How should a nurse interpret a tuberculin skin test with a firm, raised area greater than 15 mm in diameter?

A tuberculin skin test, also known as the Mantoux test, is used to determine if a person has been exposed to the bacteria that causes tuberculosis (TB). In interpreting the results of this test, the size of the induration—which is the firm, raised area—is crucial. An induration greater than 15 mm in diameter is considered a positive result in individuals who are at low risk for TB. This suggests that the individual has likely been exposed to the TB bacteria and may have an active or latent infection.

The significance of the 15 mm benchmark is particularly important because it is a threshold that indicates a significant immune response to the TB antigen injected during the test. This response signifies that the individual's immune system has recognized and reacted to the TB proteins, supporting the likelihood of TB infection.

In contrast, smaller indurations would indicate varying responses based on risk factors and previous exposure, with thresholds lower than 15 mm applied to certain high-risk populations. Therefore, a firm, raised area greater than 15 mm indicates a definite positive response, prompting further evaluation for tuberculosis, such as a chest X-ray or symptoms assessment.