A patient with a recent pulmonary tuberculosis diagnosis is admitted. Which finding is most closely associated with an increased risk of transmission?

• A. A 4+ positive acid-fast bacilli (AFB) in a sputum smear
• B. A 1+ AFB smear
• C. Negative AFB smear with positive culture
• D. Positive PPD only

Answer: (A)

In pulmonary tuberculosis, how contagious a patient is largely depends on how many bacilli are being shed in the sputum, which is reflected by the acid-fast bacilli smear result. A sputum smear that is strongly positive (4+) means a large number of bacilli are present and readily dispersed into the air when the patient coughs, greatly increasing the likelihood of transmission.

If the smear is only weakly positive (1+), there are fewer bacilli, so the transmission risk is present but lower than with a 4+ smear. A negative smear with a positive culture means there are viable bacilli, but they are not being shed in large numbers in the sputum, so the infectious risk is lower than a smear-positive case. A positive PPD (tuberculin skin test) only indicates prior exposure or latent infection and does not reflect active disease or contagiousness.

Therefore, the finding most closely associated with increased transmission is a sputum smear that is strongly positive for acid-fast bacilli.