T spot in diagnosis of TB

An alternative to the tuberculin skin test (TST) e.g. Mantoux test has emerged in the form of a new type of in-vitro T-cell-based assay: the interferon-gamma assay

• interferon-gamma assays are based on the principle that T cells of individuals sensitised with tuberculosis antigens produce interferon gamma when they re-encounter mycobacterial antigens
  • a high level of interferon-gamma production, therefore, is presumed to be indicative of tuberculosis infection
    • assays use antigens specific to M tuberculosis, such as the early secretory antigenic target 6 (ESAT6) and culture filtrate protein 10 (CFP10)
      • these proteins, encoded by genes located within the region of difference 1 (RD1) of the M tuberculosis genome, are not shared with any BCG substrains or most non tuberculosis mycobacterium (NTM) species (with the exception of Mycobacterium kansasii, Mycobacterium marinum, and Mycobacterium szulgai)
      • because these antigens are not present in most non-tuberculous mycobacteria or in any BCG vaccine variant, these tests can distinguish latent tuberculosis infection

NICE have given guidance of the use of interferon gamma in the diagnosis of latent tuberculosis (1)

NICE define a Dual strategy in the diagnosis of TB as:

• a Mantoux test followed by an interferon-gamma test if the Mantoux is positive

Reference:

• 1) NICE (March 2011) Tuberculosis: clinical diagnosis and management of tuberculosis, and measures for its prevention and control.