HIV

Know more about this test

An HIV test checks a sample of blood, oral fluid, or urine for signs of HIV infection - either antibodies your immune system makes against the virus, the p24 antigen the virus itself produces, or the virus's genetic material directly. Which sign a test looks for determines how soon after a possible exposure it can pick up an infection. The CDC recommends everyone between 13 and 64 be tested for HIV at least once as a routine part of health care, regardless of perceived risk, and more often for anyone with an ongoing chance of exposure. A positive screening result is never treated as a final diagnosis on its own; it always leads to a separate confirmatory test.

What this test measures

Antibody-only tests can take up to about 90 days after exposure to reliably detect infection. Combination antibody/antigen blood tests narrow that window to roughly 18 to 45 days. A nucleic acid test (NAT), which looks for the virus's genetic material directly, can detect infection as early as 10 to 33 days after exposure and is the fastest of the three, though it is used more selectively because of cost and turnaround time. Which test is used, and when, depends on how recently the possible exposure happened.

Understanding HIV

Why the same test can miss an infection too early

None of these tests can detect HIV the moment it enters the body - your immune system needs time to make antibodies, and even the virus itself needs time to reach detectable levels. Testing too soon after a possible exposure can give a falsely reassuring negative result, which is why the specific window period for the test used matters as much as the result itself.

Screening versus confirmation

A positive result on any screening test - antibody, antigen/antibody, or NAT - is followed by a separate confirmatory test before a diagnosis is given. This two-step process exists because no single screening test is completely free of false positives, and a diagnosis with lifelong treatment implications is not made on one result alone.

Ongoing care after a confirmed diagnosis

- A CD4 count and HIV viral load are used to guide when to start treatment and to check it is working - Antiretroviral therapy is generally started as soon as a diagnosis is confirmed - Regular monitoring continues for as long as someone is on treatment

Testing negative during a suspected window period is not the end of the process - your doctor may recommend repeat testing after the relevant window period has passed, particularly after a specific recent exposure.

References

1. 1. HIV Screening Test - MedlinePlus https://medlineplus.gov/lab-tests/hiv-screening-test
2. 2. Screening and diagnosis for HIV - MedlinePlus Medical Encyclopedia https://medlineplus.gov/ency/article/003538.htm