Yes, a 50% error due to measurement. The next question is how much should we weigh such that our percent of error falls within the acceptable parameters in health care (ie, 1%-5%)? This question addresses the least weighable amount that the balance can measure. To solve

Therefore, a Class A balance with a sensitivity requirement of 6 mg can be used to weigh a minimum of 120 mg with a 5% error. A 1% error would require you to weigh at least 600 mg on the same balance (sensitivity of 6 mg).

Aliquots

Pharmacists may be required to prepare dosage units of 120 mg or less (ie, less than the least weighable amount). This is accomplished using the aliquot method of preparation. This requires the pharmacist to weigh no less than the least weighable amount of the medication in question. So if a prescription called for a 2 mg dose, 120 mg would be weighed out which is 60 times the amount needed for the dose. (This is n times the amount of drug needed.)

Next, the active drug is diluted with excipient (usually lactose) such that taking 1/nth of the drug/excipient mixture (in this case 1/60th) would be greater than or equal to the least weighable amount and would contain the amount of drug originally called for.

In this case, 125 mg is selected (yes, out of the blue) as the amount to weigh of final mixture. We need n × 125 mg of final mixture (in this case, 60 × 125 mg = 7500 mg). The amount of drug used is subtracted from the final mixture to yield the amount of excipient needed (7500 mg – 120 mg = 7380 mg lactose). Then 1/nth. of the final mixture is the aliquot needed of the drug. In this case 120 mg was mixed into lactose for a total mixture of 7500 mg. 120/60 = 2 mg active drug; then 7500/60 = 125 mg mixture containing 2 mg of the active drug.

Practice Problems

1. What is the least weighable amount of a Class A balance with a sensitivity requirement of 4 mg and no more than a 5% error?

Answer

or

2. What is the percent error of weighing 85 mg on a Class A balance with a sensitivity requirement of 8 mg?

Answer

3. Assuming that the least weighable amount is 150 mg, what is the sensitivity requirement of a Class A balance with an error rate of 5%?

Answer