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A firm bump measuring 10 mm on a purified protein derivative (PPD) test, commonly used for tuberculosis (TB) screening, indicates a positive reaction, which can suggest an infection with TB. The interpretation of the size of the induration (bump) is influenced by the individual’s risk factors for TB.

For individuals classified within a moderate-risk group (including those who may have recent exposure to an infectious TB patient or certain medical conditions that slightly increase the risk), an induration of 10 mm or greater is considered a positive test result, signifying the presence of TB infection. This threshold is based on public health guidelines that stratify risk levels and their corresponding induration measurements for interpretation.

This finding does not apply to low-risk groups, where the criteria for a positive result is higher (generally requiring a measurement greater than 15 mm for individuals with no known risk factors), and also does not correlate with the high-risk category criteria as those generally require a smaller induration to suggest a positive result. Therefore, in the context of the moderate-risk group, a 10 mm induration is significant and warrants further investigation or treatment based on additional clinical factors.