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A 33-year-old male asked:

What is the window period for modern day hiv 4th generation tests. many hiv specialists consider this conclusive at 28 days.? and cdc says 3 months?

1 doctor answer1 doctor weighed in
Dr. Hunter Handsfield
Infectious Disease 54 years experience
Concensus is 4 weeks: Both biological principles of the test and expert opinion favor 28 days. Few if any HIV experts report ever having a patient with new HIV who didn't have a positive 4th generation test by 4 weeks. The main exception may be when post exposure prophylaxis with anti-HIV drugs is given but doesn't work. Even then, it's probably rare. Many official agencies take conservative stances and always have.
Dr. Hunter Handsfield
Infectious Disease 54 years experience
Provided original answer
A problem is that this is difficult to study. Few people who catch HIV know exactly when they were exposed, and following them for testing at precise times is difficult. So we're left with relatively small numbers plus biologcal principles and animal research. Small numbers means the margin of error is wide, as in political polling. CDC usually sticks with the worst case interpretation.
Jan 20, 2016
Dr. Hunter Handsfield
Infectious Disease 54 years experience
Provided original answer
Above is a year old. Since then, evolving data show the antigen-antibody ("duo", "combo", "4th generation") tests detect ~98% of infections by 4 weeks, needs 6 weeks for negative results to be 100% conclusive. Positive results always are conclusive no matter when done. Links to review and CDC endorsement: https://www.ncbi.nlm.nih.gov/pubmed/29140890, https://www.ncbi.nlm.nih.gov/pubmed/29140891
May 2, 2018
Last updated Nov 21, 2020

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