TB Test Diagnostic Approaches 3 — Questions and Answers
Question 1: In extrapulmonary TB, which diagnostic approach is most reliable for confirming TB lymphadenitis?
- Tuberculin skin test alone
- Fine needle aspiration cytology (FNAC) with culture (Correct answer)
- Blood IGRA test only
- MRI without biopsy
Correct answer: Fine needle aspiration cytology (FNAC) with culture
FNAC of the affected lymph node with smear, culture, and histology provides direct microbiological and pathological evidence for confirming TB lymphadenitis.
Question 2: Which characteristic CSF finding is most consistent with tuberculous meningitis?
- High glucose with low protein
- Neutrophilic pleocytosis with low glucose
- Lymphocytic pleocytosis with elevated protein and low glucose (Correct answer)
- Normal CSF with positive culture only
Correct answer: Lymphocytic pleocytosis with elevated protein and low glucose
TB meningitis classically presents with CSF lymphocytic pleocytosis, elevated protein, and low glucose due to meningeal inflammation and impaired glucose transport.
Question 3: What is the primary advantage of using liquid culture media (MGIT) over solid Lowenstein-Jensen media for TB diagnosis?
- Lower cost per test
- Faster detection time (1-3 weeks vs. 3-8 weeks) (Correct answer)
- Higher specificity for M. tuberculosis
- No requirement for biosafety containment
Correct answer: Faster detection time (1-3 weeks vs. 3-8 weeks)
Liquid culture systems like MGIT detect mycobacterial growth in 1-3 weeks compared to 3-8 weeks for solid media, significantly reducing time to diagnosis and treatment.
Question 4: A healthcare worker tests TST-negative on initial screening. Under the two-step protocol, a repeat TST 2 weeks later measures 12mm. This is best interpreted as:
- New TB infection acquired between tests
- Boosting effect of waned prior immunity (Correct answer)
- False positive due to BCG vaccination
- Indeterminate result requiring IGRA confirmation
Correct answer: Boosting effect of waned prior immunity
A positive result on a second TST shortly after a negative first test in a person with prior TB exposure represents boosting of previously waned delayed hypersensitivity, not new infection.
Question 5: Which molecular test is recommended by WHO as the initial diagnostic test for presumptive TB due to its simultaneous detection of M. tuberculosis and rifampicin resistance?
- Line probe assay (LPA)
- Xpert MTB/RIF (GeneXpert) (Correct answer)
- Loop-mediated isothermal amplification (LAMP)
- Standard PCR without resistance detection
Correct answer: Xpert MTB/RIF (GeneXpert)
Xpert MTB/RIF is WHO-endorsed as the preferred initial diagnostic test as it simultaneously detects M. tuberculosis DNA and rifampicin resistance mutations within 2 hours.
Question 6: In a patient with HIV and suspected TB, what modification to standard TST interpretation is recommended?
- Use a higher cutoff of 15mm
- Use a lower cutoff of 5mm regardless of exposure risk (Correct answer)
- TST is contraindicated in HIV patients
- Use 10mm cutoff same as healthcare workers
Correct answer: Use a lower cutoff of 5mm regardless of exposure risk
In HIV-infected individuals, a TST induration of ≥5mm is considered positive because impaired cellular immunity may limit the skin test response despite true infection.
Question 7: Which diagnostic test would be most useful for monitoring treatment response in pulmonary TB?
- Serial IGRAs every 2 months
- Monthly TST readings
- Sputum smear and culture at designated intervals (Correct answer)
- Repeat chest CT after each drug dose
Correct answer: Sputum smear and culture at designated intervals
Serial sputum smear microscopy and culture at 2, 5, and 6 months of treatment assess mycobacterial clearance and are the standard for monitoring treatment response and detecting treatment failure.
TB Test Diagnostic Approaches 3
In extrapulmonary TB, which diagnostic approach is most reliable for confirming TB lymphadenitis?