Reconstitution and Displacement Value Questions: CBT

Powdered drugs (most IV antibiotics) have to be dissolved before they can be drawn up, and the powder takes up space. That space is the displacement value, and it’s the piece of this topic that’s new to most CBT candidates.

The rule: final volume = diluent added + displacement value.

A 500 mg vial with a displacement value of 0.4 mL, reconstituted with 9.6 mL of water for injection, gives 10 mL of solution, not 9.6 mL. The concentration is therefore 500 ÷ 10 = 50 mg/mL.

The mistake displacement exists to prevent: assuming the final volume equals the diluent you added. Do that here and you’d believe the concentration was 52 mg/mL. Every partial dose drawn from the vial would then be slightly wrong, and in paediatrics, where partial vials are the norm and doses are small, slightly wrong compounds fast.

One reassurance before the practice: when the whole vial is given, displacement changes nothing, because the total drug is the vial’s stated content either way. Displacement earns its keep only for partial doses. That distinction is itself a CBT question.

The three question shapes

Find the concentration. Add diluent to displacement, divide the vial strength by the true volume. This is the base skill.

Find the diluent. The question fixes the final volume (“reconstitute to exactly 5 mL”) and you work backwards: diluent = final volume minus displacement. Flucloxacillin 250 mg with 0.2 mL displacement needs 4.8 mL of diluent, not 5.

Find the partial-dose volume. The full chain: true volume, then concentration, then want-over-have. Ceftriaxone 1 g (displacement 0.5 mL) plus 9.5 mL of diluent makes 10 mL at 100 mg/mL; a 750 mg dose is 7.5 mL. Once the true concentration exists, the last step is the same liquid formula from earlier in Part A.

Where the numbers come from

You never memorise displacement values. They vary by drug, brand and even vial size, and they live in the package insert, the manufacturer’s SPC, or the NHS Injectable Medicines Guide that UK wards keep at the drug cupboard. The CBT always prints the value in the stem. Your job is only to use it, which mostly means refusing to skip the small addition at the start.

Write the true volume down before anything else. Every wrong option in this topic is built from 9.6 where the working needed 10.

Five questions below, including the diluent-backwards shape. Then on to a drug class with its own units entirely: insulin and heparin.