPLIFICATION TECHNOLOGY (P-018)
P.S.P. Scuracchio1, A.G. Oliveira Filho2, M.M.M. Lemos2, M.C.C. Poli2, R. Colella2, M.M.C. Magri3,
N.J.F. Cavalcanti3, E.C. Sabino4, N.A. Salles4, D.A.F. Chamone5
1 Banco de Sangue de Sao Paulo, Sao Paulo, Brazil
2 Banco de Sangue de Sao Paulo, Sao Paulo, Brazil
3 Centro de Referencia e Treinamento AIDS, Sao Paulo, Brazil
4 Funda莽ao Pr贸-Sangue Hemocentro de Sao Pa, Sao Paulo, Brazil
5 Universidade de Sao Paulo, Sao Paulo, Brazil
Background: Individual Nucleic Acid-Amplification Testing (NAT) has been recently introduced in blood banks
in the city of S茫o Paulo, Brazil, in attempts to reduce the transfusion transmission risk of HIV and Hepatitis C
viruses. This screening test can identify donations made during the immunological window period before
seroconversion.
Aims: To investigate the impact of this technology in our blood donors and transfusion routine.
Methods: From March 2004 to November 2005, when the NAT testing has been implanted, 64,248 donations
were tested using two approved enzyme immunoassays (EIA) for HIV antibodies (Abbott Murex and Biom茅rieux),
although 47,866 were also tested for individual NAT (Procleix by Chiron Healthcare). Confirmatory tests: Western
blot (Genelabs Diagnostics), p24 antigen (Vironostica) and quantitative PCR-HIV-1 (Roche).
Results: Among the samples screened, two were non-reactive in enzyme immunoassays but positive for HIV-1
RNA with negative confirmatory p24 antigen, as described: Donor A, male, 34 years old, married, was a repeat
donor with a previous blood donation on 06/21/04. After the results of the index donation on 11/17/05, he was
recalled to collect a new blood sample in order to confirm the HIV-1 positivity. The sample was collected 25 days
after the index donation and confirmatory tests showed quantitative PCR-HIV 16,800 cp/ml, reactive antibody
detection and negative p24 antigen. During interview with the infectious diseases specialist, the donor admitted,
between the two donations, many occasions of male homosexual intercourse without using a protective. Two
weeks before the index donation, he had low grade fever, sore throat, myalgia and malaise. Donor B, male, 30
years old, single, first time donor on 10/01/05. Four days after the blood donation, a new sample was collected
remaining serological tests non-reactive, negative p24 antigen and quantitative PCR-HIV showing 122.000 cp/ml.
The donor developed fever and diffuse body pain one week after the donation. He was recalled many times for
counselling and testing. Unfortunately, he has never returned again.
Conclusions: Although serologic analysis for HIV is a primary tool for diagnostic testing, the introduction of
NAT has made possible the identification and preventing of transfusion of two HIV-positive blood donations in a
18 months period. Interesting to note that while the prevalence of HCV is higher than HIV, in Brazil HIV incident
cases are more common than HCV using NAT technology. The screening of donors reduced the immunological
window period, permitting the identification of very early stage HIV infections. In addition, these case reports
emphasized the fact that the risk of HIV transmission is not limited to the first time donors.
8 minutes ago - 3 days left to answer. - 1 answer - Report It
You can't answer your own question. #4 the rapid test is slightly less accurate then the elisa test its still 99.8% or something like that, its in the 99 percentile though, not enough to worry about, just get checked after 3 months, if u get negative then ur pretty much safe..6 months is to be 100% sure. If ur were in the same boat as i was, and was scared all the time thought i had symptoms, only use the elisa at the 3 and 6 month mark if u want to make sure, completely sure that u dont have hiv, the rapid test is good to relieve stress and does the job just as well, some people prefer the elisa because of its higher accuracy, i didnt care, just whichever test clinic i could get to during my free time
if u want REAL answers............go to MEDHELP.ORG or AIDSMEDS.com or org..dont know if its a com at the end. I checked them out, and they have real people going through the same situations and who have contracted hiv. you can ask them questions about hiv, and get honest and knowledgable feedback. these sites r lifesavers. dont depend on yahoo for answers....theyre often wrong. |
