Tuberculin Skin Test

Introduction

The Mantoux tuberculin skin test (TST) is the standard method of determining whether a person is infected with Mycobacterium tuberculosis. Reliable administration and reading of the TST requires standardization of procedures, training, supervision, and practice. The diagnosis of tuberculosis in children by demonstration of the bacilli is difficult as the disease is usually paucibacillary. Hence indirect evidence of tests like the Tuberculin test becomes all the more important.

History

Tuberculin has come a long way from Koch’s Old Tuberculin (OT) in the 1890s to the current PPDs in use. The Purified Protein Derivative (PPD) was initially derived by TCA precipitation of culture filtrates by Florence Siebert in 1941. That was accepted by the WHO as the standard preparation PPD-S (PPD-Standard). Another preparation of PPD was made by Siebert at the Statens Serum Institute, Copenhagen in 1958, the PPD-RT23. This was accepted as the international standard. Later a detergent, Tween 80 was added to the PPD to prevent its adsorption to glass vials & syringes. Hence, currently, there are only two standard preparations of PPD: PPD-S and PPD RT23. 1 tuberculin unit (TU) of PPD RT23 is equivalent to 2.5 TU of PPD-S. In India as PPD RT 23 is available, 2 TU of PPD RT23 should be used, as an equivalent to 5 TU of PPD-S, as per international recommendations.