Immunology and serological tests

45 Lab technician recruitment practice questions on Immunology and serological tests, 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 · Hard

   A man treated with benzathine penicillin for primary syphilis had a VDRL titre of 1:32 before treatment. Six months later it is 1:8. How should this be interpreted?

   1. ATreatment failure
   2. BReinfection with syphilis
   3. CBiological false positive
   4. DAdequate response to treatment

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   Answer: D · Adequate response to treatment

   A fall from 1:32 to 1:8 is two doubling dilutions, a fourfold decline. A fourfold fall within 6-12 months of treating early syphilis shows an adequate response. Treatment failure or reinfection is suggested by a fourfold rise or by no fall. The same nontreponemal test should be used for follow-up, because VDRL and RPR titres are not interchangeable.

   • A: Treatment failure is suggested by a titre that does not fall fourfold.
   • B: Reinfection is suggested by a fourfold rise, not a fall.
   • C: A known treated syphilis with a falling titre is not a biological false positive.

2. Question 2 · Hard

   A 28-year-old woman has had fever for 2 days with myalgia and platelets of 1,10,000/µL. Dengue tests: NS1 positive, IgM negative, IgG strongly positive. What is the best interpretation?

   1. AFalse-positive NS1 due to IgG
   2. BEarly primary dengue infection
   3. CPast dengue, current illness unrelated
   4. DSecondary dengue infection

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   Answer: D · Secondary dengue infection

   A positive NS1 antigen shows that dengue virus is replicating now. In a primary infection, IgG does not appear until about the second week, so strong IgG on day 2 means she has met dengue before. Current infection plus early IgG indicates a secondary infection, which carries a higher risk of severe dengue. She needs close monitoring of haematocrit and platelets.

   • A: NS1 is an antigen test and is not made falsely positive by IgG.
   • B: A primary infection would not show strong IgG on day 2.
   • C: NS1 positivity shows the present illness is dengue.

3. Question 3 · Hard

   Paired Widal results from a febrile man, taken on day 7 and day 17 of illness, are: TO 1:40 then 1:80; TH 1:40 then 1:40; AH 1:40 then 1:40; BH 1:160 then 1:640. What is the most likely diagnosis?

   1. AAnamnestic response from other fever
   2. BParatyphoid fever due to S. Paratyphi B
   3. CParatyphoid fever due to S. Paratyphi A
   4. DTyphoid fever due to S. Typhi

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   Answer: B · Paratyphoid fever due to S. Paratyphi B

   Only the BH titre shows a fourfold rise, from 1:160 to 1:640, pointing to infection with S. Paratyphi B. The TO titre rises only one tube, partly because Typhi and Paratyphi B share O antigen factor 12. The TH and AH titres are unchanged, so typhoid and paratyphoid A are unlikely. An anamnestic response usually raises H titres briefly and does not give a selective fourfold rise in one antigen.

   • A: An anamnestic response is not a selective fourfold rise in BH alone.
   • C: Paratyphi A would raise AH, which is unchanged.
   • D: S. Typhi infection would raise TO and TH, which stay low here.

4. Question 4 · Medium

   For the qualitative VDRL slide test on serum, the slide is rotated mechanically at:

   1. A100 rpm for 8 minutes
   2. B60 rpm for 2 minutes
   3. C180 rpm for 4 minutes
   4. D120 rpm for 15 minutes

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   Answer: C · 180 rpm for 4 minutes

   In the VDRL slide test, heated serum and antigen suspension are rotated at 180 rpm for 4 minutes and then read at once under the low-power microscope. Correct speed and time let the antigen particles clump with reagin. Too little rotation gives weak or missed reactions, and too much can break up flocculation or dry the drop. The RPR card test uses 100 rpm for 8 minutes.

   • A: This is the rotation for the RPR card test, not the VDRL slide test.
   • B: This is too slow and too short for the antigen to flocculate.
   • D: This speed and time are not those of any standard syphilis slide test.

5. Question 5 · Medium

   In an ELISA that uses an alkaline phosphatase conjugate, the usual substrate gives a yellow colour read at 405 nm. The substrate is:

   1. Ap-Nitrophenyl phosphate
   2. BHydrogen peroxide alone
   3. CTetramethylbenzidine
   4. DOrtho-phenylenediamine

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   Answer: A · p-Nitrophenyl phosphate

   Alkaline phosphatase splits p-nitrophenyl phosphate into p-nitrophenol, which is yellow in alkaline solution and read at 405 nm. The reaction can be stopped with sodium hydroxide. TMB and OPD are chromogens for horseradish peroxidase, which also needs hydrogen peroxide. The substrate must always match the enzyme label in the kit.

   • B: Hydrogen peroxide is the peroxidase substrate and gives no colour alone.
   • C: TMB is a substrate for peroxidase, read at 450 nm after acid stop.
   • D: OPD is also a peroxidase chromogen and gives an orange colour.

6. Question 6 · Medium

   A 38-year-old woman with early symmetric joint pain has a weakly positive rheumatoid factor. Which test is more specific for rheumatoid arthritis?

   1. AAnti-CCP antibody
   2. BC-reactive protein
   3. CSerum uric acid
   4. DAntistreptolysin O

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7. Question 7 · Easy

   Rubella-specific IgM found in a newborn's cord blood points to congenital infection because IgM:

   1. AIs made only by the mother
   2. BIs destroyed by the placenta
   3. CDoes not cross the placenta
   4. DLasts for life after birth

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8. Question 8 · Medium

   In a one-step sandwich immunoassay, an extremely high antigen level gives a falsely low result. This is called the:

   1. AMatrix effect of serum
   2. BHigh-dose hook effect
   3. CEdge effect of the plate
   4. DProzone of antibody excess

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9. Question 9 · Easy

   Rheumatoid factor is an antibody, usually of the IgM class, directed against:

   1. AThe Fc portion of IgG
   2. BCitrullinated peptides
   3. CDouble-stranded DNA
   4. DStreptolysin O

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10. Question 10 · Hard

    A 24-year-old man is on day 8 of fever with a falling platelet count. Dengue tests: NS1 negative, IgM positive, IgG negative. What is the best interpretation?

    1. ADengue excluded as NS1 is negative
    2. BPast dengue, current illness unrelated
    3. CRecent primary dengue infection
    4. DSecondary dengue infection

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11. Question 11 · Easy

    A reactive RPR must be confirmed with a test that detects antibody to the organism itself. Which test is used?

    1. AVDRL
    2. BKahn test
    3. CTPHA
    4. DWidal

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12. Question 12 · Easy

    The solid phase in a standard microplate ELISA is usually a:

    1. ANitrocellulose membrane
    2. BAgarose gel slab
    3. CGlass microscope slide
    4. DPolystyrene microtitre plate

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13. Question 13 · Easy

    A blood donor's sample is repeatedly reactive for HBsAg and the result is confirmed. This marker shows:

    1. AHepatitis C co-infection
    2. BPast infection with recovery
    3. CImmunity after vaccination
    4. DCurrent hepatitis B infection

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14. Question 14 · Medium

    The latex slide test for rheumatoid factor uses latex particles coated with:

    1. ARabbit IgM
    2. BStreptolysin O
    3. CCardiolipin
    4. DHuman IgG

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15. Question 15 · Medium

    On an ELISA plate, what is the main purpose of the blank well that receives only substrate and stop solution?

    1. ATo check that the conjugate is active
    2. BTo detect a prozone in strong samples
    3. CTo set the cut-off for reactive results
    4. DTo set the reader to zero absorbance

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16. Question 16 · Medium

    A 22-year-old has had fever for 4 days. The doctor asks for a Widal test. Why is the Widal test of limited value at this stage?

    1. AAgglutinins rise only from the end of the first week
    2. BThe organism is not in the blood in the first week
    3. CAntibiotics have usually destroyed the antibodies
    4. DO antigens disappear from the blood after three days

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17. Question 17 · Easy

    The H antigen of Salmonella, detected in the Widal test, is located on the:

    1. AFlagella
    2. BFimbriae
    3. CCapsule
    4. DCell wall

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18. Question 18 · Hard

    A pregnant woman was fully treated for secondary syphilis three years ago. Her RPR has stayed at 1:2 on four tests since then, and TPHA is reactive. She has no lesions or new exposure. How is this best interpreted?

    1. ASerofast state after adequate treatment
    2. BReinfection with a new syphilis episode
    3. CActive untreated syphilis needing treatment
    4. DBiological false-positive RPR reaction

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19. Question 19 · Easy

    In the Widal test, the 'AH' and 'BH' suspensions contain the flagellar antigens of:

    1. ASalmonella Typhi and Shigella
    2. BSalmonella Typhi O and Vi
    3. CSalmonella Typhimurium and Enteritidis
    4. DSalmonella Paratyphi A and B

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20. Question 20 · Medium

    To prepare the O antigen suspension for the Widal test, the Salmonella culture is treated with:

    1. APhenol saline
    2. BAbsolute alcohol
    3. CMerthiolate
    4. DFormalin

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21. Question 21 · Hard

    In a peroxidase-labelled HBsAg ELISA, no blue colour develops in any well, including the positive control. The technician made the wash buffer from saline preserved with sodium azide. What is the most likely cause?

    1. AThe plate was read at the wrong filter
    2. BAzide inhibited the peroxidase label
    3. CToo few washes left unbound conjugate
    4. DA high-dose hook in the positive control

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22. Question 22 · Easy

    In many Indian laboratories, the upper limit of normal for ASO in adults is taken as:

    1. A2,000 IU/mL
    2. B200 IU/mL
    3. C20 IU/mL
    4. D6 IU/mL

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23. Question 23 · Easy

    On a rapid card test for HBsAg, only the control band appears. What does the control band itself tell the technician?

    1. AThe patient has no infection at all
    2. BThe sample and reagents flowed correctly
    3. CThe patient sample contains the antigen
    4. DThe sample contained enough red cells

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24. Question 24 · Medium

    Why must the streptolysin O reagent in the classical ASO haemolysis-inhibition test be kept in the reduced state?

    1. AStreptolysin O lyses cells only in the cold
    2. BReduction removes the lipoproteins from serum
    3. CReduction converts it into a stable toxoid
    4. DStreptolysin O is inactivated by oxygen

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25. Question 25 · Medium

    In a precipitation test, a false-negative result caused by a large excess of antigen, so that no lattice forms, is called the:

    1. AProzone
    2. BZone of equivalence
    3. CZone of inhibition
    4. DPostzone

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