Qualification of Working Standards, Assigning potency of Working standards
When Qualifying a working standard (secondary standard) against Pharmacopeia standard by six replcate assay determination, the average assay was above 100% (say 100.3%). But the batch of API has say 0.3% related impurities and 0.21% residue on Ignition. Is it acceptable to consider the Potency for working standard as 100%, since the replicate assay against Pharmacopeia standard is 100.3%
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Of course not, it is not correct to assign potency of Working standard as 100%, based on replicate assay results when it has impurities present. A working standard (secondary standard) with organic and inorganic impurities present cannot have potency of 100%. The best way to qualify a working standard is following the mass balance approach. Potency = 100-(%Inorganic impurities+%Organic Impurities+%Water%+%Organic Volaties (Residual Solvets)]. This addresses the issue of assigning potency for the working standard, when impurities are present. The assay against Pharmacopeia standard shall be one acceptance criteria for the working standard (it should comply as per specification, when tested against current lot of official Pharmacopeia standard).
Please also refer following for the Mass balance approach for certifying working standards: