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Rapid test detect HIV before Western Blot?

Oct 24, 2008

Could a rapid HIV test (specifically Uni-Gold which claims to be able to detect HIV as soon as 3-4 weeks post exposure), detect HIV before a Western Blot is capable of detecting HIV? Asked another way, could a person have a reactive rapid test result 3-4 weeks post exposure and a negative Western Blot, but actually be infected? Thanks for your help!

Response from Dr. Frascino


No. If someone had a reactive rapid test and negative Western Blot, they would be HIV negative. The Western Blot test is the more "specific" test, while the ELISA or rapid test is more "sensitive." See below.

Dr. Bob


On previous posts you say that the western blot test is more sensetive and other informaton that I have read says it is the more specific test. What is the difference between the western blot and the elisa test? Thank You

Response from Dr. Frascino


You are correct. I was using the word sensitive in its generic sense. That is trying to reassure the questioner that the Western Blot was the test to rely on to confirm a positive ELISA or determine the ELISA was falsely reactive.

The technical terms sensitivity and specificity are defined below:

The specificity is the number of true negatives (disease-free) divided by the number of all negative cases in the population.

The sensitivity is the number of true positives (with disease) divided by the number of all positive cases in the population.

Consequently, using the technical definitions, what you read is correct. Western Blot (WB) tests would be more "specific." They would be able to determine "true negatives" better than an ELISA, which is more sensitive (i.e. able to determine more true positives than the WB).

Hope that helps. I know it's a bit confusing.

Dr. Bob

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