Saunders Respiratory Practice

The correct answer is recent antibiotic use. When a patient suspected of having tuberculosis (TB) has recently been treated with antibiotics, this can suppress the growth of mycobacteria that would otherwise be detected in a sputum culture. Antibiotics can eliminate or significantly reduce the number of bacteria present, leading to a false normal result even in cases where TB is actually present.

In contrast, tobacco smoking, the presence of asthma, and chronic respiratory illness may have varying effects on the respiratory system but are not typically associated with yielding a false normal sputum culture for TB. Tobacco smoking can affect lung health and increase the risk of respiratory infections, but it does not directly inhibit the growth of mycobacteria in sputum cultures. Asthma, while it can affect sputum production, does not usually impact the accuracy of TB cultures, and chronic respiratory illnesses may complicate the clinical picture but do not specifically lead to a false negative result in sputum cultures for TB. Thus, the most significant influence in this context causing a false negative sputum culture is recent antibiotic use.