At a residence-visa screening center, applicant A's sample, collected during a busy morning session and labeled by hand after the applicants had left the room, gives a fourth-generation HIV antigen/antibody S/CO of 386 (reactive at 1.00 or more), and the HIV-1/HIV-2 differentiation assay is HIV-1 positive, HIV-2 negative. Under the center's policy, a second sample is collected 5 days later and labeled in front of the applicant after a photo-ID check. It gives an S/CO of 0.14 on the same analyzer, and HIV-1 RNA is not detected. What is the most likely explanation?

The first sample came from another applicant

Antibody cleared by antiretroviral therapy

HIV-2 infection missed by the second screen

Acute infection still in the window period

A visa screening laboratory tests about 250 samples a day with a fourth-generation HIV antigen/antibody chemiluminescence assay (reactive at S/CO 1.00 or more). Besides the manufacturer's controls, it runs a third-party low-positive control whose acceptable range, set over 20 runs, is S/CO 2.6-4.2; it read 3.1 and 3.3 earlier this week. This morning a new reagent lot was loaded. Today the negative and manufacturer's positive controls are in range, but the low-positive control reads 1.42. Of 248 samples, 2 are reactive and 3 give S/CO 0.70-0.95. What should the laboratory do?

Reject the run and retest after troubleshooting

Release nonreactives; retest only the three near cutoff

Accept the run, as the low control is still reactive

Widen the low control's range to fit the new lot

Under the WHO HIV testing strategy for national programs (2019), a person is diagnosed HIV-positive when:

Three different assays in sequence are all reactive

One rapid test is reactive on two separate days

Two runs of the same assay are both reactive

The first assay is reactive with a high signal

After HIV-1 infection, in what order do the laboratory markers usually become detectable?

RNA, then p24 antigen, then antibody

Antibody, then p24 antigen, then RNA

p24 antigen, then RNA, then antibody

RNA, then antibody, then p24 antigen

Compared with a laboratory fourth-generation assay, an oral-fluid rapid HIV test detects infection later mainly because it detects only:

IgG antibody, at lower levels than in blood

p24 antigen, at lower levels than in blood

IgM antibody, which fades within weeks

HIV RNA, which is scarce in saliva

HIV testing

14 DOH Abu Dhabi (HAAD) lab licensing practice questions on HIV testing, part of Immunology and Serology. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
At a residence-visa screening center, applicant A's sample, collected during a busy morning session and labeled by hand after the applicants had left the room, gives a fourth-generation HIV antigen/antibody S/CO of 386 (reactive at 1.00 or more), and the HIV-1/HIV-2 differentiation assay is HIV-1 positive, HIV-2 negative. Under the center's policy, a second sample is collected 5 days later and labeled in front of the applicant after a photo-ID check. It gives an S/CO of 0.14 on the same analyzer, and HIV-1 RNA is not detected. What is the most likely explanation?
1. AAntibody cleared by antiretroviral therapy
2. BHIV-2 infection missed by the second screen
3. CAcute infection still in the window period
4. DThe first sample came from another applicant
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Answer: D · The first sample came from another applicant
Antibodies in established HIV-1 infection, strong enough to give an S/CO of 386 and a positive differentiation assay, do not disappear within 5 days. A nonreactive fourth-generation screen with undetectable RNA on a properly identified sample therefore means the two tubes came from different people, and labeling tubes away from the applicant is a classic cause of this error. The window period comes before antibodies appear, not after; antiretroviral therapy suppresses RNA but antibodies persist; and fourth-generation screens detect HIV-2 antibodies too. WHO advises retesting a second specimen before a diagnosis is given for exactly this reason. The laboratory must also trace the whole session, because another applicant may have been wrongly reported as negative.
- A: Treatment suppresses RNA, but antibodies stay detectable, so the screen stays reactive.
- B: Fourth-generation screens detect HIV-2 antibodies too, and the first sample was HIV-1.
- C: The window period comes before antibody appears; a strong antibody cannot vanish in 5 days.
Question 2 · Hard
A visa screening laboratory tests about 250 samples a day with a fourth-generation HIV antigen/antibody chemiluminescence assay (reactive at S/CO 1.00 or more). Besides the manufacturer's controls, it runs a third-party low-positive control whose acceptable range, set over 20 runs, is S/CO 2.6-4.2; it read 3.1 and 3.3 earlier this week. This morning a new reagent lot was loaded. Today the negative and manufacturer's positive controls are in range, but the low-positive control reads 1.42. Of 248 samples, 2 are reactive and 3 give S/CO 0.70-0.95. What should the laboratory do?
1. ARelease nonreactives; retest only the three near cutoff
2. BReject the run and retest after troubleshooting
3. CAccept the run, as the low control is still reactive
4. DWiden the low control's range to fit the new lot
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Answer: B · Reject the run and retest after troubleshooting
A low-positive control sits just above the cut-off so that it reveals any loss of sensitivity, and its result must fall inside its own range, not merely read reactive. A fall from about 3.2 to 1.42 after a lot change means signals are roughly half their usual size, so a weakly reactive sample from early infection could read nonreactive, and the three results of 0.70-0.95 could be such samples. The run is rejected and no results are released; the new lot, its calibration and reagent storage are checked, and all samples are retested once the controls are back in range. Releasing nonreactive results, even with selective retesting, risks missed infections, and ranges are never widened to make a failing control pass. A new lot should be checked against the old one with the controls before it is put into use.
- A: With sensitivity reduced, any nonreactive result in this run may be a false negative.
- C: Being reactive is not enough; the control is far below its range, showing lost sensitivity.
- D: Ranges are never widened to make a failing control pass; the cause must be found.
Question 3 · Medium
Under the WHO HIV testing strategy for national programs (2019), a person is diagnosed HIV-positive when:
1. AThree different assays in sequence are all reactive
2. BOne rapid test is reactive on two separate days
3. CTwo runs of the same assay are both reactive
4. DThe first assay is reactive with a high signal
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Answer: A · Three different assays in sequence are all reactive
WHO (2019) advises a three-test strategy: a sensitive first assay, followed by two further assays of different design or antigen, each run only if the previous one is reactive. A positive diagnosis needs all three to be reactive, which keeps the positive predictive value high even where prevalence is low. Repeating the same assay repeats its own false reactions, so it adds little. Some discordant patterns are reported as inconclusive, with retesting after 14 days.
- B: Repeating one rapid test repeats its own false reactions.
- C: Running the same assay twice checks the technique, not the diagnosis.
- D: A high signal on one assay is not enough for a diagnosis.
Question 4 · Medium
After HIV-1 infection, in what order do the laboratory markers usually become detectable?
1. AAntibody, then p24 antigen, then RNA
2. Bp24 antigen, then RNA, then antibody
3. CRNA, then p24 antigen, then antibody
4. DRNA, then antibody, then p24 antigen
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Answer: C · RNA, then p24 antigen, then antibody
HIV-1 RNA appears first, about 10 days after infection, and p24 antigen follows a few days later as the viral load rises. IgM and then IgG antibodies appear after that, about 3-4 weeks after infection. This order sets the window periods: nucleic acid tests detect infection first, fourth-generation antigen/antibody assays next and antibody-only tests last. p24 can become undetectable once antibodies bind it in immune complexes.
- A: Antibody is the last of the three to appear.
- B: RNA is detectable a few days before p24 antigen.
- D: p24 antigen appears before antibody, not after it.
Question 5 · Medium
Compared with a laboratory fourth-generation assay, an oral-fluid rapid HIV test detects infection later mainly because it detects only:
1. Ap24 antigen, at lower levels than in blood
2. BIgM antibody, which fades within weeks
3. CIgG antibody, at lower levels than in blood
4. DHIV RNA, which is scarce in saliva
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Answer: C · IgG antibody, at lower levels than in blood
Oral-fluid tests detect antibody, mostly IgG, in the fluid that seeps from the gums, where antibody levels are much lower than in blood. They detect no p24 antigen and miss the early IgM response, so their window period is longer than that of blood antibody tests and much longer than fourth-generation assays. A reactive oral test needs laboratory confirmation, and a nonreactive result after a recent exposure does not exclude infection. These tests suit self-testing and outreach.
- A: Oral-fluid rapid tests detect antibody, not p24 antigen.
- B: These tests rely on IgG and miss the early IgM response.
- D: Rapid oral tests are antibody tests, not nucleic acid tests.
Question 6 · Medium
At a residence-visa medical screening center, an applicant's fourth-generation HIV screen is reactive. What should the laboratory do before any result is reported?
1. AReport HIV-positive based on the screen alone
2. BReport it as negative if the applicant is low risk
3. CAsk the applicant to come back in 3 months
4. DComplete the supplemental tests of the algorithm
Answer and explanation in LearnFree account
Question 7 · Easy
The p24 antigen detected by fourth-generation HIV screening assays is the viral:
1. AMatrix protein
2. BOuter envelope glycoprotein
3. CCapsid (core) protein
4. DTransmembrane glycoprotein
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Question 8 · Medium
In the CDC laboratory HIV testing algorithm, a specimen reactive on the fourth-generation antigen/antibody screen is next tested with:
1. AA repeat fourth-generation screen in 3 months
2. BAn HIV-1/HIV-2 antibody differentiation immunoassay
3. CAn HIV-1 p24 antigen neutralization test
4. DAn HIV-1 Western blot or immunofluorescence assay
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Question 9 · Hard
A specimen is reactive on a fourth-generation HIV antigen/antibody screen, and the HIV-1/HIV-2 differentiation immunoassay is nonreactive for both types. In the CDC algorithm, what is the next test?
1. ACD4 lymphocyte count
2. BHIV-1 Western blot assay
3. CHIV-1 nucleic acid test
4. DHIV-2 DNA PCR
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Question 10 · Hard
A 40-year-old woman applying for a work visa says she has taken antiretroviral therapy for 6 years. Fourth-generation screen reactive; HIV-1/HIV-2 differentiation assay HIV-1 positive, HIV-2 negative; HIV-1 RNA not detected. The best interpretation is:
1. AHIV-2 infection missed by the RNA test
2. BHIV-1 infection controlled by treatment
3. CHIV infection cured by long treatment
4. DFalse-positive screen; HIV is excluded
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Question 11 · Hard
A 28-year-old man has fever, sore throat, rash and lymphadenopathy 10 days after a high-risk sexual exposure. His fourth-generation HIV antigen/antibody screen is nonreactive. What is the best next step?
1. AReport that HIV infection is excluded
2. BRepeat the same screen in 12 months
3. CDo an HIV-1/HIV-2 differentiation assay
4. DTest for HIV-1 RNA now
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Question 12 · Hard
Results: fourth-generation HIV antigen/antibody screen reactive; HIV-1/HIV-2 differentiation immunoassay reactive for both HIV-1 and HIV-2. Under the CDC algorithm, the best interpretation is:
1. AHIV antibodies present, type not yet determined
2. BConfirmed dual infection with HIV-1 and HIV-2
3. CAcute HIV-1 infection before seroconversion
4. DFalse-positive screen; no HIV infection
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Question 13 · Hard
A laboratory confirms HIV with an HIV-1 immunoblot, read as positive when at least two of the p24, gp41 and gp120/160 bands are present. A screen-reactive sample shows a p24 band only. How should it be reported?
1. APositive; HIV-1 infection confirmed
2. BIndeterminate; follow with HIV-1 RNA
3. CNegative; no further testing needed
4. DPositive; HIV-2 infection confirmed
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Question 14 · Hard
A fourth-generation HIV screen with 100% sensitivity and 99.9% specificity is used on 20,000 visa applicants, in whom HIV prevalence is 0.1%. About what proportion of reactive screens will be true positives?
1. AAbout 0.1%
2. BAbout 50%
3. CAbout 99.9%
4. DAbout 100%
Answer and explanation in LearnFree account

HIV testing

Under the WHO HIV testing strategy for national programs (2019), a person is diagnosed HIV-positive when:

After HIV-1 infection, in what order do the laboratory markers usually become detectable?

Compared with a laboratory fourth-generation assay, an oral-fluid rapid HIV test detects infection later mainly because it detects only:

At a residence-visa medical screening center, an applicant's fourth-generation HIV screen is reactive. What should the laboratory do before any result is reported?

The p24 antigen detected by fourth-generation HIV screening assays is the viral:

In the CDC laboratory HIV testing algorithm, a specimen reactive on the fourth-generation antigen/antibody screen is next tested with:

A specimen is reactive on a fourth-generation HIV antigen/antibody screen, and the HIV-1/HIV-2 differentiation immunoassay is nonreactive for both types. In the CDC algorithm, what is the next test?

A 40-year-old woman applying for a work visa says she has taken antiretroviral therapy for 6 years. Fourth-generation screen reactive; HIV-1/HIV-2 differentiation assay HIV-1 positive, HIV-2 negative; HIV-1 RNA not detected. The best interpretation is:

A 28-year-old man has fever, sore throat, rash and lymphadenopathy 10 days after a high-risk sexual exposure. His fourth-generation HIV antigen/antibody screen is nonreactive. What is the best next step?

Results: fourth-generation HIV antigen/antibody screen reactive; HIV-1/HIV-2 differentiation immunoassay reactive for both HIV-1 and HIV-2. Under the CDC algorithm, the best interpretation is:

A laboratory confirms HIV with an HIV-1 immunoblot, read as positive when at least two of the p24, gp41 and gp120/160 bands are present. A screen-reactive sample shows a p24 band only. How should it be reported?

A fourth-generation HIV screen with 100% sensitivity and 99.9% specificity is used on 20,000 visa applicants, in whom HIV prevalence is 0.1%. About what proportion of reactive screens will be true positives?