Which test result most strongly suggests active TB requiring isolation?

• A. Normal chest X-ray
• B. 4+ acid-fast bacilli smear
• C. Negative AFB smear
• D. No isolation indicated

Answer: (B)

The key idea is how contagious active pulmonary TB is based on microbiology results and how that guides isolation. A sputum smear showing four or more acid-fast bacilli means a very heavy burden of Mycobacterium tuberculosis in the airways, so the patient is highly infectious. That level of contagiousness is what most strongly prompts immediate airborne isolation to prevent transmission.

A normal chest X-ray does not rule out active TB and, in some cases, TB can be present with a normal radiograph or with findings that aren’t obviously diagnostic of contagious disease. A negative AFB smear means no organisms were seen in that specimen, but TB can be present even if smears are negative, especially in paucibacillary disease or early infection, so it doesn’t indicate a high likelihood of contagious TB. Saying no isolation is indicated would ignore established infection-control practices for suspected active TB.