3.8 Tuberculin skin test (TST)


Cutaneous hypersensitivity to tuberculin reflects a delayed hypersensitivity reaction to some M. tuberculosis antigens.

A positive reaction signifies that an infection has occurred, but it does not determine if the TB is latent or active. It does not differentiate between infection by M. tuberculosis and hypersensitivity due to mycobacterium other than TB.

The TST is done by injecting 5 international units of tuberculin intradermally on the ventral surface (side of arm exposed with palm facing up) of the forearm.

The test is read by a trained health care worker, 48 to 72 hours after the injection. The reaction is the area of induration (swelling that can be felt) around the injection. The diameter of induration is measured with a ruler in millimetres across the forearm. The erythema (redness) around the indurated area is not measured, because the presence of redness does not indicate a reaction.

Tuberculin used for the skin test is also known as purified protein derivative, PPD. The TST is sometimes called PPD test or Mantoux test.

BCG vaccination induces a state of hypersensitivity which can result in a false positive TST, such that the average diameter 1 year after BCG vaccination is 10 mm, with extremes ranging from 4 to 20 mm. A false positive TST due to a vaccine reaction has a tendency to be less reactive with time and disappears 5 to 10 years post-vaccination.

Positive TST

A TST is considered as positive if28:
– Induration is ≥ 5 mm in HIV infected individuals, immunocompromised patients, including those receiving prednisolone therapy of ≥ 15 mg/day for ≥ 1 month, and malnourished children;
– Induration is ≥ 10 mm in all other adults or children (BCG vaccinated or not).

A reaction that appears several minutes or several hours after injection (occasionally even after 24 hours) but which disappears on the day after its appearance is of no value.

In practice, TST has little value as a diagnostic tool when the annual rate of infection and BCG vaccine coverage are high. It can only be used as an element among a body of arguments to establish the diagnosis of active TB, and it is usually only used to help with the diagnosis in children (Chapter 5).

A highly positive (induration diameter > 20 mm) or phlyctenular reaction should be considered as an argument in favour of active TB, but insufficient in itself for deciding on treatment.

Negative TST

Negative reactions in patients that previously presented positive reactions signify a loss of hypersensitivity. These are considered false negative reactions and can be observed:

Temporally:
– In viral (influenza, measles) or bacterial (whooping cough) infections;
– At the start of the evolution of TB meningitis or miliary TB;
– In patients in poor general condition (e.g. malnutrition);
– During immunosuppressive treatment (e.g. corticosteroids).

Permanently:
– With natural extinction of post-vaccination reaction, observed from the fifth year that follows BCG;
– In a person with a weak immune response, such in very elderly persons;
– In persons with diseases that result in anergy: AIDS, haemopathies, sarcoidosis.

Approximately 30% of children with active TB have negative or doubtful TST when diagnosed.

TST has an essential role in identifying candidates for isoniazid prophylaxis therapy, see Chapter 16.