In the pharmaceutical field, calculating aliquots is necessary when compounding medications. Scales have a minimum amount, called the sensitivity, that they can measure accurately. Industry standards dictated by the United States Pharmacopoeia state that scales must have an error rate of 5 per cent or lower. The aliquot method must be used when measuring an amount at or below the sensitivity.

Find the sensitivity point of the balance being used by referring to its manual and dividing by .05. This will give you the minimum weighable quantity or MWQ.

For example if the scale has a sensitivity of 6 mg, than the MWQ would be:

6 mg / .05 = 120 mg

Multiply the needed amount of drug per dosage by a number, or multiplication factor, that allows you to weigh at or above the minimum weighable quantity.

For example if the prescription calls for 10 mg of drug and the patient is to take 1 capsule daily for 6 days you will have to make 6 capsules. Both the 10 mg and the 60 mg--6 x 10mg--are below the MWQ so you can multiply the 10 mg by a multiplication factor of 12,

10 mg X 12 = 120 mg

which is the same as the MWQ and the amount of drug you can accurately weigh out. This will also give you 12 capsules which provides room for spillage and refills.

An inert ingredient called a diluent is added to drugs as a binder, a thinner or a flavour enhancer. Decide how much diluent needs to be added to the drug to fulfil the minimum weighable amount for each dose and then multiply by the multiplication factor. The best way to determine how much diluent to add is to subtract the drug dose from the MWQ,

Example:

If the MWQ is 120 mg and the drug dose is 10 mg,

120 mg - 10 mg = 110 mg

then this is how much diluent you will need for each dose.

Now multiply by the multiplication factor of 12:

110 mg x 12 = 1320 mg

This is the total amount of diluent to be weighed out.

Add the totals from Step 2 and Step 3 and divide by the multiplication factor to determine how much of the mixture to put in each dose. This is referred to as an aliquot.

Example:

120 mg + 1320 mg = 1440 mg

This is the total weight of drug and diluent when mixed together.

Divide by multiplication factor:

1440 mg / 12 = 120 mg

This is the amount of mixture that should be placed in each capsule.

#### Tip

Choose the lowest multiplication factor you can to avoid wastage. Save any remainder mixture for refills if possible. When determining amount of diluent remember that each completed dose must weigh more than the minimum allowable quantity and that the dosage container such as a capsule can only hold a specific amount.

#### Warning

Double check all calculations and weights to prevent over or under doses.