The
Tuberculin Skin Test (TST) is a diagnostic tool used to detect exposure to
Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). It is particularly important in Pediatrics to identify children who may have been exposed to TB, as they are at higher risk for developing severe forms of the disease.
The TST involves the intradermal injection of 0.1 mL of purified protein derivative (
PPD) into the inner surface of the forearm. A small, pale elevation of the skin (a wheal) should appear, indicating correct administration. The test site is then evaluated 48 to 72 hours later for the presence of induration (a raised, hardened area).
In Pediatrics, the TST is especially recommended for:
Children who have had close contact with individuals with active TB.
Children born in or who frequently travel to countries with high TB prevalence.
Children with compromised immune systems, such as those with
HIV infection.
Children who are showing symptoms suggestive of TB, such as a prolonged cough or unexplained weight loss.
The TST is read by measuring the diameter of the induration (not erythema) in millimeters using a ruler. The interpretation depends on the size of the induration and the child's risk factors:
≥5 mm: Positive in children with recent close contact with TB cases, immunocompromised children, or children with abnormal chest X-rays consistent with TB.
≥10 mm: Positive in children under 5 years old, children with medical risk factors, or children from high-prevalence countries.
≥15 mm: Positive in children 5 years and older without any risk factors.
The TST has several limitations:
False-positive results can occur, especially in children who have received the
BCG vaccine or have been exposed to non-tuberculous mycobacteria.
False-negative results may occur in children with severe TB, recent TB exposure (within 8-10 weeks), or those who are immunocompromised.
It requires a follow-up visit for reading the result, which can be challenging in certain settings.
Interferon-gamma release assays (IGRAs) are blood tests that can also be used to detect TB infection. They have the advantage of not being affected by prior BCG vaccination and require only one visit to obtain results. However, they are more expensive and may not be as widely available.
A positive TST should prompt further evaluation, which may include:
A thorough clinical examination and medical history review.
A chest
X-ray to look for evidence of active TB disease.
Additional tests such as sputum cultures or gastric aspirates, especially in symptomatic children.
Children with latent TB infection (LTBI) but no signs of active disease may be treated with
preventive therapy to reduce the risk of developing active TB in the future.
Conclusion
The Tuberculin Skin Test remains a crucial tool in the early detection of TB exposure in children. Despite its limitations, it helps identify those at risk and ensures timely intervention. Understanding the proper administration, interpretation, and follow-up of the TST is essential for healthcare providers working in Pediatrics.
