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There are several methods for preparing the 'dilutions' of the vaccine
for the potency test.
The first (favoured for many years by the British) involves making a dilution series of the vaccine with adjuvant. Because only the antigen is diluted, the dose response line is shallow.
The second (favoured for many years by the Europeans and until recently the official test of the European Pharmacopoeia) involves diluting the vaccine with an inert diluent. Because both the antigen and adjuvant are diluted the dose response line is steep.
The third (now the official test of the European Pharmacopoeia) involves administration of different volumes of vaccine, thereby achieving the same objective as the inert diluent method.
Ahl has shown that the relationship between the two basic methods is approximately a square root function. That is:
The official pass mark of 3 PD50 according to the EP is equivalent to
9 PD50 by the British method.
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