Virology: serological diagnosis

27 Kerala PSC Lab Technician Gr II practice questions on Virology: serological diagnosis, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   NS1 antigen testing is used in the early diagnosis of which infection?

   1. AMeasles
   2. BMumps
   3. CRabies
   4. DDengue

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   Answer: D · Dengue

   NS1 is a non-structural protein released into the blood by dengue virus, and it can be detected from the first day of fever until about day 5 to 7, before IgM appears. Laboratories therefore run NS1 with IgM capture ELISA so that early and later cases are both caught. A positive NS1 test confirms current dengue infection, while IgM may reflect a recent one. RT-PCR is the other early test.

   • A: Measles is diagnosed by IgM ELISA or RT-PCR.
   • B: Mumps is diagnosed by IgM ELISA or RT-PCR on saliva.
   • C: Rabies is confirmed by antigen detection in brain or skin, not by NS1.

2. Question 2 · Hard

   A patient in the seventh day of fever has dengue NS1 negative, dengue IgM positive and dengue IgG strongly positive. The best interpretation is:

   1. ASecondary dengue infection
   2. BOld dengue with no current illness
   3. CPrimary dengue infection
   4. DFalse positive from rheumatoid factor

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   Answer: A · Secondary dengue infection

   NS1 is usually negative after about day 5, so its absence on day 7 does not argue against dengue. In a primary infection IgM rises first and IgG appears late and low; a strong IgG alongside IgM this early means a memory response, that is a repeat infection with another serotype. Secondary infection carries the higher risk of plasma leakage, so the platelet count and haematocrit are watched. An old infection would give IgG without IgM, and rheumatoid factor interference is unlikely when IgM and IgG results agree.

   • B: An old infection gives IgG alone, without IgM.
   • C: A primary infection would show IgM with little or no IgG at this stage.
   • D: Interference is improbable when both antibody classes fit the illness.

3. Question 3 · Medium

   IgM capture ELISA (MAC-ELISA) on cerebrospinal fluid is the test of choice for Japanese encephalitis because:

   1. AIgM is more stable in CSF than in serum
   2. BIt also measures the viral load
   3. CCSF needs no dilution before testing
   4. DIgM in CSF is made within the CNS

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   Answer: D · IgM in CSF is made within the CNS

   The wells are coated with anti-human IgM, which captures the patient's IgM and keeps IgG out of the reaction, so interference from past flavivirus antibody is reduced. IgM does not cross an intact blood-brain barrier, so finding it in CSF shows antibody made within the central nervous system and confirms acute encephalitis. Serum IgM is tested alongside, as CSF may be negative very early. The assay is used in the national surveillance network for acute encephalitis syndrome.

   • A: IgM is no more stable in CSF than in serum.
   • B: An antibody assay measures antibody, not viral load.
   • C: CSF is diluted in the kit's buffer like any other sample.

4. Question 4 · Medium

   In HIV testing, what is the role of the Western blot?

   1. AScreen large numbers of sera cheaply
   2. BMeasure the CD4 cell count
   3. CDetect viral RNA in plasma
   4. DConfirm a reactive screening test

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   Answer: D · Confirm a reactive screening test

   The Western blot separates HIV proteins by electrophoresis on a strip, and the patient's antibodies show as bands against p24, gp41 and gp120/160, so the pattern tells which viral proteins are recognised. It was the classic confirmatory test after a reactive ELISA, although national programmes now confirm with three different rapid or ELISA tests on different principles. It is too slow and costly for screening. CD4 counting is done by flow cytometry and viral load by PCR.

   • A: It is far too slow and costly for screening.
   • B: CD4 counts are measured by flow cytometry.
   • C: Viral RNA is measured by nucleic acid amplification.

5. Question 5 · Easy

   What does specific IgM antibody in a single serum sample suggest?

   1. ASuccessful vaccination
   2. BPast infection
   3. CRecent infection
   4. DA chronic carrier state

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   Answer: C · Recent infection

   IgM is the first antibody made in a primary infection; it appears within days, peaks in a week or two and usually disappears within 2 to 3 months. Its presence in one sample therefore points to a recent or current infection, which is how measles, dengue, hepatitis A and Japanese encephalitis are diagnosed. IgG appears later and persists for years, so a single IgG result cannot date an infection. Rheumatoid factor and cross-reacting antibodies can give false IgM results.

   • A: Vaccination usually leaves IgG; IgM, if any, is short lived.
   • B: Past infection is shown by IgG alone, without IgM.
   • D: A carrier state is shown by persisting antigen, such as HBsAg.

6. Question 6 · Medium

   A four-fold or greater rise in antibody titre between acute and convalescent serum samples indicates:

   1. AImmunity left by a past infection
   2. BRecent infection by that agent
   3. CFailure of the vaccine
   4. DA cross-reacting antibody

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7. Question 7 · Easy

   The hepatitis B vaccine contains only the surface antigen. A vaccinated person who was never infected therefore has:

   1. AAnti-HBc alone
   2. BHBsAg and anti-HBe
   3. CAnti-HBs and anti-HBc
   4. DAnti-HBs alone

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8. Question 8 · Hard

   An HIV ELISA kit defines the cut-off as the mean OD of the negative controls plus 0.200. The negative controls read 0.040, 0.050 and 0.060, and a patient's sample reads 0.780. What is the sample's S/CO ratio and result?

   1. A3.12; reactive
   2. B15.6; reactive
   3. C2.23; reactive
   4. D0.32; non-reactive

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9. Question 9 · Easy

   A man on antiretroviral therapy for a year is tested to see how well the treatment is suppressing the virus. Which test is used?

   1. Ap24 antigen ELISA
   2. BQuantitative RT-PCR for HIV RNA
   3. CCD4 count by flow cytometry
   4. DHIV-1/2 antibody ELISA

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10. Question 10 · Medium

    A 9-month-old baby of an HIV-positive mother has a reactive HIV antibody test. What does this result show?

    1. AThe baby is certainly not infected
    2. BThe baby is certainly infected
    3. CA false positive from infant IgM
    4. DPossibly maternal antibody; do DNA PCR

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11. Question 11 · Easy

    In haemagglutination inhibition, how does antibody in the patient's serum show?

    1. AAs a colour change of an indicator
    2. BAs complete lysis of the red cells
    3. CAs a compact button of red cells
    4. DAs a smooth mat of red cells

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12. Question 12 · Medium

    In a virus neutralisation test, the patient's antibody is detected by its ability to:

    1. APrecipitate the viral antigen in gel
    2. BMake infected cells fluoresce
    3. CAgglutinate red cells in a tube
    4. DStop the virus from infecting cells

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13. Question 13 · Hard

    A 28-year-old man has jaundice and ALT 1,450 U/L. Results: HBsAg positive, HBeAg positive, IgM anti-HBc positive, anti-HBs negative. The best interpretation is:

    1. ARecovered from past hepatitis B
    2. BChronic hepatitis B, low infectivity
    3. CAcute hepatitis B, highly infectious
    4. DImmunity from hepatitis B vaccination

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14. Question 14 · Easy

    Which hepatitis B marker indicates active viral replication and high infectivity?

    1. AIgG anti-HBc
    2. BHBeAg
    3. CAnti-HBe
    4. DAnti-HBs

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15. Question 15 · Easy

    Which red cells are agglutinated by the heterophile antibodies detected in the Paul-Bunnell test?

    1. AHuman group O red cells
    2. BSheep red cells
    3. CGuinea pig red cells
    4. DFowl red cells

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16. Question 16 · Medium

    What does a fourth-generation HIV ELISA detect in addition to antibody, which shortens the window period?

    1. ACD4 T cells
    2. BHIV RNA
    3. Cp24 antigen
    4. DReverse transcriptase

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17. Question 17 · Medium

    After a full course of hepatitis B vaccine, a health worker is regarded as protected if the anti-HBs level is at least:

    1. A100 IU/mL
    2. B1 mIU/mL
    3. C0.5 mIU/mL
    4. D10 mIU/mL

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18. Question 18 · Easy

    By NACO strategy for diagnosing HIV in an asymptomatic adult, a person is reported HIV positive only when the sample is reactive in:

    1. AOne rapid test performed twice
    2. BOne ELISA with a high optical density
    3. CTwo tests from the same manufacturer
    4. DThree tests of different principles or antigens

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19. Question 19 · Medium

    What are the microtitre wells coated with in a sandwich ELISA for HBsAg?

    1. AHBsAg antigen
    2. BAnti-human globulin
    3. CAnti-HBs antibody
    4. DEnzyme-labelled HBsAg

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20. Question 20 · Medium

    The hemagglutination inhibition test can be used to measure antibody to rubella and influenza viruses because these viruses:

    1. AHave DNA genomes
    2. BProduce syncytia in cell culture
    3. CGrow in embryonated eggs
    4. DAgglutinate red cells

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21. Question 21 · Easy

    In an ELISA with horseradish peroxidase and TMB substrate, the reaction is stopped with acid and the absorbance read at:

    1. A340 nm
    2. B450 nm
    3. C405 nm
    4. D540 nm

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22. Question 22 · Hard

    A pregnant woman is screened at 20 weeks. HBsAg positive (also positive 2 years ago), HBeAg negative, anti-HBe positive, IgG anti-HBc positive, IgM anti-HBc negative, ALT 28 U/L. Her status is:

    1. AWindow period of acute hepatitis B
    2. BResolved hepatitis B with immunity
    3. CAcute hepatitis B, early phase
    4. DChronic HBV infection, low replication

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23. Question 23 · Easy

    For how long must HBsAg stay positive before a person is called a chronic hepatitis B carrier?

    1. A6 months
    2. B2 years
    3. C3 months
    4. D1 month

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24. Question 24 · Medium

    In dengue, from about which day of illness does IgM antibody usually become detectable?

    1. ADay 1
    2. BDay 14
    3. CDay 5
    4. DDay 30

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25. Question 25 · Medium

    A blood donor's sample is anti-HCV reactive on screening. Which test tells current infection from past, cleared infection?

    1. ARepeat anti-HCV ELISA
    2. BHCV RNA by PCR
    3. CSerum ALT level
    4. DHCV IgG avidity

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