Unit 8.4: Unit Conversions and Measurement

• Topic 01

  SI Unit Conversions

  45 questions
• Topic 02

  Concentration Expressions

  45 questions
• Topic 03

  Interpreting Drug Labels and Prescriptions

  39 questions
• Topic 04

  Common Measurement Errors and How to Avoid Them

  75 questions

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

1. What does the SI prefix "milli" mean in terms of its numerical value relative to the base unit?

   • A
     One billionth ($\div 1,000,000,000$, or $\times 10^{-9}$)
   • B
     One thousandth ($\div 1000$, or $\times 10^{-3}$)
     Correct answer
   • C
     One hundredth ($\div 100$, or $\times 10^{-2}$)
   • D
     One thousand times ($\times 1000$)
   Explanation

   The prefix "milli" means one thousandth of the base unit (10⁻³). So 1 milligram = 0.001 gram, and 1 millilitre = 0.001 litre. To convert from a base unit to milli-units, multiply by 1000; to convert back, divide by 1000. This is distinct from "micro" (10⁻⁶) and "centi" (10⁻²).

2. What does the prescription abbreviation "prn" mean, and what additional information must accompany it on a valid prescription?

   • A
     "prn" means "when required" (pro re nata); the prescription must also specify a minimum dosing interval and a maximum daily dose
     Correct answer
   • B
     "prn" means "once per night" and the timing is at the nurse's discretion
   • C
     "prn" means "repeat prescription" — the dose is repeated every time it runs out
   • D
     "prn" means "as prescribed by nurse" — it delegates dosing decisions to the nurse
   Explanation

   "prn" (pro re nata) means "when required" — the dose should only be given when the patient needs it, based on clinical assessment. A valid prn prescription must include: (1) the drug and dose, (2) the minimum interval between doses (e.g. every 4–6 hours), and (3) the maximum daily dose. Without these, a nurse cannot safely administer the drug as the frequency and total dose would be unrestricted. For example: "paracetamol 1 g prn, maximum every 4 hours, maximum 4 g in 24 hours".

3. During an anaphylaxis emergency, a nurse prepares 0.5 mL of adrenaline IM. Later it is discovered that 1:10000 adrenaline was used instead of 1:1000. What dose in mcg was actually administered, what was the intended dose in mcg, and what is the magnitude of the error?

   • A
     Administered: 50 mcg; intended: 5000 mcg; a hundred-fold underdose
   • B
     Administered: 500 mcg; intended: 500 mcg; both preparations deliver the same dose if the same volume is given
   • C
     Administered: 50 mcg; intended: 500 mcg; a ten-fold underdose — insufficient to treat anaphylaxis
     Correct answer
   • D
     Administered: 500 mcg; intended: 50 mcg; a ten-fold overdose — risk of hypertensive crisis
   Explanation

   1:10000 = 0.1 mg/mL = 100 mcg/mL. Dose administered = $0.5 mL \times 100 mcg/mL = 50 mcg. Intended dose from 1:\1000 = 0.5 mL \times 1000 mcg/mL = 500 mcg$. Error: ten-fold underdose. Using the wrong (more dilute) preparation is a critical patient safety error in anaphylaxis — the patient may not receive adequate treatment. Adrenaline 1:1000 is the correct preparation for IM administration in anaphylaxis per NICE/BNF guidance.