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Today’s Theme: Minimum Accurate Weighable Quantity (MAWQ)

In compounding, getting the calculation right is only part of the job.

You also have to be able to measure and weigh the calculated amounts of ingredients accurately.

Balances have limits. If the quantity of an ingredient is too small relative to the sensitivity of the balance, the potential percentage error becomes too large.

The minimum accurate weighable quantity (MAWQ) is the smallest quantity that can be weighed on a balance while staying within a specified percentage error.

Two relationships to know

The sensitivity requirement (SR) describes the smallest mass that produces a measurable response on a balance. Traditional pharmacy calculations commonly use an SR of 6 mg for a Class III prescription balance.

Minimum accurate weighable quantity:

MAWQ = (Sensitivity Requirement × 100) ÷ Allowable % Error

Maximum potential % error:

Error = (Sensitivity Requirement ÷ Quantity Weighed) × 100

Remember

The smaller the allowable percentage error, the larger the minimum amount that must be weighed.

And if the amount of drug required is below the MAWQ, simply placing that amount on the balance does not solve the problem.

You may need to use another technique, such as the aliquot method, to measure the required amount accurately.

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Calculation Corner: MAWQ

Example 1: Finding the Minimum Accurate Weighable Quantity

A balance has a sensitivity requirement of 6 mg.

The maximum allowable error is 5%.

What is the minimum accurate weighable quantity?

Step 1: Identify the relationship

MAWQ = (Sensitivity Requirement × 100) ÷ Allowable % Error

Step 2: Substitute the values

MAWQ = (6 mg × 100) ÷ 5

MAWQ = 600 mg ÷ 5

Answer: 120 mg

At least 120 mg should be weighed if the potential error is to remain at or below 5%.

Why this matters

Suppose a compounded prescription requires only 30 mg of an active ingredient.

The calculation may correctly tell you that 30 mg is needed.

But if the MAWQ of the balance is 120 mg, weighing 30 mg directly would exceed the desired accuracy limit.

The next question becomes:

How can you accurately obtain 30 mg without directly weighing only 30 mg?

That is where techniques such as the aliquot method become important.

Example 2: Finding the Maximum Potential Error

A balance has a sensitivity requirement of 6 mg.

A student weighs 150 mg of powder.

What is the maximum potential error?

Step 1: Identify the relationship

% Error = (Sensitivity Requirement ÷ Quantity Weighed) × 100

Step 2: Substitute the values

% Error = (6 mg ÷ 150 mg) × 100

% Error = 0.04 × 100

Answer: 4%

Try It Yourself

Try this before looking below.

A balance has a sensitivity requirement of 6 mg.

The maximum allowable error is 5%.

A compounded preparation requires 40 mg of an active ingredient.

• What is the MAWQ of the balance?
• Can 40 mg be weighed directly while remaining within the 5% error limit?
• What type of technique could be considered if an accurately measurable quantity smaller than the MAWQ is required?

Answer

Step 1: Calculate the MAWQ

MAWQ = (6 mg × 100) ÷ 5

MAWQ = 120 mg

Step 2: Compare the required amount with the MAWQ

Required amount = 40 mg

MAWQ = 120 mg

Because 40 mg is less than 120 mg, it should not be weighed directly if the goal is to remain within the stated 5% error limit.

High-yield check

An aliquot method may be used to accurately obtain a quantity smaller than the MAWQ, provided the aliquot itself weighs at least the MAWQ.

Compounding Spotlight: The Aliquot Method

What happens when the amount of drug you need is smaller than the amount your balance can accurately weigh?

One solution is the aliquot method.

The basic idea is to:

1. Weigh an amount of the active ingredient that is at least the MAWQ.
2. Mix it thoroughly with a measured amount of diluent.
3. Take a calculated portion, or aliquot, of that mixture containing the desired amount of active ingredient.

Example

Suppose you need 30 mg of Drug A, but the MAWQ is 120 mg.

Weigh 120 mg of Drug A. Add diluent to a total mixture weight of 480 mg. Weigh a 120 mg aliquot. That portion is one-fourth of the mixture, so it contains 30 mg of Drug A, and 120 mg is at the MAWQ, so it can be weighed accurately.

The MAWQ helps determine the minimum size of the aliquot that must be weighed. Work backward from an aliquot that can be measured accurately.

If the final mixture is prepared so that one-fourth of the mixture contains 30 mg of Drug A, you can then accurately weigh the calculated aliquot rather than trying to weigh 30 mg of Drug A directly.

The key principle:

Do not solve an accuracy problem by simply weighing an amount your balance cannot accurately measure.

Compounding Technique: Geometric Dilution

Once a small amount of active ingredient has been weighed, another challenge appears:

How do you distribute it evenly throughout a much larger amount of powder?

One common technique is geometric dilution.

Start with the ingredient present in the smallest quantity.

Add an approximately equal amount of diluent and mix thoroughly.

Then add an amount of diluent approximately equal to the mixture already present and mix again.

Continue this process until all of the diluent has been incorporated.

Think of the pattern

Small amount of drug

→ add approximately equal amount of diluent

→ mix thoroughly

→ add approximately an amount equal to the mixture

→ mix again

→ repeat

Geometric dilution helps improve the uniform distribution of a small quantity of drug throughout a larger powder mixture.

Compounding Pearl

A correct calculation does not automatically produce an accurate preparation.

A pharmacist or technician must also consider:

• Can this amount actually be measured accurately?
• Is the equipment appropriate for the quantity?
• Is the active ingredient distributed uniformly?
• Was the preparation made according to the formula and procedure?

The mathematics tells you what quantity you need.

Compounding technique determines whether you can prepare that quantity accurately and uniformly.

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Safety Check

When preparing a nonsterile compounded preparation:

• Verify the formula and calculations before beginning.
• Confirm the identity and strength of each ingredient.
• Determine whether each calculated quantity can be measured accurately.
• Use equipment appropriate for the quantity being measured.
• Do not directly weigh an amount below the applicable MAWQ when the required accuracy cannot be achieved.
• Use an appropriate mixing technique, such as geometric dilution when indicated.
• Document the quantities actually used.
• Follow the applicable compounding procedure, labeling requirements, storage conditions, and beyond-use dating requirements.

One-Minute Knowledge Check

1.A balance has a sensitivity requirement of 4 mg. A student weighs 160 mg of powder. What is the potential percentage error?

A. 1%
B. 2.5%
C. 4%
D. 6.25%

2.A compounded preparation requires 50 mg of an active ingredient, but the MAWQ of the balance is 100 mg. What is the best approach?

A. Weigh 50 mg directly
B. Estimate 50 mg visually
C. Use an aliquot method
D. Round the amount to 100 mg

3.Which technique is used to help uniformly distribute a small amount of active ingredient throughout a larger amount of powder?

A. Levigation
B. Geometric dilution
C. Trituration
D. Spatulation

Answers below.

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Liz Fredrickson, PharmD, BCPS Founder, RelayEd liz@relayedllc.com

RelayEd provides independent educational resources and is not affiliated with, endorsed by, or sponsored by NABP or USP. NAPLEX® is a registered trademark of the National Association of Boards of Pharmacy. USP® is a registered trademark of the United States Pharmacopeial Convention. Always verify medication information using current product labeling, course materials, and workplace policies.