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test confidence

Oct 28, 2001

Dear Ryan,

In response to several questions about HIV testing, I have read you and others say the following -- after 3 or 4 weeks the majority of people show dectactable antibodies; after 3 months the vast majority, and in only rare cases would it take 6 months. Not to criticize, because I think the service you are providing is wonderful, but this language is ambiguous. I would like to know the confidence scientists place in the tests with regard to the different window periods. Is it something like 60 percent after 1 month, 80 percent after 3 months, and 99 percent after 6 months; or is it 90 percent after 1 month, 99 percent after 3 months, and 99.9 percent after 6 months. Can you clarify this situation or is the science still so confusing that the vague words of majority, vast majority and rare all we really have to go with?

The "window" period issue is a source of great worry I imagine in many cases. This combined with the broad notion of symptoms really leads to a situation of wreaking havoc on the human mind for those who might have been exposed.

Response from Mr. Kull

The language is ambiguous because there are no exact answers. Different sources will vary to certain degrees on the length of the window period; this depends on clinical experience and opinion (which is usually variable), and conclusions drawn from different research studies (which are limited in their own right). It is difficult to generalize studies to the entire population.

The three month window period is a safety net to avoid missing any individuals that might take longer than the usual three weeks it takes to seroconvert. For the needs of the general population, the exact numbers at three weeks shouldn't matter: you can only rely on a test performed at least three months following exposure.

My knowledge of the window period is drawn from the Centers for Disease Control, the New York Department of Health, GMHC, professionals in the field, and available research literature. The recommendation stands that testing is accurate for the general population three months following exposure (approximately 99.9% accuracy). The following excerpt from the publication Medical Management of HIV Infection by John G. Bartlett, MD at the Johns Hopkins AIDS Service, states the situation well:

"Seroconversion with positive HIV serology generally takes place at an average of 3 weeks after transmission with the recombinant antigens now used in standard third generation EIA (AIDS 1995;9:597). A healthcare worker with occupationally-acquired HIV infection was EIA negative at 9 months and seroconverted at 11 months post exposure; this is one of the longest delays in seroconversion reported to the CDC (NEJM 1997;336:919"

Ultimately, each individual is going to need to do what they are comfortable with, regardless of what the numbers say.


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