Parasitology: serological and rapid tests

11 Kerala PSC Lab Technician Gr II practice questions on Parasitology: serological and rapid tests, part of Microbiology. The first 11 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 · Medium

   Why can the card test for Wuchereria bancrofti circulating antigen be done on blood taken during the day?

   1. AMicrofilariae enter day blood in mild cases
   2. BThe antigen comes from adult worms at all hours
   3. CA dose of DEC is given before the test
   4. DThe card detects antibody to microfilariae

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   Answer: B · The antigen comes from adult worms at all hours

   The immunochromatographic card (or filariasis test strip) detects circulating filarial antigen released by living adult W. bancrofti worms. Its level does not follow the nocturnal periodicity of microfilariae, so a finger-prick sample can be taken at any time. This makes it practical for surveys in the national elimination programme. It does not detect Brugia malayi, for which an antibody test is used.

   • A: Nocturnal periodicity holds whatever the intensity of infection.
   • C: DEC provocation is used with blood films, not with the antigen card.
   • D: The card detects antigen, not antibody to microfilariae.

2. Question 2 · Hard

   A 32-year-old migrant worker in Perumbavoor has fever with chills. A combination malaria RDT shows: control line present, HRP-2 (P. falciparum) line absent, pan-pLDH line present. The most likely interpretation is:

   1. AInvalid test that must be repeated
   2. BPlasmodium falciparum malaria only
   3. CNon-falciparum malaria, probably P. vivax
   4. DMixed P. falciparum and P. vivax

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   Answer: C · Non-falciparum malaria, probably P. vivax

   HRP-2 is made only by P. falciparum, while pan-pLDH is made by all human Plasmodium species. A valid test with the pan line alone therefore points to a non-falciparum species, which in India is mostly P. vivax. P. falciparum, alone or mixed, would normally make the HRP-2 line positive, unless the parasite lacks the hrp2 gene. A blood film should confirm the species before treatment, including primaquine, is chosen.

   • A: The control line appeared, so the test is valid.
   • B: P. falciparum would usually give a positive HRP-2 line.
   • D: A mixed infection would normally show both the HRP-2 and pan lines.

3. Question 3 · Easy

   Which disease is diagnosed with the rK39 rapid dipstick test?

   1. AAmoebic liver abscess
   2. BVisceral leishmaniasis
   3. CLymphatic filariasis
   4. DFalciparum malaria

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   Answer: B · Visceral leishmaniasis

   The rK39 test detects antibody to a recombinant kinesin antigen of the Leishmania donovani complex on an immunochromatographic strip, using a drop of blood or serum. It is highly sensitive and specific in India and is used in the kala-azar elimination programme for diagnosis at the point of care. A positive result in a patient with fever for over 2 weeks and splenomegaly supports treatment. Antibody persists after cure, so the test does not diagnose relapse.

   • A: Amoebic liver abscess is diagnosed by E. histolytica antibody tests.
   • C: Filariasis rapid tests detect circulating filarial antigen or Brugia antibodies.
   • D: Falciparum malaria is detected by HRP-2 antigen tests.

4. Question 4 · Medium

   Two weeks after successful treatment of falciparum malaria, a patient is well and his blood film is negative, but an HRP-2 rapid test is still positive. The best explanation is:

   1. AThe parasites are drug-resistant
   2. BHRP-2 antigen persists after cure
   3. CThe test cross-reacts with P. vivax
   4. DThe test detects the patient's antibody

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   Answer: B · HRP-2 antigen persists after cure

   HRP-2 is released by P. falciparum and stays in the blood for several weeks after the parasites are cleared. An HRP-2 test can therefore stay positive after cure and should not be used to check treatment response. Follow-up is by blood film; pLDH, made only by living parasites, clears faster. Drug resistance would show parasites on the film.

   • A: Resistant parasites would still be seen on the blood film.
   • C: HRP-2 is specific for P. falciparum.
   • D: Malaria rapid tests detect parasite antigen, not antibody.

5. Question 5 · Easy

   Which infection is diagnosed with the Sabin-Feldman dye test?

   1. AAmoebiasis
   2. BToxoplasmosis
   3. CMalaria
   4. DVisceral leishmaniasis

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   Answer: B · Toxoplasmosis

   In the dye test, live Toxoplasma tachyzoites are mixed with patient serum and an accessory factor. Antibody damages the parasites, so they no longer take up alkaline methylene blue and stay unstained. It is a reference test because it needs live parasites. Routine laboratories use ELISA for Toxoplasma IgG and IgM.

   • A: Amoebiasis antibody is detected by IHA or ELISA.
   • C: Malaria is diagnosed by blood films and antigen rapid tests.
   • D: Visceral leishmaniasis is diagnosed with rK39 or direct agglutination tests.

6. Question 6 · Medium

   In an endemic area, a positive Entamoeba histolytica antibody test in a patient with diarrhoea is hard to interpret because:

   1. AAntibodies appear in E. dispar infection
   2. BAntibodies form only in healthy carriers
   3. CAntibodies persist long after past infection
   4. DThe test detects cysts, not trophozoites

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7. Question 7 · Medium

   Which serological test is preferred for confirming neurocysticercosis?

   1. AImmunoblot (EITB)
   2. BWidal tube agglutination
   3. CCasoni intradermal test
   4. DSabin-Feldman dye test

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8. Question 8 · Medium

   A patient has many P. falciparum rings on the smear, but the HRP-2 rapid test is negative. A likely cause is:

   1. ADeletion of the pfhrp2 gene in the parasite
   2. BMixed infection with P. vivax
   3. CAntigen persisting after earlier treatment
   4. DCross-reaction with rheumatoid factor in serum

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9. Question 9 · Easy

   The Casoni intradermal test was used in the diagnosis of:

   1. AToxoplasmosis
   2. BKala-azar
   3. CHydatid disease
   4. DFilariasis

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10. Question 10 · Hard

    A malaria RDT cassette shows a clear P. falciparum test line but no control line after 20 minutes. The buffer was added correctly. How should it be reported?

    1. AMixed infection is likely; report both
    2. BPositive for P. falciparum malaria
    3. CNegative for malaria
    4. DInvalid; repeat with a new cassette

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11. Question 11 · Easy

    Malaria rapid diagnostic tests work on the principle of:

    1. AComplement fixation
    2. BFlocculation
    3. CImmunochromatography
    4. DHaemagglutination

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