For health care workers, the baseline tuberculin skin testing should employ the two-step method after how many months without a negative result?

The two-step method for tuberculin skin testing in health care workers is particularly beneficial to detect latent tuberculosis infection that may not be evident in a single test. This method is typically used when an individual has not been tested for tuberculin in the previous 12 months and is essential for those who may have had a false negative due to various reasons, such as "booster phenomenon."

After the initial skin test, if the result is negative, a second test is usually conducted 1 to 3 weeks later to ensure that any potential latent TB infection is identified. If there hasn’t been a negative result after 12 months, it is advisable to re-evaluate the individual with the two-step method. This approach helps to ensure that any delayed reactions in those who may have been exposed to TB are adequately identified and monitored.

Given this protocol, the baseline tuberculin skin testing should employ the two-step method specifically after 12 months without a negative result to ensure adequate screening and management of potential TB infections among health care workers.