TB Test Assessment and Evaluation 2

Question 1

A patient with a 14mm induration on TST has no risk factors and was born in a low-prevalence country. How should this result be interpreted?

Accepted Answer

Negative — threshold for low-risk individuals is 15mm

Answer

Positive — any induration ≥10mm is positive

Answer

Positive — any induration ≥5mm is positive

Answer

Inconclusive — requires IGRA confirmation

Question 2

Which IGRA result pattern indicates a borderline or indeterminate finding that requires retesting?

Accepted Answer

IFN-γ nil ≥8 IU/mL causing invalid background

Answer

IFN-γ nil <0.35 IU/mL with TB antigen response ≥0.35 IU/mL

Answer

TB antigen minus nil ≥0.35 IU/mL with mitogen response ≥0.5 IU/mL

Answer

TB antigen minus nil <0.35 IU/mL with mitogen response ≥0.5 IU/mL

Question 3

A chest X-ray shows upper lobe fibrotic changes in a patient with a positive TST. What is the most appropriate next step?

Accepted Answer

Order sputum smear and culture to rule out active TB

Answer

Repeat TST in 8 weeks

Answer

Initiate LTBI treatment immediately without further workup

Answer

Perform IGRA to confirm TST positivity

Question 4

During a TB contact investigation, which classification describes a person exposed to an infectious TB case who has a negative TST but should be retested in 8–10 weeks?

Accepted Answer

Window period contact

Answer

Confirmed LTBI contact

Answer

Non-reactor contact

Answer

Recently converted contact

Question 5

Which of the following conditions can cause a false-negative TST result despite true LTBI?

Accepted Answer

Advanced HIV infection with CD4 count <200 cells/µL

Answer

Recent BCG vaccination within the past year

Answer

Prior positive TST documented 5 years ago

Answer

Testing performed more than 72 hours after placement

Question 6

A two-step TST is performed. The first test is negative at 72 hours, and a second test two weeks later shows 12mm induration. How should this second result be interpreted?

Accepted Answer

Boosted reaction — the patient likely has remote LTBI, not new infection

Answer

New positive — the patient recently converted and has LTBI

Answer

False positive due to repeat antigen exposure

Answer

Indeterminate — requires IGRA confirmation

Question 7

When evaluating a healthcare worker for annual TB surveillance, which scenario would prompt an immediate evaluation regardless of prior test results?

Accepted Answer

New onset of cough lasting 3 weeks plus night sweats and weight loss

Answer

Annual TST showing induration 2mm larger than last year

Answer

Positive IGRA result in a worker who had BCG vaccination

Answer

Negative IGRA result following a known TB exposure 4 weeks ago