Tuberculin skin tests
Introduction
The Tuberculin skin test (TST), also known as the Mantoux test, is a diagnostic tool used to detect Mycobacterium tuberculosis infection. It is a widely used method for screening for tuberculosis (TB) and is particularly important in identifying latent TB infections. The test involves the intradermal injection of purified protein derivative (PPD) and the subsequent measurement of the induration (swelling) at the injection site after 48 to 72 hours. This article delves into the history, methodology, interpretation, and clinical significance of the tuberculin skin test.
History
The development of the tuberculin skin test dates back to the late 19th century. The test was first conceptualized by Robert Koch, who discovered the tubercle bacillus in 1882. Koch's initial attempts involved using tuberculin, a protein derivative of the bacterium, as a therapeutic agent. However, it was Charles Mantoux, a French physician, who refined the test in 1907 by introducing the intradermal injection technique, which significantly improved its diagnostic accuracy.
Methodology
Preparation
The tuberculin skin test requires the use of purified protein derivative (PPD), which is a standardized antigen preparation. PPD is derived from cultures of Mycobacterium tuberculosis and is used to elicit a delayed-type hypersensitivity reaction in individuals previously sensitized to the bacterium. The standard dose for the test is 0.1 mL of PPD, containing 5 tuberculin units (TU).
Administration
The test is administered by injecting PPD intradermally into the volar aspect of the forearm. The injection should produce a small, pale wheal of 6 to 10 mm in diameter. Proper technique is crucial to ensure accurate results, as subcutaneous injection can lead to false-negative results.
Interpretation
The interpretation of the tuberculin skin test is based on the size of the induration measured in millimeters at the site of injection, not the erythema (redness). The reading is typically done 48 to 72 hours post-injection.
Induration Measurement
The induration is measured transversely to the long axis of the forearm using a flexible ruler. The measurement should be recorded in millimeters, and the absence of induration is recorded as 0 mm.
Criteria for Positive Test
The criteria for a positive TST result vary depending on the individual's risk factors and history of exposure to tuberculosis. General guidelines include:
- **5 mm or more**: Considered positive in individuals with HIV infection, recent contacts of TB cases, individuals with fibrotic changes on chest radiograph consistent with prior TB, and organ transplant recipients.
- **10 mm or more**: Considered positive in recent immigrants from high-prevalence countries, injection drug users, residents and employees of high-risk congregate settings, and children under 4 years of age.
- **15 mm or more**: Considered positive in individuals with no known risk factors for TB.
Clinical Significance
The tuberculin skin test is a critical tool in the diagnosis and management of tuberculosis. It is used to identify individuals with latent TB infection who are at risk of developing active disease. The test is particularly valuable in high-risk populations, such as healthcare workers, individuals with compromised immune systems, and those living in crowded conditions.
Limitations
Despite its widespread use, the TST has several limitations. False-positive results can occur in individuals who have received the Bacillus Calmette-Guérin (BCG) vaccine or have been exposed to non-tuberculous mycobacteria. Conversely, false-negative results may occur in individuals with weakened immune systems, such as those with HIV/AIDS or malnutrition.
Alternatives
In recent years, Interferon-gamma release assays (IGRAs) have emerged as an alternative to the tuberculin skin test. IGRAs are blood tests that measure the immune response to specific TB antigens and are not affected by prior BCG vaccination. However, they are more expensive and require laboratory facilities.
Conclusion
The tuberculin skin test remains a cornerstone in the global effort to control tuberculosis. Its simplicity, low cost, and effectiveness in identifying latent TB infections make it an indispensable tool in public health. However, the limitations of the test necessitate careful interpretation and, in some cases, the use of supplementary diagnostic methods.