Rabies vaccines for veterinary use: difficulties in establishing potency acceptability thresholds.

The variability of a control test is sometimes difficult to assess. In fact this variability should be considered as a common basis of any biological test. The example of the NIH or PER test is given for veterinary rabies vaccines. The same acceptability threshold is accepted by everybody, both producers and control laboratory. But biological variability should induce the producers to determine a "working acceptability threshold" higher than the conventional one. This safety margin should be determined by the producer to counterbalance the variability of the test both in the producer and control laboratories so that most batches submitted to the official control are accepted. On the other hand, control laboratories should fulfil two "fair" requirements: reveal their flow charts and their margin of error to allow the producer to make the needed adjustments; reduce as far as possible the uncertainty of control tests by using the most accurate statistical interpretation to allow the producers to establish a "working acceptability threshold" not too much higher than the official one.