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HIV is a virus that attacks cells in the immune system. When it’s not treated, the virus can seriously weaken the immune system over time.

Several types of tests are available to determine whether someone has HIV. Testing is important both in preventing HIV and in helping people who have contracted the virus to seek treatment.

A person is said to be HIV-negative when a test doesn’t detect HIV. However, a negative result doesn’t necessarily mean that they don’t have HIV.

Continue reading below to learn more about the different types of HIV tests and what a negative result means.

Receiving a negative result from an HIV test means that the test didn’t detect HIV. However, a negative result doesn’t always mean that a person hasn’t contracted HIV.

This is because each type of HIV test has a different window period. A test’s window period is the amount of time that passes between exposure to HIV and when a test can accurately detect that a person has contracted HIV.

If a person is tested within a test’s window period, the test may not be able to detect the presence of the virus, and they may have a negative result.

When a negative result is received within a test’s window period, it’s important to get another test after that window period has passed. This can help to confirm the negative result.

Additionally, HIV exposure is possible during a window period. In this situation, HIV testing may not be accurate.

There are three different types of HIV tests including:

Antibody tests

An antibody test looks for the presence of antibodies to HIV in a sample of blood or saliva. Antibodies are specialized proteins made by the immune system in response to disease-causing agents like HIV.

Many HIV rapid or self-tests are antibody tests. Results can be available in 30 minutes or less, depending on the type of test.

The window period for antibody tests is broad. It can take anywhere between 23 to 90 days after a potential exposure for an antibody test to accurately detect an HIV infection.

Antigen/antibody tests

An antigen/antibody test detects antibodies to HIV as well as a specific viral protein (antigen) called p24.

Typically, a blood sample from a vein in the arm is used for this test. However, there are some rapid tests available that use a blood sample from a finger prick.

How long it takes to get results can depend on the type of test. If a sample is sent to a laboratory, it can take several days to get results. Results from a rapid antigen/antibody test can be available in 30 minutes or less.

Because p24 can be found before antibodies to HIV are made, antigen/antibody tests may detect an HIV infection earlier than an antibody test.

The window period for a standard antigen/antibody test is 18 to 45 days. Rapid antigen/antibody tests may take longer — between 18 and 90 days.

Nucleic acid tests

Nucleic acid tests (NATs) look for the presence of the virus’s genetic material in a blood sample.

A NAT can detect HIV in a person’s blood anywhere between 10 and 33 days after potential exposure. However, while these tests are more sensitive, they’re also expensive and aren’t typically used for screening.

Blood samples for a NAT are sent away to a lab for processing. Because of this, it can take several days to receive a test result.

NATs are also used after an HIV diagnosis to detect the amount of virus in a person’s blood. This is called a viral load test.

HIV-1 is one of the two types of HIV. It’s the most common type of HIV worldwide. In fact, it’s estimated that about 95 percent of people currently living with HIV have HIV-1.

Because HIV-1 is so prevalent, most HIV tests are geared toward detecting HIV-1. These include all three types of tests that we’ve discussed above.

HIV-2 is much less common than HIV-1. HIV-2 is estimated to make up less than 0.1 percent of HIV diagnoses in the United States.

On a genetic level, HIV-2 is over 55 percent different from HIV-1. This means that many types of HIV tests may not be able to accurately detect HIV-2.

Testing guidelines from the Centers for Disease Control and Prevention (CDC) help to address this. These guidelines recommend using an antigen/antibody test initially to screen for HIV.

If this test comes back positive, a second antibody test is then ordered to determine if a person has HIV-1 or HIV-2. If test results are indeterminate, a NAT is done to either confirm or exclude a diagnosis of HIV-1.

Due to the fact that HIV-2 is uncommon in the United States, NATs for HIV-2 aren’t always readily available. However, if HIV-2 is suspected, there are testing centers across the United States that can perform this screening to help confirm a diagnosis.

It’s possible that an HIV test result may come back as “indeterminate.” Simply put, this means that the results of the test are unclear. Tests that come back indeterminate typically show a weak positive result.

This type of result isn’t that uncommon. Researchers estimate that the indeterminate HIV test results happen less than 5 percent of the time.

There are several things that can potentially cause an indeterminate result, including if:

  • a person has contracted HIV very recently
  • an antibody cross-reaction has occurred, which can be due to things like a recent tetanus vaccination or an underlying health condition like lupus or rheumatoid arthritis
  • the sample was contaminated
  • a technical error happened during testing

If the results of an HIV test come back indeterminate, a repeat test is typically recommended.

There are some situations where a healthcare professional may recommend retesting. This includes cases of:

  • a negative test result within a test’s window period
  • another potential HIV exposure during a test’s window period
  • an indeterminate test result

Repeat testing based on risk

While the CDC recommends that all people between the ages of 13 and 64 get tested for HIV at least once, they also recommend yearly HIV testing for the following people:

Exactly when a person is retested can depend on the specific situation. Let’s examine some potential scenarios in more detail:

  • Negative result within the window period. To confirm a negative result, a healthcare professional will recommend retesting after the test’s window period has passed.
  • Exposure during the window period. If another potential exposure has occurred within a test’s window period, a healthcare professional may recommend waiting at least 4 weeks before retesting, depending on a test’s window period.
  • Indeterminate result. Testing may be repeated immediately to rule out things like technical errors or sample contamination. If results from this test are still indeterminate, another test is typically done 2 weeks later.

There are many steps that can be taken to prevent contracting HIV. These include:

  • Using a condom. Using a condom during sex can help prevent transmission of the virus.
  • Not sharing needles. It’s important not to share needles or other injection drug equipment.
  • Taking pre-exposure prophylaxis (PrEP). PrEP is a daily oral medication. It can help to prevent contracting HIV during sex or when using injection drugs.
  • Getting tested. Being aware of HIV status is a valuable tool in prevention. Testing for other STIs is also important because some STIs can increase the risk of HIV transmission. Encourage sexual partners to be tested as well.
  • Asking about post-exposure prophylaxis (PEP). PEP is an emergency medication that can help prevent HIV after potential exposure. To be effective, it must be started within 72 hours after a possible exposure.

Where to get an HIV test

There are many locations where HIV testing is available. These can include:

  • a healthcare professional’s office
  • sexual health clinics
  • community health centers
  • VA medical centers
  • pharmacies
  • community outreach organizations
  • state or local health departments
  • facilities that treat substance use disorders

It’s also possible to obtain a self-test that can be taken at home. Some of these tests provide results in less than 20 minutes. For other types of self-tests, a sample may have to be mailed to a lab for testing.

The CDC has a search tool to help find locations where HIV testing is available. They also have a list of available self-testing services in each state.

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There are several different types of HIV tests. Each type has a different window period for which it can accurately detect an HIV infection.

A negative HIV test result means that the specific test that was used didn’t detect HIV. However, a negative result doesn’t always mean that a person hasn’t contracted the virus.

If an individual received their result within a test’s window period or had another HIV exposure within the window period, retesting is recommended. Another scenario where retesting can happen is in the case of an indeterminate result.

Getting tested is a valuable tool for both preventing HIV transmission and for ensuring that those who have contracted the virus get timely treatment. As such, it’s important that anyone concerned that they may have been exposed to HIV receive testing.