Skip to content

Add your earlier progress to your account?

From before you signed in, this browser has:

Which plan do you want to keep?

You answered the plan questions before you signed in, and those answers don't match the plan saved on your account.

SettingSaved planNew answers

Study progress is waiting to be saved

The HIV testing algorithm

16 min

Skills you'll build3 skills
  • Choose the next laboratory test in the HIV diagnostic algorithm
  • Recognize when recent exposure or antiretroviral use limits HIV detection
  • Choose virologic testing for an infant with possible perinatal HIV exposure

At the bench: Relate viral serology findings to the stage of infection

Try first

Try first

A serum HIV-1/HIV-2 antigen/antibody screen is repeatedly reactive. What happens next?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

The three-step algorithm

Laboratory HIV testing of serum or plasma follows a set sequence. The laboratory reports each assay result with one combined interpretation.1

  1. Antigen/antibody screen. A fourth-generation assay detects HIV-1 p24 antigen with HIV-1 and HIV-2 antibody. A nonreactive result shows no laboratory evidence of HIV infection. A reactive result is repeated as the manufacturer directs and is presumptive.1,2
  2. Antibody differentiation. The HIV-1/HIV-2 differentiation immunoassay reports HIV-1 positive, HIV-2 positive or HIV positive untypable. An antibody-negative or indeterminate result goes to step 3.1
  3. Nucleic acid amplification test (NAAT). Detected HIV-1 RNA supports acute HIV-1 infection and is reported promptly to the provider and the health department. HIV-1 RNA not detected after an antibody-negative or HIV-1 indeterminate differentiation result is interpreted as HIV negative.1

Window periods

Each assay becomes reactive some days after infection. Before that, a nonreactive result cannot exclude infection.

TestUsual detection window after exposure
Laboratory antigen/antibody test on venous blood18 to 45 days
Fingerstick antigen/antibody rapid test18 to 90 days
Antibody-only test23 to 90 days
Nucleic acid test (NAT)10 to 33 days

3

A nonreactive result after a recent exposure is followed by the full algorithm on a later specimen. A negative rapid test during a suspected recent exposure is repeated with laboratory testing.1,3 Antiretroviral drugs taken for treatment or pre-exposure prophylaxis (PrEP) can suppress viremia and delay antibody. For a person taking PrEP, CDC recommends a laboratory antigen/antibody test and an HIV RNA test at the same time.1

Infants need virologic tests

Maternal IgG crosses the placenta. Antibody and antigen/antibody tests therefore cannot diagnose HIV in an infant younger than 18 months.4 An exposed infant has HIV RNA or DNA tests at birth, at 14 to 21 days, at 1 to 2 months and at 4 to 6 months. A positive virologic test is confirmed on a second specimen as soon as possible. Cord blood is never used.4

Follow the bench demonstration

Sources for this step
References
  1. Association of Public Health Laboratories. Suggested Reporting Language for the HIV Laboratory Diagnostic Testing Algorithm. 4th ed. October 2025. Accessed September 27, 2026. https://aphl.org/docs/default-source/technical/id-hiv-suggested-reporting-language.pdf
  2. Centers for Disease Control and Prevention and Association of Public Health Laboratories. Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations. June 27, 2014. doi:10.15620/cdc.23447
  3. Centers for Disease Control and Prevention. HIV testing. Updated August 17, 2026. Accessed September 27, 2026. https://www.cdc.gov/hivpartners/php/hiv-testing/index.html
  4. Panel on Treatment of HIV During Pregnancy and Prevention of Perinatal Transmission. Recommendations for the Use of Antiretroviral Drugs During Pregnancy and Interventions to Reduce Perinatal HIV Transmission in the United States. Updated June 25, 2026. Accessed September 27, 2026. https://clinicalinfo.hiv.gov/en/guidelines/perinatal/management-infants-diagnosis-hiv-infection-children

Watch one

Read each decision and its explanation.

A 41-year-old woman has a routine HIV screen. She reports no recent exposure. Her results:

What is the algorithm's interpretation?

TestResultReference intervalFlag
HIV-1/HIV-2 antigen/antibody screenReactive, repeated in duplicateNonreactiveNonreactive
HIV-1/HIV-2 antibody differentiationHIV-1 Indeterminate
HIV-1 RNA, qualitative NAATNot detectedNot detectedNot detected

Specimen: H 4, L 10, I 1. Plasma, EDTA

  1. The screen was reactive initially and on repeat, so the specimen went on to step 2.

    A repeatedly reactive screen is what sends a specimen on to supplemental testing.

  2. The differentiation assay result is HIV-1 Indeterminate, so the specimen goes to NAAT.

    An indeterminate differentiation result cannot confirm or exclude antibody, so it goes to step 3.

  3. HIV-1 RNA is not detected.

    The indeterminate result is for HIV-1, so an HIV-1 NAAT resolves it.

  4. An HIV-1 Indeterminate result with HIV-1 RNA not detected is interpreted as HIV negative.

    The algorithm defines this combination.

  5. Report each assay result together with the interpretation. If recent exposure is later suspected, request a new specimen and repeat the algorithm.

    The provider needs every assay result that led to the interpretation.

HIV negative. The reactive screen was not confirmed by antibody differentiation, and HIV-1 RNA was not detected.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A laboratory HIV-1/HIV-2 antigen/antibody test on venous blood is nonreactive 14 days after a suspected exposure. What does this result mean?

Incorrect. p24 detection shortens the window period but does not eliminate it. This test usually becomes reactive 18 to 45 days after exposure.

Incorrect. The nucleic acid test (NAT) window is 10 to 33 days, so a negative NAT at day 14 does not exclude infection.

Correct. This test's usual detection window is 18 to 45 days, so a day-14 specimen can precede detectable p24 antigen and antibody. Repeat the full algorithm on a later specimen.

Hint
  1. Find the laboratory antigen/antibody test in the window table.
  2. Compare day 14 with the earliest day that test usually turns reactive.

Review Window periods

Problem 2 of 3 · Hints available

Which approach is used to diagnose HIV in an infant under 18 months with possible perinatal exposure?

Correct. HIV RNA or DNA tests detect the virus directly. They are scheduled at birth, 14 to 21 days, 1 to 2 months, and 4 to 6 months, and a positive result is confirmed on a second specimen.

Incorrect. Maternal IgG crosses the placenta, so antibody tests, including antigen/antibody combination assays, cannot diagnose infection before 18 months.

Incorrect. A negative birth test does not complete exclusion of perinatal infection. Definitive exclusion in a non-breastfed infant needs negative virologic tests at 1 month or later and at 4 months or later.

Hint
  1. Ask whose antibody is in the infant's blood.
  2. A test for the virus itself avoids the mother's antibody. Think about how many results exclusion needs.

Review Testing in infants under 18 months

Problem 3 of 3 · On your own

A plasma antigen/antibody screen is repeatedly reactive, and the differentiation assay reads HIV-1 Positive. What is the next laboratory action?

NAAT resolves negative and indeterminate differentiation results. An HIV-1 Positive result already confirms HIV-1 antibody.

The algorithm is complete, and waiting delays a confirmed result.

The Western blot is no longer part of the algorithm.

A reactive screen with an HIV-1 Positive differentiation result is laboratory evidence of HIV-1 infection. Both assay results appear with the combined interpretation.

Review The recommended testing algorithm

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Tomas Triplett, 23, had a condomless sexual exposure 24 days ago. He takes no antiretroviral drugs.
  • A fingerstick rapid antibody test at a community clinic was nonreactive 2 days ago.
  • The clinic then sent venous plasma to your laboratory.
TestResultReference intervalFlag
HIV-1/HIV-2 antigen/antibody screenReactive, repeated in duplicateNonreactiveNonreactive
HIV-1/HIV-2 antibody differentiationHIV antibody negative

Specimen: H 3, L 9, I 1. Plasma, EDTA, collected 24 days after exposure

Decision 1 of 3

The clinic asks whether their nonreactive rapid test means your reactive screen is false. What do you tell them?

A fingerstick antibody test has a window of up to 90 days. At day 22 it could easily miss an infection.

Excluded recent infection with one nonreactive rapid test

Fingerstick rapid tests have windows of up to 90 days, and antiretroviral treatment or PrEP can suppress viremia and delay antibody. A single nonreactive rapid result after a recent exposure can miss infection that a later laboratory specimen would detect.

The laboratory screen can become reactive from about day 18. The rapid antibody test often needs weeks longer.

A third screen adds no information. The next step of the algorithm resolves the result.

Review Window periods

Decision 2 of 3

What is the next step for this specimen?

A reactive screen with no antibody can be p24 antigen in acute infection. The algorithm is not complete.

The screen is presumptive. Only the supplemental steps confirm infection.

Reported HIV infection from a reactive screen

A reactive antigen/antibody screen is presumptive, because nonspecific reactivity occurs and the screen does not type the virus. Reporting infection from it skips the differentiation assay and nucleic acid test (NAT) that confirm infection, separate HIV-1 from HIV-2, and detect acute infection before antibody forms.

An antibody-negative differentiation result after a reactive screen goes to NAAT, which detects acute infection before antibody forms.

The current specimen can go on to the next step now.

Review The recommended testing algorithm

Decision 3 of 3

The HIV-1 RNA NAAT is detected. How is the result handled?

Detected RNA with no antibody yet fits acute HIV-1 infection. Viremia is high at this stage, so the result is reported without delay.

The algorithm is complete with detected RNA. Waiting for antibody would delay reporting of an infection at its most transmissible stage.

The NAAT detected HIV-1 RNA. Absent antibody reflects how early the specimen was drawn.

Review The recommended testing algorithm

The exposure 24 days before collection explains the results. At that point the rapid antibody test can be nonreactive and the laboratory screen can detect p24 antigen before antibody appears. NAAT completes the algorithm.

For reference

Sources checked