Which TB skin test threshold is associated with individuals who have no known risk factors?

Which TB skin test threshold is associated with individuals who have no known risk factors?

• A. 5 mm
• B. 10 mm
• C. 15 mm
• D. More than 15 mm

Answer: (D)

The key idea here is that how big a tuberculin skin test reaction must be to be considered positive depends on the person’s risk factors. People with no known risk factors have a low pretest probability of TB infection, so a larger induration is required before we call it positive. In practice, the threshold used for this group is 15 mm or more. The option stating “more than 15 mm” captures that concept of a high cutoff, reflecting that larger reactions are needed to indicate infection in low-risk individuals. Remember, an induration of 15 mm or greater is considered positive in those with no risk factors, and smaller thresholds apply to groups with higher risk.

The key idea here is that how big a tuberculin skin test reaction must be to be considered positive depends on the person’s risk factors. People with no known risk factors have a low pretest probability of TB infection, so a larger induration is required before we call it positive. In practice, the threshold used for this group is 15 mm or more. The option stating “more than 15 mm” captures that concept of a high cutoff, reflecting that larger reactions are needed to indicate infection in low-risk individuals. Remember, an induration of 15 mm or greater is considered positive in those with no risk factors, and smaller thresholds apply to groups with higher risk.