Antibiotic Weighing Calculation (Simplified Version)
1. Objective
Standardize the calculation for weighing antimicrobial salts (powder) to prepare
stock solutions at specific active drug (base) concentrations.
This procedure corrects the mass to be weighed based on the
potency
of the drug, which is the actual amount of active ingredient present in the salt.
2. Key Concepts
-
Active Drug (or Base):
The pure substance with antimicrobial activity (e.g., Ceftazidime). This is the
unit in which we express the desired final concentration (e.g., 2000 µg/mL).
-
Drug Salt (or Powder):
The form in which the antibiotic is commercially supplied (e.g., Ceftazidime
Pentahydrate). This is the powder we
actually
weigh on the balance.
The weight of the salt is not equal to the weight of the active drug.
-
Potency (P):
The correction factor that tells us how much "Active Drug" exists in each
milligram of "Salt".
3. Formula and Calculation Steps
The general formula to find the mass of salt to be weighed is:
Mass to Weigh (mg) = Total Active Mass (µg) / Potency (µg/mg)
Step 1: Calculate Total Active Mass (in µg)
Formula:
Active Mass (µg) = Desired Concentration (µg/mL) × Final Volume (mL)
Step 2: Find Potency (P) (in µg/mg)
Check the label or certificate of your antibiotic.
Step 3: Calculate Mass of Salt to Weigh (in mg)
Formula:
Mass to Weigh (mg) = Active Mass (µg) / Potency (µg/mg)
4. Practical Example
-
Drug:
Kefadim® 1g ABL (Ceftazidime)
-
Info:
1.165 g of ceftazidime pentahydrate (Salt) equals 1 g of ceftazidime (Active)
-
Objective:
Prepare 5 mL of stock solution at 2000 µg/mL
Conclusion:
Weigh 11.655 mg of Kefadim® powder and dissolve in 5 mL of solvent to obtain a
stock solution of 2000 µg/mL.