The window period is the time from infection until a test can detect any change. The average window period with HIV-1 antibody tests is 25 days for subtype B. Antigen testing cuts the window period to approximately 16 days and nucleic acid testing (NAT) further reduces this period to 12 days.
Performance of medical tests is often described in terms of:
sensitivity: The percentage of the results that will be positive when HIV is present
specificity: The percentage of the results that will be negative when HIV is not present.
All diagnostic tests have limitations, and sometimes their use may produce erroneous or questionable results.
False positive: The test incorrectly indicates that HIV is present in a non-infected person.
False negative: The test incorrectly indicates that HIV is absent in an infected person.