tuberculine

Tuberculine is an extract derived from Mycobacterium tuberculosis used to diagnose infection with the tuberculosis complex. The term historically covered a range of tuberculin preparations, but in modern practice it most often refers to purified protein derivative (PPD), the standard reagent used in the tuberculin skin test (TST).

Preparation and administration

Tuberculin reagents are typically administered intradermally. A common modern protocol uses about 0.1 mL of a PPD preparation containing 5 tuberculin units. The injection is made into the forearm, and the test site is examined after 48 to 72 hours. The result is read as the size of the induration (a raised, hardened area), not the surrounding redness.

Interpretation

A positive reaction suggests prior exposure to Mycobacterium tuberculosis or related mycobacteria, including vaccination with BCG in some cases. Whether a result is considered positive depends on the individual’s risk factors and guidelines, with induration thresholds typically ranging from roughly 5 to 15 millimeters. A negative result may occur in people who are immunosuppressed or anergic. False positives can arise from BCG vaccination or environmental mycobacteria; false negatives can occur in recent infection or immune-compromised states.

Uses and limitations

The tuberculin skin test detects latent TB infection but does not distinguish latent infection from active disease. Its specificity is affected by prior BCG vaccination and exposure to non-tuberculous mycobacteria. In some settings, interferon-gamma release assays are used as adjuncts or alternatives to improve specificity.

Other applications

In veterinary medicine, tuberculin tests using bovine or avian tuberculin are employed to screen livestock for tuberculosis, aiding control and eradication programs.

History

Tuberculine was described in the late 19th century by Robert Koch. Modern PPD-based skin testing evolved from those early preparations and remains in use worldwide, subject to local guidelines.