Comparison testing of current (PPD-S1) and proposed (PPD-S2) reference tuberculin standards

Area of Science:

• Immunology
• Microbiology
• Public Health

Background:

• The tuberculin purified protein derivative (PPD)-S1 has been the U.S. reference standard for tuberculosis testing since 1951.
• A shortage of PPD-S1 necessitates the development and validation of a new standard, PPD-S2.

Purpose of the Study:

• To evaluate the equivalence of the newly manufactured PPD-S2 to the existing PPD-S1 standard.
• To determine if PPD-S2 can effectively replace PPD-S1 in tuberculosis diagnostics and surveys.

Main Methods:

• A double-blind clinical trial was conducted involving 69 tuberculosis patients and 1,189 low-risk subjects.
• Participants received four skin tests, including PPD-S1 and PPD-S2, with reactions measured by two observers.
• Statistical analysis compared immunogenicity in TB patients and specificity in low-risk individuals.

Main Results:

• PPD-S1 and PPD-S2 demonstrated statistically indistinguishable immunogenicity in tuberculosis patients (mean reaction sizes 15.6 mm vs. 14.8 mm).
• Both PPD-S1 and PPD-S2 exhibited high specificity (98.7% and 98.5%, respectively) in low-risk subjects using a 10-mm cutoff.
• The rate of discordant interpretations for PPD-S2 was low and comparable to inter-observer variability.

Conclusions:

• PPD-S2 is immunogenically equivalent to PPD-S1.
• PPD-S2 demonstrates comparable specificity to PPD-S1 in low-risk populations.
• PPD-S2 is qualified to serve as the new U.S. reference standard for PPD tuberculin, ensuring continuity in tuberculosis testing.