what is a two step tb skin test

3 min read 31-12-2024

The two-step tuberculin skin test (TST), also known as the two-step Mantoux test, is a method used to detect infection with Mycobacterium tuberculosis, the bacteria that causes tuberculosis (TB). It's particularly useful in identifying individuals who may have been exposed to TB but haven't mounted a strong enough immune response to show up on a single test. This article will explain the procedure, why it's used, and its limitations.

Understanding the Tuberculin Skin Test (TST)

The TST involves injecting a small amount of purified protein derivative (PPD) under the skin of the forearm. This PPD is a substance derived from M. tuberculosis. If a person has been exposed to TB, their immune system will react to the PPD. This reaction is measured 48-72 hours later by checking for a raised, hardened area (induration) at the injection site. The size of this induration determines whether the test is considered positive or negative.

Why is a Two-Step Test Necessary?

A single TST might produce a false-negative result in some individuals, especially those with prior exposure to TB or the BCG (Bacillus Calmette-Guérin) vaccine. The BCG vaccine is given in many parts of the world to prevent TB, but it can interfere with the interpretation of a single TST.

The two-step approach aims to address this issue. It involves:

First Test: A standard TST is performed.
Second Test (if necessary): If the first test is negative, a second TST is administered one to three weeks later.

The rationale behind the two-step approach is that a negative first test in someone with prior exposure can sometimes be followed by a positive second test. This "booster" effect is thought to be due to the initial injection stimulating a delayed immune response.

Interpreting the Results of a Two-Step TST

The interpretation of a two-step TST relies on the size of the induration (in millimeters) measured at the injection site 48-72 hours after each injection. Interpreting the results requires considering several factors including the individual's risk factors for TB, recent exposure, and any prior BCG vaccination.

First Test Negative, Second Test Positive: This is considered a positive result, indicating possible TB infection.
Both Tests Negative: This usually indicates no TB infection.
Both Tests Positive: This is also indicative of possible TB infection. The size of the indurations are taken into account.
First Test Positive, Second Test Negative: This can be interpreted in several ways and needs further evaluation by a healthcare professional.

Important Note: A positive TST does not mean that the individual has active TB disease. It only indicates that they have been exposed to the bacteria and have developed an immune response. Further testing, such as a chest X-ray and sputum culture, is necessary to diagnose active TB.

Limitations of the Two-Step TST

The two-step TST, while valuable, isn't perfect. It's important to acknowledge its limitations:

False Positives: Certain medical conditions and other infections can sometimes cause a false positive reaction.
False Negatives: Individuals with weakened immune systems may not have a detectable response even if they are infected.
BCG Vaccine Interference: The BCG vaccine can lead to both false-positive and false-negative results.

When is a Two-Step TST Used?

Healthcare providers often recommend a two-step TST for individuals who:

Are at high risk for TB exposure (healthcare workers, immigrants from high-TB-prevalence countries).
Have received the BCG vaccine.
Have previously had a negative TST.

Conclusion

The two-step TST is a valuable tool in screening for TB infection, particularly when dealing with individuals who may have had prior exposure or BCG vaccination. However, it is crucial to remember that a positive result does not confirm active TB disease and that further investigation is needed to establish a definitive diagnosis. Always consult with a healthcare professional to interpret the results and determine the appropriate course of action. They can help assess your risk factors and determine the necessary follow-up tests.