PTCB Certified Pharmacy Technician (PTCE)MedicationsMedium
A pharmacy technician is asked to prepare a non-sterile compound that requires the use of a Class A torsion balance. The formula calls for 15 mg of a potent active pharmaceutical ingredient (API). The minimum weighable quantity (MWQ) for the balance is 120 mg. Which of the following is the most appropriate action for the technician?
- AWeigh the 15 mg directly on the Class A balance, as it is a potent ingredient.
- BUse a digital analytical balance instead, as it is more accurate for small quantities.
- CRound up the required quantity to 120 mg to meet the MWQ.
- DPerform an aliquot weighing to obtain the required 15 mg.
Show answer & explanationAnswer & explanation
Correct answer: D. Perform an aliquot weighing to obtain the required 15 mg.
Since 15 mg is below the minimum weighable quantity (MWQ) of 120 mg for the Class A torsion balance, direct weighing would lead to an unacceptable error. The most appropriate action is to use aliquot weighing, which involves weighing a larger, diluting, and then taking a portion (aliquot) that contains the desired amount of the API.
Why the other options are wrong
- A. Directly weighing less than the MWQ results in significant error and is unsafe.
- B. While a digital balance might be more accurate, the question specifies using a Class A torsion balance, and aliquot method is designed for this scenario.
- C. Rounding up to 120 mg would result in a significantly incorrect dose and is dangerous.
Aliquot Weighing
A technique used to accurately measure quantities of an ingredient that are below the minimum weighable quantity (MWQ) of a balance.
- Involves weighing a larger amount of the ingredient mixed with an inert diluent.
- A portion (aliquot) of this mixture is then taken to provide the desired amount of the ingredient.
- Ensures accuracy when using balances with specific MWQs.
Memory trick: Small Amounts Need Aliquot Accuracy