TB skin tests

Tuberculin Skin Test
Tuberculin_skin_test.jpg
Synonyms	Mantoux test, PPD test
Pronounce	N/A
Specialty	Infectious disease
Symptoms	Induration at the site of injection
Complications	False positives, false negatives
Onset	48 to 72 hours after administration
Duration	Induration can last for several days
Types	N/A
Causes	Infection with Mycobacterium tuberculosis
Risks	Recent exposure to TB, immunocompromised state
Diagnosis	Based on size of induration
Differential diagnosis	N/A
Prevention	N/A
Treatment	Further testing if positive
Medication	N/A
Prognosis	N/A
Frequency	N/A
Deaths	N/A

The Tuberculin Skin Test (TST), also known as the Mantoux test or PPD test, is a diagnostic tool for tuberculosis (TB). It is used to determine if a person has been exposed to the Mycobacterium tuberculosis bacteria.

History

The tuberculin skin test was developed by Charles Mantoux in 1907, building on the work of Robert Koch, who discovered the tuberculin protein. The test has been widely used since the early 20th century as a method to screen for tuberculosis infection.

Procedure

The test involves the intradermal injection of 0.1 mL of purified protein derivative (PPD) into the inner surface of the forearm. The injection should be made with a 27-gauge needle, and the bevel should be facing upward. A small, pale elevation of the skin (a wheal) should appear, measuring 6 to 10 mm in diameter.

Reading the Test

The test is read 48 to 72 hours after administration. The result is based on the size of the induration (swelling) at the site of injection, not the redness. The induration is measured in millimeters using a ruler.

Interpretation

The interpretation of the TST depends on the size of the induration and the individual's risk factors for TB. The following guidelines are commonly used:

• ≥5 mm is considered positive in:

 * HIV-infected persons
 * Recent contacts of TB cases
 * Persons with fibrotic changes on chest radiograph consistent with prior TB
 * Organ transplant recipients
 * Immunosuppressed patients

• ≥10 mm is considered positive in:

 * Recent immigrants from high-prevalence countries
 * Injection drug users
 * Residents and employees of high-risk congregate settings
 * Mycobacteriology laboratory personnel
 * Persons with clinical conditions that place them at high risk
 * Children under 4 years of age

• ≥15 mm is considered positive in:

 * Persons with no known risk factors for TB

Limitations

The TST has several limitations. It can produce false-positive results in individuals who have been vaccinated with the Bacillus Calmette-Guérin (BCG) vaccine or who have been exposed to non-tuberculous mycobacteria. False-negative results can occur in individuals who are immunocompromised or have recently been infected with TB.

Alternatives

An alternative to the TST is the Interferon Gamma Release Assay (IGRA), which is a blood test that measures the immune response to specific TB antigens. IGRAs are not affected by prior BCG vaccination and have fewer false positives.

Complications

Complications from the TST are rare but can include severe local reactions, such as blistering or ulceration, particularly in individuals with a strong hypersensitivity to tuberculin.

Follow-up

A positive TST should be followed by further evaluation, including a chest X-ray and clinical assessment, to determine if the individual has active TB disease or latent TB infection.

See Also

• Tuberculosis
• Interferon Gamma Release Assay
• Bacillus Calmette-Guérin

External Links

• [CDC Tuberculosis Testing Guidelines](https://www.cdc.gov/tb/topic/testing/default.htm)