Appendix 9. Tuberculin skin test

Update: January 2022

9.1 Introduction

A delayed hypersensitivity reaction occurs after an intradermal injection of tuberculin (tuberculin skin test, TST) in persons infected by M. tuberculosis or vaccinated with BCG.

The test is performed by injecting 5 IU of tuberculin (purified protein derivative, PPD) intradermally on the ventral surface of the forearm (side of forearm exposed with palm facing up) a Citation a. For more information on injection technique: WHO operational handbook on tuberculosis. Module 1: prevention - tuberculosis preventive treatment. Geneva: World Health Organization; 2020.
https://apps.who.int/iris/rest/bitstreams/1272664/retrieve .

The test, which should be performed by a trained healthcare worker, requires 2 visits. The reading is done on the second visit, 48 to 72 hours after the tuberculin injection.
If the patient does not return within 72 hours, another TST should be performed.

The result is determined by the diameter of the reaction and individual characteristics of the person being tested. It should be recorded in millimetres, not as "positive" or "negative".

The reaction is the area of induration (swelling that can be felt) around the injection site.
Using a ruler, the diameter of induration is measured transversely. The erythema (redness) around the indurated area is not the reaction and should not be measured.
A reaction that appears several minutes, hours or even 24 hours after injection, but disappears on the day after its appearance, is of no significance.

There is no correlation between the diameter of the induration and:

• likelihood of active TB,
• risk of developing active TB,  
• protection against TB disease in vaccinated people.

9.2 Positive TST

A positive TST result (table below) signifies that a M. tuberculosis infection has occurred.
However, TST cannot differentiate between active and latent infection.
A positive test supports the diagnosis of latent TB infection (LTBI) when other diagnostic tools have been used to rule out active TB.
In children, a positive TST may be one element among many to establish the diagnosis of active TB.

A reaction highly positive (induration diameter > 20 mm) or phlyctenular (with vesicle) should be considered as an argument in favour of active TB but is not enough to decide on treatment.

Some persons may have a positive TST result even if they have not been infected with M. tuberculosis. Causes of false positive results include:

• Errors in tuberculin administration
• Previous BCG vaccination
• Infection with non-tuberculosis mycobacteria
• Low specificity of TST

BCG is given at birth so previous BCG vaccination has limited impact on the interpretation of TST results, except in small children. The average diameter of the TST reaction 1 year after BCG vaccination is 10 mm, with extremes ranging from 4 to 20 mm. The reaction becomes weaker over time and disappears 5 to 10 years post-vaccination.

9.3 Negative TST

Usually, a negative TST result signifies that no M. tuberculosis infection has occurred. However, a negative TST result does not rule out TB infection. Causes of false negative results include:

• Errors in tuberculin administration
• Recent viral illness or live virus vaccination (e.g. measles)
• Severe TB disease (e.g. TB meningitis or miliary TB)
• Recent (< 12 weeks) or very old (many years) TB infection
• Immunodepression or a weak immune response (e.g. the very elderly, children < 5 years, malnutrition, patients taking corticosteroids or immunosuppressants)
• Persons with diseases that result in anergy (e.g. AIDS, haemopathy, sarcoidosis)
• Natural extinction of post-vaccination reaction from the 5^th year following BCG