Unit 8.1: Drug Dosage Calculations

• Topic 01

  Tablet and Capsule Calculations

  45 questions
• Topic 02

  Liquid Medication Calculations

  39 questions
• Topic 03

  Weight-Based Dosing

  36 questions
• Topic 04

  Paediatric Drug Calculations

  43 questions

A few questions from this unit, with the answer and a full explanation. The complete bank is available when you start practising.

1. Haloperidol 5 mg/mL injection is available. The prescribed dose is 2.5 mg IM. What volume in mL should be drawn up?

   • A
     5 mL
   • B
     2 mL
   • C
     0.5 mL
     Correct answer
   • D
     1 mL
   Explanation

   Applying the formula: $(2.5 \div 5) \times 1 = 0.5 mL$. The stock concentration is 5 mg per 1 mL, so 2.5 mg requires half a mL. A 1 mL syringe is appropriate for this volume.

2. What common error occurs when a nurse confuses units/mL with mg/mL for heparin preparations?

   • A
     Confusing units and mg for heparin is not clinically significant as they are proportionally equivalent
   • B
     The error only affects the volume drawn up, not the dose delivered to the patient
   • C
     The nurse may treat heparin 5000 units/mL as if it were 5000 mg/mL, leading to a dose expressed in mg being administered as if it were units, causing a gross under- or overdose
     Correct answer
   • D
     Units and mg are interchangeable for heparin because its molecular weight is constant
   Explanation

   Heparin is expressed in units (international units of anticoagulant activity), not milligrams. Heparin 5000 units/mL contains 5000 units per mL — this is NOT equivalent to 5000 mg/mL. If a nurse confuses these and calculates a dose in mg using units/mL as the denominator (or vice versa), the volume drawn up will be wrong by an unpredictable factor. The MHRA and NHS England have both issued guidance specifically warning against unit-to-mg conversion errors for heparin after fatal incidents.

3. Morphine sulphate injection is available as 10 mg/mL in 1 mL ampoules and 15 mg/mL in 1 mL ampoules. A patient is prescribed 7.5 mg SC. Which preparation should be used and what volume is required?

   • A
     10 mg/mL preparation: volume = 0.75 mL, which is measurable with a 1 mL syringe
     Correct answer
   • B
     Either preparation is equally acceptable; the nurse should choose whichever is easier to measure
   • C
     10 mg/mL preparation: volume = 0.75 mL, but this must be rounded to 1 mL for a 1 mL syringe
   • D
     No injection should be given as 7.5 mg is not a standard morphine dose
   Explanation

   Both preparations could technically deliver 7.5 mg: 10 mg/mL gives 0.75 mL, 15 mg/mL gives 0.5 mL. However, 0.5 mL from a 15 mg/mL ampoule wastes the remaining 7.5 mg in the ampoule and increases risk of a 10-fold concentration error if the wrong strength is accidentally used. The 10 mg/mL preparation gives 0.75 mL, which is easily and accurately drawn up in a 1 mL syringe. Rounding 0.75 mL to 1 mL would administer 10 mg instead of 7.5 mg — a 33% overdose and not acceptable. Use the most appropriate concentration to minimise waste and error.