Commercially produced tuberculin should be bio-equivalent with 5 TU PPD-S.  One of the most widely used commercial tuberculins is PPD RT23.  A large batch of this tuberculin was produced by the Danish Statens Serum Institut in Copenhagen upon a request by UNICEF in 1955.

During standardizations it was noted that tuberculin could be stabilized by adding an agent.  This increased the potency of tuberculin and the amount required to induce the same reaction as 5 TU PPD-S had to be reduced.  The above study demonstrated that 2 TU Tuberculin PPD RT23 were bioequivalent with 5 TU Tuberculin PPD-S.

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