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Test sensitivity and specificity
I wonder if anyone has an answer to my question about tests. As you may know, there are two ways to quantify the accuracy of a diagnostic test. SENSITIVITY is the probability of a test being positive when a person actually is positive. E.g., the Oraquick in-home oral fluids test has 92% sensitivity according to the FDA (http://www.fda.gov/BiologicsBloodVaccines/BloodBloodProducts/ApprovedProducts/PremarketApprovalsPMAs/ucm311895.htm). SPECIFICITY is the probability of a test being negative when a person actually is negative. E.g., the Oraquick in-home oral fluids test has 99.98% specificity according to the FDA.
Without getting into a debate on appropriate window periods (yes, the CDC recommendation is three months, we know that), I am wondering whether it is the sensitivity, specificity, or both that is reduced when a test is taken earlier. For example, on the expert forum the doctors have said things such as the oral fluids rapid antibody test is about 90% accurate at six weeks. Does that mean that the six-week specificity is 90% x 92% (to use the example above)? Likewise for sensitivity? Both?
According to my logic, a reduction in accuracy of the nature the doctors state would affect test sensitivity (probability of testing positive if actually infected). Not sure about specificity because that's about testing negative if actually not infected, and the test isn't detecting anything there--it is negative because of the absence of antibodies that would indicate an infection.
Would appreciate thoughts on this from the community. Otherwise maybe this is a question for the doctors in the forum. Thanks.
Without getting into a debate on appropriate window periods (yes, the CDC recommendation is three months, we know that), I am wondering whether it is the sensitivity, specificity, or both that is reduced when a test is taken earlier. For example, on the expert forum the doctors have said things such as the oral fluids rapid antibody test is about 90% accurate at six weeks. Does that mean that the six-week specificity is 90% x 92% (to use the example above)? Likewise for sensitivity? Both?
According to my logic, a reduction in accuracy of the nature the doctors state would affect test sensitivity (probability of testing positive if actually infected). Not sure about specificity because that's about testing negative if actually not infected, and the test isn't detecting anything there--it is negative because of the absence of antibodies that would indicate an infection.
Would appreciate thoughts on this from the community. Otherwise maybe this is a question for the doctors in the forum. Thanks.
To determine the sixth week specificity you would definitely NOT multiply the 92 percent and the ninety percent, because these two figures represent the sensitivity of the oraquick test under TWO ENTIRELY DIFFERENT SETS OF CONDITIONS. The 92 percent refers to the specifity of the oraquick test WHEN IT IS SELF ADMINISTERED AT HOME BY THE AVERAGE USER AFTER THE 3 MONTH WINDOW PERIOD and the 90 percent refers to the specificity of the test when it is administered at no further than the six week mark following the potential exposure. ie, after six weeks, ninety percent of people will produce detectable antibodies. However, there is a caveat to the 92 percent statistic: it refers specifically to the reliability of results obtained by the AVERAGE, AT HOME, USER. Subsequent research has revealed that when the test is administered by a TRAINED HEALTHCARE PROFESSIONAL, (aftet the three month window periof) than the test has at least 99.3 percent specificity, and other studies, if i recall, cite a higher specificity. So as long as you are testing after the three month mark, can read and understand english, and can follow directions, you shouldnt worry about a false negative result.
Correct you are! The "average" user i.e, "general public" must be told not to drink the solution in the vial !!
Hence, the discrepancy of test results.
If you get my drift......emphasis on my liberal use of quotation marks.
Hence, the discrepancy of test results.
If you get my drift......emphasis on my liberal use of quotation marks.
From a survey conducted in the U.K HIV prevalence , sensitivity of OraQuick was 93.64% and specificity 99.87% (99.28-100%), positive predictive value 97.78% (88.27-99.94%) and negative predictive value 99.61% (98.87-99.92%). This includes three false-negatives considered to be due to observer error and now rectified by further training. This has increased test sensitivity to 100%. The observed test performance of OraQuick compares well with EIA and with other rapid tests.
In a nutshell, if you test negative with an Oraquick swab test after 12 weeks you're negative. It is as accurate as a lab based blood test.
In a nutshell, if you test negative with an Oraquick swab test after 12 weeks you're negative. It is as accurate as a lab based blood test.
The sensitivity of a test is the percentage of results that will be (correctly) positive when HIV is actually present. Lower rates of sensitivity will produce more false negative results.
The specificity of a test is the percentage of results that will be (correctly) negative when HIV is not present. Lower rates of specificity will produce more false positive results.
The specificity of a test is the percentage of results that will be (correctly) negative when HIV is not present. Lower rates of specificity will produce more false positive results.
I don't think you answered the question. The poster is asking whether tests taken before the official window period have lower sensitivity, lower specificity, or both. This is a perfectly valid question and not about the official guidelines. I don't have an answer but maybe someone here does?
The Oraquick HIV rapid tests is just as accurate as any other form of testing once performed after 3 months post--exposure.
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