In the Widal test, a raised H agglutinin titre with a low O titre is of little diagnostic value because H antibodies:

Persist long after vaccination or past infection

React only with Brucella antigens

Appear only in the first few days of fever

Are IgM and vanish within a few days

A 26-year-old man has a rash on his palms and soles and moist, flat warty lesions near the anus. His undiluted serum VDRL is nonreactive, although the reactive control gives the expected result. What should be done next?

Repeat the test on diluted serum

Reheat the serum at 56 °C for 10 minutes

Repeat with a fresh antigen suspension

A 30-year-old antenatal woman, treated for syphilis five years ago and with no lesions now, has: RPR nonreactive, TPHA reactive. This is best interpreted as:

Adequately treated past syphilis

Biological false-positive reaction

Bacteriology: serological and rapid tests

14 Kerala PSC Lab Technician Gr II practice questions on Bacteriology: serological and rapid tests, part of Microbiology. 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 · Medium
In the Widal test, a raised H agglutinin titre with a low O titre is of little diagnostic value because H antibodies:
1. APersist long after vaccination or past infection
2. BReact only with Brucella antigens
3. CAppear only in the first few days of fever
4. DAre IgM and vanish within a few days
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Answer: A · Persist long after vaccination or past infection
O (somatic) antibodies are mainly IgM; they rise early in typhoid and fall within months. H (flagellar) antibodies are largely IgG and stay raised for months to years after whole-cell typhoid vaccine or an earlier infection, and may rise again during unrelated fevers (anamnestic response). A high O titre, or a fourfold rise in paired sera, is therefore more meaningful. Titres must be judged against the baseline of the local population.
- B: H agglutinins are specific to Salmonella flagellar antigens, not Brucella.
- C: H antibodies rise later than O antibodies, not only in the first days.
- D: It is O antibody that is mainly IgM and falls sooner.
Question 2 · Hard
A 26-year-old man has a rash on his palms and soles and moist, flat warty lesions near the anus. His undiluted serum VDRL is nonreactive, although the reactive control gives the expected result. What should be done next?
1. AReport the VDRL as nonreactive
2. BReheat the serum at 56 °C for 10 minutes
3. CRepeat the test on diluted serum
4. DRepeat with a fresh antigen suspension
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Answer: C · Repeat the test on diluted serum
A rash on the palms and soles with condylomata lata suggests secondary syphilis, when reagin titres are at their highest. Very high antibody levels can block lattice formation and give a falsely negative or rough, weak reaction in undiluted serum: the prozone phenomenon. Testing serial dilutions of the serum gives a clearly reactive result. The working reactive control shows that the antigen is fine, and reheating does not correct antibody excess.
- A: Reporting nonreactive would miss secondary syphilis in a patient with typical signs.
- B: Reheating is needed only for serum heated more than 4 hours earlier; it does not fix antibody excess.
- D: The reactive control worked, so the antigen suspension is satisfactory.
Question 3 · Easy
The VDRL test for syphilis is a:
1. AComplement fixation test
2. BLatex agglutination test
3. CTube agglutination test
4. DSlide flocculation test
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Answer: D · Slide flocculation test
In the VDRL test, heat-inactivated serum is mixed on a slide with a cardiolipin-lecithin-cholesterol antigen, and reagin antibody clumps the fine antigen particles into floccules seen under the microscope. It is a nontreponemal test, used for screening and for following treatment by titre. The RPR card test is a similar flocculation test with carbon particles, read by eye. Biological false positives occur, so reactive results are confirmed with a treponemal test such as TPHA.
- A: Complement fixation was used in the old Wassermann reaction.
- B: Latex agglutination is used for tests such as CRP, ASO and RA factor.
- C: Tube agglutination is the format of the Widal test.
Question 4 · Medium
A 30-year-old antenatal woman, treated for syphilis five years ago and with no lesions now, has: RPR nonreactive, TPHA reactive. This is best interpreted as:
1. ABiological false-positive reaction
2. BProzone effect in the RPR
3. CAdequately treated past syphilis
4. DActive untreated syphilis
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Answer: C · Adequately treated past syphilis
TPHA detects antibody to specific treponemal antigens and usually stays reactive for life, even after cure. The nontreponemal RPR measures reagin antibody, which falls and usually becomes nonreactive after successful treatment, so it reflects disease activity. A biological false positive shows the opposite pattern: RPR reactive with TPHA nonreactive. Her history and the absence of lesions fit treated past infection.
- A: A biological false positive is RPR reactive with TPHA nonreactive.
- B: Prozone happens with very high reagin titres, as in secondary syphilis with lesions; it is unlikely here.
- D: Active syphilis would give a reactive RPR as well as a reactive TPHA.
Question 5 · Easy
Which organism provides the antigens for the Weil-Felix test in rickettsial infections?
1. ABrucella
2. BEscherichia coli
3. CSalmonella
4. DProteus
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Answer: D · Proteus
The Weil-Felix test is a heterophile agglutination test using strains of Proteus (OX19, OX2 and OXK) that share polysaccharide antigens with rickettsiae. Scrub typhus, which occurs in Kerala, gives agglutination with OXK alone. The test is cheap but has low sensitivity and specificity. IgM ELISA or other specific tests are preferred where they are available.
- A: Brucella antigens are used in the standard agglutination test for brucellosis.
- B: E. coli strains are not used in rickettsial serology.
- C: Salmonella antigens are used in the Widal test.
Question 6 · Medium
After a streptococcal skin infection, the ASO titre often stays low. Which antibody test is preferred instead?
1. AAnti-DNase B
2. BC-reactive protein
3. CAntistaphylolysin
4. DAnti-streptolysin S
Answer and explanation in LearnFree account
Question 7 · Medium
Unlike the VDRL test, the RPR card test can be read with the naked eye. This is because its antigen:
1. AIs coloured with a dye
2. BIs coated on latex beads
3. CContains charcoal particles
4. DIs made from Treponema pallidum
Answer and explanation in LearnFree account
Question 8 · Hard
A 24-year-old man in Palakkad has had fever for 6 days. Widal: TO 1:80, TH 1:160. Ten days later: TO 1:640, TH 1:320. Blood culture was not done. The best interpretation is:
1. APast typhoid vaccination
2. BParatyphoid A fever
3. CActive typhoid fever
4. DPast typhoid, now cured
Answer and explanation in LearnFree account
Question 9 · Medium
Which patient is most likely to have a reactive VDRL without having syphilis?
1. AA man with iron deficiency anemia
2. BA woman with systemic lupus
3. CA man with a peptic ulcer
4. DA girl with acute appendicitis
Answer and explanation in LearnFree account
Question 10 · Medium
A 22-year-old in Kozhikode has had a step-ladder fever for 4 days. Which test is most likely to confirm enteric fever at this stage?
1. AStool culture
2. BWidal test
3. CUrine culture
4. DBlood culture
Answer and explanation in LearnFree account
Question 11 · Easy
The antistreptolysin O (ASO) test detects antibody against:
1. AAn oxygen-labile hemolysin of Streptococcus pyogenes
2. BThe capsule of Streptococcus pneumoniae
3. CAn oxygen-stable hemolysin of streptococci
4. DThe M protein of Streptococcus pyogenes
Answer and explanation in LearnFree account
Question 12 · Easy
C-reactive protein, an acute-phase protein, is produced mainly by the:
1. ALymph nodes
2. BLiver
3. CBone marrow
4. DSpleen
Answer and explanation in LearnFree account
Question 13 · Easy
Which test is used to follow the response to treatment of syphilis?
1. AFTA-ABS test
2. BSerial TPHA titres
3. CQuantitative VDRL titre
4. DDark-field microscopy
Answer and explanation in LearnFree account
Question 14 · Hard
An 8-year-old boy has puffy eyes and smoky urine 2 weeks after a sore throat. Urine: protein 2+, many red cells and red cell casts. ASO 800 IU/mL (reference below 200), serum C3 low. These results support:
1. AAcute glomerulonephritis
2. BAcute pyelonephritis
3. CAcute rheumatic fever
4. DNephrotic syndrome
Answer and explanation in LearnFree account

Bacteriology: serological and rapid tests

In the Widal test, a raised H agglutinin titre with a low O titre is of little diagnostic value because H antibodies:

A 26-year-old man has a rash on his palms and soles and moist, flat warty lesions near the anus. His undiluted serum VDRL is nonreactive, although the reactive control gives the expected result. What should be done next?

The VDRL test for syphilis is a:

A 30-year-old antenatal woman, treated for syphilis five years ago and with no lesions now, has: RPR nonreactive, TPHA reactive. This is best interpreted as:

Which organism provides the antigens for the Weil-Felix test in rickettsial infections?

After a streptococcal skin infection, the ASO titre often stays low. Which antibody test is preferred instead?

Unlike the VDRL test, the RPR card test can be read with the naked eye. This is because its antigen:

A 24-year-old man in Palakkad has had fever for 6 days. Widal: TO 1:80, TH 1:160. Ten days later: TO 1:640, TH 1:320. Blood culture was not done. The best interpretation is:

Which patient is most likely to have a reactive VDRL without having syphilis?

A 22-year-old in Kozhikode has had a step-ladder fever for 4 days. Which test is most likely to confirm enteric fever at this stage?

The antistreptolysin O (ASO) test detects antibody against:

C-reactive protein, an acute-phase protein, is produced mainly by the:

Which test is used to follow the response to treatment of syphilis?

An 8-year-old boy has puffy eyes and smoky urine 2 weeks after a sore throat. Urine: protein 2+, many red cells and red cell casts. ASO 800 IU/mL (reference below 200), serum C3 low. These results support: