Design and Analysis Of Trials For A New Tuberculin Standard

Introduction
Potencies of tuberculin batches need to be assessed relative to the Bovine International Standard (BIS, potency defined as 32,500 IU/ml), produced in the 1970’s and recognised as the international standard in 1986 by the WHO. Currently both BIS stocks and quality are diminishing. Therefore, a new bovine standard is required which should be of comparable potency to BIS while its estimated potency must be highly precise.
Materials and Methods
A trial was designed using one concentration (100μl of 1mg PPD/ml) of BIS and of a ‘working standard’ (WS) which each was applied 4 times to the same animal. The number of bovines to be included in the trial was simulated using varying estimates of between- and within-bovine variation in skin response. The
trial was conducted with naturally infected steers, showing a positive skin response to SICTT and a positive result to Gamma Interferon assay, selected from herds in which infection was confirmed. Results were analysed using GLMM with standard (BIS, WS) as fixed effect and animal as random effect.
Results
Sample size was set to 30 animals assuming a between-bovine variation of 3.0mm and a within-bovine variation of 2.0mm. Then the 95% CI of the potency estimate is ± 3000IU. In the trial itself, the between and within-bovine variations were 2.4 and 1.3 respectively, resulting in a potency estimate of 34,700 ±
1,400IU/ml for WS in bovines.
Conclusion
Potency estimate of WS and its 95%CI indicate that WS is a good candidate to replace BIS.