B. Interferon-γ – Explanation
Tuberculin skin tests are an example of type IV (delayed) hypersensitivity reactions. These are
largely mediated by interferon-γ secreted by Th1 cells which in turn stimulates macrophage activity.
Tuberculosis: screening
The Mantoux test is the main technique used to screen for latent tuberculosis. In recent years the
interferongamma blood test has also been introduced. It is used in a number of specific situations
such as:
- the Mantoux test is positive or equivocal
- people where a tuberculin test may be falsely negative (see below)
Mantoux test
- 0.1 ml of 1:1,000 purified protein derivative (PPD) injected intradermally
- result read 2-3 days later
| Diameter of induration |
Positivity | Interpretation |
| < 6mm | Negative – no significant hypersensitivity to tuberculin
protein |
Previously unvaccinated individuals may be given the BCG |
| 6 – 15mm | Positive – hypersensitive to tuberculin protein | Should not be given BCG. May be due to previous
TB infection or BCG |
| > 15mm | Strongly positive – strongly hypersensitive to tuberculin
protein |
Suggests tuberculosis infection |
False negative tests may be caused by:
- miliary TB
- sarcoidosis
- HIV
- lymphoma
- very young age (e.g. < 6 months)
Heaf test
The Heaf test was previously used in the UK but has been since been discontinued. It involved
injection of PPD equivalent to 100,000 units per ml to the skin over the flexor surface of the left
forearm. It was then read 3-10 days later.
