Overview

Desired over have — sometimes written D/H x Q — is the single formula behind most oral and injectable dose calculations. Desired is what the prescriber ordered. Have is the strength printed on the label. Quantity is the volume or number of tablets that labelled strength comes in. Divide desired by have, multiply by the quantity, and the result is the amount to administer. Two things make it go wrong: running it before the units match, and misreading the quantity as the size of the container rather than the volume the strength is stated in.

Interpretation

Desired = the ordered dose Hallmark

Have = the strength on the label Hallmark

Quantity = the volume that strength comes in Hallmark

5 mL on a label reading 250 mg per 5 mL — not the bottle size

Quantity is 1 for a tablet Hallmark

which is why tablet problems look like plain division

Both dose figures must share a unit Hallmark

The answer carries the quantity's unit

millilitres in, millilitres out

Technique

Worked example: 375 mg ordered, label reads 250 mg per 5 mL

Desired375 mg
Have250 mg
Quantity5 mL
Divide then multiply375 / 250 x 5
Answer7.5 mL

During — Monitoring

Compare the answer with the strength you have Hallmark

if desired is larger than have, the answer must exceed the quantity

Check the tablet can be split the way the answer requires Hallmark

only scored tablets halve; nothing splits into thirds

Have a second nurse verify high-alert calculations

Re-read the label rather than working from memory

Patient Teaching

Measure the calculated volume with the supplied syringe Hallmark

Do not split a tablet that is not scored

Check that a refill has the same strength as the last bottle Hallmark

a new strength changes the volume to give

Ask before halving a capsule or extended-release tablet

Report Nowescalate immediately

An answer that needs a tablet cut into thirds Hallmark

An intramuscular volume larger than the site can hold

An answer far larger than the quantity on the label Hallmark

usually an unmatched unit

A refill bottle with a different strength than expected

Any high-alert dose not independently double-checked

Clinical Pearl

Desired over have, times the quantity it comes in. Match the units first, or the formula will hand you a wrong answer with total confidence.

Desired-Over-Have Formula

Overview

Interpretation

Technique

During — Monitoring

Patient Teaching

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