Montoux Test (Tuberculin Skin Test)

The Montoux Test, also known as the Tuberculin Skin Test (TST), is a diagnostic tool used to determine whether a person has been infected with Mycobacterium tuberculosis, the bacteria that cause tuberculosis (TB).

Clinical Significance

Diagnosis of Latent TB Infection (LTBI): Helps identify individuals who are infected with TB bacteria but do not have active TB disease.

Screening Tool: Used for screening high-risk populations such as healthcare workers, people with weakened immune systems, and individuals who have been in contact with TB patients.

Public Health Surveillance: Assists in controlling and preventing TB by identifying and treating latent infections before they progress to active disease.

Principle

The test involves the intradermal injection of a purified protein derivative (PPD) of the TB bacterium. If the person has been exposed to TB bacteria, their immune system will recognize the PPD and mount a localized reaction at the site of injection. This reaction is measured 48-72 hours after injection to determine if it is positive or negative.

Requirements

- Tuberculin PPD: Standardized dose of 5 tuberculin units (TU) in 0.1 mL solution.

- Syringe and Needle: A 1 mL syringe with a short (26 or 27 gauge) needle.

- Skin Marker: To mark the injection site.

- Ruler or Calipers: For measuring the induration.

- Antiseptic Solution: To clean the injection site.

Procedure

1. Patient Preparation:

   - Explain the procedure to the patient.

   - Confirm there are no contraindications, such as a history of severe reaction to a previous TST.

2. Injection:

   - Clean the inner surface of the forearm with an antiseptic.

   - Draw 0.1 mL of tuberculin PPD into the syringe.

   - Stretch the skin taut and insert the needle intradermally, bevel up, at a 5-15 degree angle.

   - Inject the PPD to form a pale, raised wheal (6-10 mm in diameter) under the skin.

3. Post-Injection Care:

   - Advise the patient not to cover the site with a bandage and not to scratch or rub the area.

   - Instruct the patient to return after 48-72 hours for the test reading.

4. Reading the Test:

   - Inspect the injection site in good light.

   - Palpate the site to identify the induration (raised, hardened area).

   - Measure the diameter of the induration across the forearm (perpendicular to the long axis) using a ruler or calipers.

   - Record the size of the induration in millimeters.

Interpretation

- 0-4 mm Induration: Generally considered negative, although in certain circumstances, such as recent exposure, this might require further evaluation.

- 5-9 mm Induration: Positive in individuals with recent TB contacts, HIV infection, chest radiograph suggestive of prior TB, or other immunocompromising conditions.

- 10-14 mm Induration: Positive in recent immigrants from high-prevalence countries, injection drug users, children under 4, or those with certain medical conditions.

- 15 mm or Greater Induration: Positive in all individuals, regardless of risk factors.

A positive test indicates TB infection but does not distinguish between latent infection and active TB disease. Further tests, such as a chest X-ray and sputum analysis, are needed to confirm active TB.

Quality Control (QC)

- Storage of PPD: Store tuberculin PPD in a refrigerator (2-8°C) and avoid exposure to light and heat.

- Calibration of Equipment: Ensure syringes and calipers are accurate and properly calibrated.

- Training: Staff performing the test should be trained and proficient in the technique and interpretation.

- Documentation: Keep accurate records of test administration and results.

- Control Testing: Periodically use control subjects with known positive and negative reactions to ensure consistency and accuracy in readings.

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