Window Period for HIV Infection
Updated February 2013
New York State Recommendations for HIV Testing and Re-testing
What is the Window Period?
When a person becomes infected, HIV multiplies in the blood and the body develops antibodies against the virus. HIV has most commonly been diagnosed in adolescents and adults using tests that detect antibodies. However, newer tests can detect both antibodies to HIV and certain proteins of the virus itself.
The “window period” is the length of time after infection that it takes for the virus to become detectable by HIV diagnostic tests. The length of the window period varies depending on the type of diagnostic test used and the method it employs to detect the virus.
The window period varies slightly from person to person. It is extremely rare for an HIV-infected person not to develop antibodies by 3 months after a potential exposure. A person who tests negative for HIV antibodies 3 months after an exposure does not require further testing unless he or she has had repeated exposures to HIV or if the antibody test results are incompatible with the person’s clinical history.
How soon after exposure to HIV can tests detect the virus?
Even among antibody tests, the window period varies. The so-called “first-generation” and “second-generation” HIV antibody tests detect one type of HIV antibody. On average, they can detect antibodies 42-60 days after infection.
Third-generation tests detect all types of antibodies, which makes them more sensitive than the first-generation and second-generation tests. These assays can detect antibodies about 21-24 days after infection.
Fourth-generation tests can simultaneously detect both HIV antibodies and antigens. Tests that look for the p24 antigen can detect it within 14-15 days. Tests can detect plasma HIV RNA (ribonucleic acid) within about 10 days of infection.
For clinical guidelines on using HIV diagnostic tests, see Diagnostic, Monitoring, and Resistance Laboratory Tests for HIV.
What is acute HIV infection and can it be detected by HIV antibody tests?
Acute HIV infection is the very early, initial stage of HIV infection when the virus is multiplying rapidly and the body has not yet developed antibodies to fight it. Symptoms of acute HIV infection include fever, fatigue or malaise, joint pain, headache, loss of appetite, rash, night sweats, myalgias, nausea or diarrhea, and pharyngitis. These symptoms can be very similar to flu symptoms. HIV antibody tests generally will not detect HIV infection during this early acute stage of infection; however, fourth-generation tests, which detect both antigens and antibodies, can be used to detect acute infection. Traditional tests used to confirm HIV antibody test results such as the Western blot cannot be used to confirm acute infection — rather, tests that can detect HIV RNA (viral load tests) need to be used.
If a patient presents with acute HIV symptoms and your facility offers only antibody testing:
- A preliminary positive antibody test for HIV infection should be followed with both a test to confirm antibodies and a HIV viral load test. If antibodies are not confirmed but RNA is detected, this indicates early-stage HIV infection.
- A negative antibody test must be followed with a viral load test if acute infection is suspected. If RNA is detected, it is likely that acute infection is present. Repeat the RNA (viral load) test to confirm this result. If RNA is not detected, no further testing is needed.
If a patient presents with acute HIV symptoms and your facility has fourth-generation antigen/antibody tests or viral load tests:
- A negative test result on an antigen/antibody test indicates that no infection was detected. There is no need for additional testing unless exposure was very recent and acute infection is strongly suspected. In this case, an RNA (viral load) test may be needed to exclude infection or you could repeat the antigen/antibody test in 5-7 days.
- A positive result on an antigen/antibody test may indicate acute infection. If antibodies are not confirmed and RNA is detected on a viral load test, the patient has acute HIV.
- A positive result on an antigen/antibody test together with a positive result on a confirmatory antibody test indicates the patient has established HIV infection beyond the acute HIV phase.
For more information, see Diagnosis and Management of Acute HIV Infection.
Why is it so important to diagnose infection during the acute HIV phase?
In most infected persons, HIV viral load increases quickly from the time of exposure, peaks about 3 weeks later, then declines over the next several months. A patient is most infectious during this acute phase because of the high viral load levels.
Tests that measure both antibodies and viral p24 antigens can detect HIV within 10-17 days of infection. Tests for acute HIV infection can identify recently infected persons so they can begin antiretroviral drug treatment when their viral load is highest.
If a pregnant patient is suspected to have acute HIV infection, consult with a provider who has expertise diagnosing and evaluating acute HIV infection. Earlier diagnosis and treatment can help reduce the risk of mother-to-child transmission. Clinicians who need support from a provider with experience in acute HIV infection may call the CEI Line at 1-866-637-2342.
For more information, see Acute HIV Infection in Pregnancy.
What is the post-exposure protocol for persons who report an exposure?
If a patient reports a possible exposure to HIV, the patient should be evaluated immediately for post-exposure prophylaxis (PEP). When PEP is begun immediately after an exposure, it can prevent HIV infection. Ideally, it should be initiated within 2 hours of the exposure. PEP is most likely to prevent infection when initiated within 24 to 36 hours of exposure.
Decisions about starting PEP beyond 36 hours post-exposure should be made jointly by the clinician and the patient, with the understanding that PEP will have lower efficacy the longer it is delayed. There is no arbitrary time period, post-exposure, after which PEP should not be given. NYS protocols recommend testing at baseline, 1 month, and 3 months post-exposure. For persons with continued high-risk behaviors, routine testing should be performed every 3 months. See HIV Prophylaxis Following Occupational Exposure and HIV Prophylaxis Following Non-Occupational Exposure.
What are the recommendations for routine HIV testing of all adults?
The NYSDOH, the U.S. Centers for Disease Control and Prevention, and the U.S. Preventive Services Task Force recommend offering HIV testing to all adults as a routine part of health care — not just to those who engage in risk behaviors.
New York State law requires that all persons between the ages of 13 and 64 be offered an HIV test at least once. The law also requires healthcare providers to offer an HIV test to any person, regardless of age, if there is evidence of risk activity. Testing should be offered annually to persons whose behavior indicates elevated risk, such as sexual or drug use activity.
For individuals with ongoing high-risk behavior:
Clinicians should recommend testing every 3 months for patients with high risk behaviors, such as unprotected anal sex or multiple or anonymous sexual partners or needle-sharing partners. In these cases, the function of testing is to ensure early access to care in the event the patient becomes infected and to prevent transmission to others.
For high-risk patients, the focus should be on continued education, behavioral counseling, and harm reduction. Risk-reduction counseling should include education about safer sex practices, condom use, safer injection practices, referral to syringe exchange programs, and drug treatment services.
Pre-exposure prophylaxis (PrEP) should be considered for these individuals. See the CDC interim guidance documents for use of PrEP in men who have sex with men (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6003a1.htm) and in heterosexually active adults (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6131a2.htm).
Since many people with high-risk behaviors choose not to disclose their risks, providers should consider adopting a low threshold for recommending the test.
For more information about New York State requirements for HIV testing, please visit: http://www.health.ny.gov/diseases/aids/testing/index.htm.
The following are articles from the medical literature on HIV testing and the window period (listed chronologically):
Update on HIV Diagnostic Testing Algorithms. J Clin Virol. 2011 Dec (Suppl 1): S1-S90. http://www.journalofclinicalvirology.com/issues?issue_key=S1386-6532%2811%29X0012-4#.
Medical Care Criteria Committee. Diagnostic, Monitoring, and Resistance Tests for HIV.
New York State Department of Health AIDS Institute. Updated February 2011: http://www.hivguidelines.org/clinical-guidelines/adults/diagnostic-monitoring-and-resistance-laboratory-tests-for-hiv.
Patel P, Mackellar D, Simmons P, et al. Detecting acute immunodeficiency virus infection using 3 different screening immunoassays and nucleic acid amplification testing for human immunodeficiency virus RNA, 2006-2008. Arch Intern Med. 2010 Jan 11;170(1):66-74. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2965267/.
Pandori MW, Hackett J Jr, Louie B, et al. Assessment of the ability of a fourth-generation immunoassay for human immunodeficiency virus (HIV) antibody and p24 antigen to detect both acute and recent HIV infections in a high-risk setting. J Clin Microbiol. 2009;47:2639-2642. http://www.ncbi.nlm.nih.gov/m/pubmed/19535523/.
Owen SM, Yang C, Spira T. et al. Alternative algorithms for human immunodeficiency virus infection diagnosis using tests that are licensed in the United States. J Clin Microbiol. 2008 May; 46(5):1588-1595. http://www.ncbi.nlm.nih.gov/pubmed/18322061.
Stekler J, Maenza J, Stevens CE, et al. Screening for acute HIV infection: Lessons learned. Clin Infect Dis. 2007;44:459-461. http://www.ncbi.nlm.nih.gov/pubmed/17205460.
Patel P, Klausner JD, Bacon OM, et al. Detection of acute HIV infections in high-risk patients in California. J Acquir Immune Defic Syndr. 2006;42:75-79. http://www.ncbi.nlm.nih.gov/pubmed/16763493.
Branson BM, Handsfield HH, Lampe MA, et al. Revised Recommendations for HIV Testing of Adults, Adolescents, and Pregnant Women in Health-Care Settings CDC Testing 2006. Morb Mortal Wkly Rep MMWR. 2006;55(RR-14):1-17: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5514a1.htm.
Speers D, Phillips P, Dyer J. Combination assay detecting both human immunodeficiency virus (HIV) p24 antigen and anti-HIV antibodies opens a second diagnostic window. J Clin Microbiol. 2005 Oct;43(10):5397-9. http://www.ncbi.nlm.nih.gov/pubmed/16208030.