How many consecutive negative sputum cultures are required to consider a person with tuberculosis no longer infectious?

• A. 2
• B. 3
• C. 1
• D. 4

Answer: (B)

In tuberculosis, infectiousness is tied to whether live bacilli are being shed in the sputum. Sputum culture detects those viable bacteria, and using multiple samples over time gives a reliable read on whether shedding has truly ceased. A single negative culture can be a fluke, and even two negatives might miss intermittent shedding or sampling error. Requiring three consecutive negative sputum cultures collected on separate days provides strong evidence that the person has stopped shedding viable bacteria, reducing the chance of missed infectiousness.

This approach helps prevent ongoing transmission while avoiding unnecessary prolonged isolation. Four consecutive negatives are more stringent than necessary given typical standards, while one or two do not adequately guard against false negatives.