Potency or assay calculation of API is a daily work in pharmaceutical industry. So as a pharmacist the API potency calculation formula must be known as a primary knowledge. Here we try to share the easy potency calculation formula for drugs.


In case of single (01) batch of Raw Materials                                                                                

Quantity of Paracetamol BP    =  Standard quantity of Paracetamol x 100 / Working Assay  (kg)
                                                                             
                                                   =                     (kg)
 

In case of two (02) batches of Raw Materials:                                                                                      

Quantity of Paracetamol BP     =  Standard quantity of Paracetamol x 100 / Working Assay of (Batch – A)  =        kg (X)
 
 
 Practical quantity of Paracetamol BP used from (Batch – A)  =        kg. (Y)
                                                                                                            
 Quantity of Paracetamol BP required from (Batch – B) =  {(X) – (Y)} × Working Assay of (Batch−A) /  Working Assay of (Batch – B) kg                                                                 
                                                 =          kg
 
      
Where, 
  • Batch−A = First Batch (according to the Batch No.) of Paracetamol BP in case of two batches.
  • Batch−B = Second Batch (according to the Batch No.) of Paracetamol BP in case of two batches.


Assay calculation (As is Basis, Dried Basis & Anhydrous Basis):

Potency/Assay on as is basis = Sample area x standard concentration/ Standard area x sample concentration x potency or Assay of Standard
or
Standard Quantity of API ×  100/(Assay on as is basis)

Assay on dried basis = Standard Quantity of API×(Assay on as is basis ×100)/((100-LOD))

Assay on anhydrous basis = Standard Quantity of API×(Assay on as is basis ×100)/((100-Water Content))

Working standard potency calculation

The primary reference standard is the one highly purified, including preparative HPLC and available in small quantities. Practically it is not possible to get potency greater than 100 for any standard when you are qualified it against official standard. If you are getting such results like 101%, we can consider it analytical deviation and we can use potency as 100%. But if you are getting potency like 102%, then it is very necessary to confirm the things with high degree of investigation.


How to calculate the potency of antibiotic supplied as a salt?

Formula to calculate the potency of antibiotic is shown as: Potency = (Assay purity) * (Active fraction) * (1-Water Content).

Difference between Assay and Potency

  • Assay is an analytical technique to determine the quantity of the drug product. 
  • Potency is how many things do you need to use to get the desired effect.

  • Assay is for evaluation of percent of purity of action or its pharmacological effects on biologicals.
  • Whereas potency is the amount of a drug required to get an effect at its maximum intensity.


Example

Assay Calculation for UV Spectroscopy Method




Assay Calculation for HPLC Method




Dye Content Calculation






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