Calculations and Independent Checks

Part of Trial Pharmacy (PHARM) Training: Prescription Review and Dose Verification.

About 2 minutes. Free, in your language, no account needed.

Why this matters

Dose calculation errors are among the most consequential mistakes a pharmacy can make, and they are almost always preventable by a second person.

The check only works if it is genuinely independent.

What independent means

The second person calculates from the source, not from your working.

If they look at your number and agree it seems right, that is not a check. It is agreement, and agreement is exactly what confirmation bias produces.

Give them the prescription, the weight or surface area, and the protocol. Let them arrive at a number, then compare.

Where errors cluster

Unit conversions. Milligrams to millilitres, per kilogram to total, micrograms to milligrams.

Body surface area formulas. Different formulas give different values; the protocol usually specifies which.

Rounding and capping. Many protocols cap a dose or round to a vial size, and the rule is protocol-specific.

Weight currency. Using a weight from three months ago when the protocol requires a current one.

Decimal placement. The classic, and the most dangerous.

Record the calculation, not just the answer

Write the inputs, the formula and the result, with both initials.

A recorded calculation lets somebody reconstruct what was done. A recorded answer only tells you what was concluded, and if it is wrong nobody can see why.

Do not calculate from memory of the protocol

Have it open. Protocols change, and dosing rules are amended more often than most sections.

Check the version while you are there.

In practice

A dose comes out at 412mg. The protocol caps it at 400mg.

You apply the cap, record both the calculated and the capped value, note the rule applied, and have the second checker verify both steps.

Recording only 400mg loses the fact that a cap was applied, which matters for exposure analysis.

What this looks like when it goes wrong

Two pharmacists check a dose. The second reviews the first's written calculation and initials it.

Both used a weight in pounds where the protocol specifies kilograms. The error is consistent, the check confirmed it, and the participant receives more than twice the intended dose.

The check was performed and it was not independent.

Key takeaways