how to read a tb mantoux test

3 min read 17-12-2024

The Mantoux tuberculin skin test (TST), also known as the PPD (purified protein derivative) test, is a crucial tool in detecting latent tuberculosis (TB) infection. Accurate interpretation of the test results is vital for appropriate diagnosis and management. This guide provides healthcare professionals with a comprehensive overview of how to read a Mantoux test.

Understanding the Mantoux Test

The Mantoux test involves injecting a small amount of tuberculin into the forearm. A positive reaction indicates exposure to *Mycobacterium tuberculosis*, but not necessarily active TB disease. A negative result suggests no exposure or a lack of a detectable immune response.

It's crucial to remember the Mantoux test only measures exposure, not active disease. Further investigations are needed to confirm an active TB infection.

Reading the Mantoux Test: A Step-by-Step Guide

1. Timing is Crucial

The Mantoux test is read 48-72 hours after injection. Readings taken outside this timeframe may be inaccurate.

2. Assessing the Reaction

Induration (hardening of the skin), not erythema (redness), is the key indicator. Measure the induration in millimeters (mm) using a ruler, measuring across the widest diameter, perpendicular to the injection site. Only measure the raised, hardened area, not the surrounding redness.

Gently palpate the area to distinguish between induration and erythema.

3. Interpreting the Results

Interpretation depends on several factors, including the individual's risk factors for TB infection, their immune status, and the specific guidelines used in your region. Generally, interpretations fall into these categories:

Interpreting Results Based on Induration (mm)

These guidelines are general. Consult your local guidelines and consider individual risk factors.

≤4 mm: Generally considered negative. However, this could be positive in high-risk individuals (e.g., recent immigrants from high-prevalence countries, individuals with HIV).
5-9 mm: Considered indeterminate or weakly positive. Further investigation may be needed depending on risk factors.
≥10 mm: Generally considered positive, indicating probable exposure to *M. tuberculosis*. Further investigation is crucial.
≥5 mm: Considered positive in high-risk individuals (e.g., those with HIV, recent close contacts of TB patients, immunocompromised individuals, patients with abnormal chest X-rays suggestive of TB).

Factors Affecting Interpretation

Prior BCG Vaccination: Bacillus Calmette-Guérin (BCG) vaccination can cause a positive reaction, complicating interpretation. Consider the individual's vaccination history.
Immune Status: Immunocompromised individuals may show a weaker or delayed response.
Previous TB Infection: Individuals with prior TB infection may show a larger reaction.
Improper Test Administration: Incorrect injection technique can lead to inaccurate results.

4. Documentation

Thoroughly document the test results, including the date of injection, date of reading, the size of induration in millimeters, and any relevant clinical information, such as risk factors and BCG vaccination history.

Further Investigations

A positive or indeterminate Mantoux test does *not* confirm active TB disease. Further diagnostic testing, such as chest X-rays, sputum cultures, and other laboratory tests, are necessary to confirm the diagnosis and assess the extent of the infection.

Conclusion

Accurate interpretation of the Mantoux test requires careful attention to detail, adherence to established guidelines, and consideration of individual risk factors. Remember, this test only indicates exposure, not active disease. Always consult with a physician to determine the appropriate course of action based on the test results and the patient's clinical presentation.

This information is for educational purposes and should not be considered medical advice. Always refer to the most up-to-date guidelines and consult with medical professionals for specific cases. Proper training is essential for accurate Mantoux test interpretation.