# Common PSA mistakes that cost marks Source: PassthePSA, PSA revision notes URL: https://passthepsa.com/psa-revision/common-mistakes/ Updated: 2026-10-01 Topic: UK Prescribing Safety Assessment (PSA) revision Short answer: The most common PSA mistakes are writing units wrongly or abbreviating micrograms, nanograms and units, unsafe decimals such as trailing zeros, ambiguous abbreviations and frequencies, giving a volume instead of a dose, using the wrong chart section, ignoring renal function or allergy, not using the BNF, and poor time management. Each is avoidable with a fixed writing routine. ## Which unit and decimal errors lose marks? Unit and decimal errors are the easiest to make and the most dangerous in practice, because a misread number can cause a tenfold or thousandfold error. Use a fixed routine every time. - Write micrograms and nanograms in full; do not use mcg, ug or ng. - Write units in full; a handwritten U can be misread as a zero. - Do not add a trailing zero: write 5 mg, not 5.0 mg, which can be read as 50 mg. - Where a decimal is unavoidable, add a leading zero: write 0.5 mL, not .5 mL, which can be read as 5 mL. - Prefer the smaller unit to a decimal: 500 mg, not 0.5 g; 100 micrograms, not 0.1 mg. These conventions follow the BNF guidance on prescription writing. Read that section once and apply it every time. Practise writing doses by hand or on screen until the safe forms feel automatic. Under time pressure you revert to habits, so the habit has to be the safe one. ## Which abbreviations and frequencies cause trouble? Use only abbreviations that are standard and unambiguous, and write the frequency so that nobody can misread it. An unclear frequency may be marked as wrong. Ambiguity is the common thread. The test is whether a nurse who has never met the patient could give the drug exactly as intended without having to ask you. - Do not leave the frequency blank when the question requires one. - Avoid non-standard shorthand for drug names; use the generic name in full. - State once daily, twice daily or a stated number of hours apart in a form the chart accepts. - For a weekly drug, make the day or the word weekly explicit. - Where the form accepts a route abbreviation, use the standard one; if in doubt, write the route in full. Some drugs are given weekly or less often than daily, and a daily frequency written for them is a serious error. Check the BNF entry when a frequency looks unusual. ## What goes wrong with volumes and chart sections? A prescription states the amount of drug, not the volume of liquid. Writing 2 mL without a strength leaves the dose unclear and loses the marks. - Write the dose in mass or units (for example mg, micrograms or units), then the form if needed. - For an infusion, state the drug, the amount and the diluent or rate as the item requires. - Write a once-only drug in the once-only section, a regular drug in the regular section, and an as-required drug in the as-required section. - Read the instruction line, because it tells you which section the marks are on. A right drug in the wrong section can lose the prescription marks even though the choice was correct. ## How do ignored patient details cost marks? The case gives you the details that change the answer, and skipping them is a classic trap. Check them before you write anything. Detail | What to check --- | --- Allergy | Does the drug or its class match the allergy? Renal function | Does the BNF advise a dose change or avoidance? Weight, age | Is the dose weight-based or age-banded? Pregnancy, breastfeeding | Is the drug unsuitable or does it need caution? Other medicines | Is there an interaction that changes the choice? Underline the patient details in the stem before opening the BNF. Renal function matters most for drugs cleared by the kidney, and the BNF monograph states when to adjust or avoid. Allergy matters for the drug and for related drugs in the same class, so check cross-reactivity as well as the named drug. ## Which exam-technique mistakes cost marks? Technique errors cost marks even when knowledge is sound. The main ones are not using the BNF and poor time management. - Answering from memory when the BNF is open; check dose, route and indication every time. - Taking the first dose listed rather than the dose for the stated indication. - Spending too long on a 2-mark item and rushing the 10-mark prescribing items. - Leaving answers blank; on a prescribing item partial credit exists. - Hedging by writing two drugs, which risks losing the box. Note: Doses: the examples above are about format only. Confirm every dose in the BNF. Log these in your error log with their cause. A look-up failure needs BNF practice, a writing error needs rehearsal of the prescription format, and a time error needs timed sets. Fixing the cause is faster than relearning the topic. ## Check yourself - How should micrograms, nanograms and units be written? - Why is 5.0 mg unsafe, and why is .5 mL unsafe? - Why is 2 mL not a dose? - What happens if a regular drug goes in the once-only section? - Which patient details should you check before writing any prescription? - How long should a 2-mark item take at exam pace? ## Questions people ask Q: Why must micrograms be written in full? A: Abbreviations such as mcg or ug can be misread as mg, which is a thousandfold error. The BNF advises writing micrograms and nanograms in full. Q: What is the safest way to write a decimal dose? A: Add a leading zero (0.5 mg, not .5 mg) and avoid trailing zeros (5 mg, not 5.0 mg). Where possible use the smaller unit instead of a decimal. Q: Can I lose marks for using the wrong chart section? A: Yes. A once-only drug written as a regular prescription, or the reverse, can lose the prescription marks even when the drug is right. Q: Should I always use the BNF even when I think I know the answer? A: Yes. It is available throughout, and a quick check of dose, route and patient modifiers catches errors that recall misses. ## Related notes - How to write a safe prescription: https://passthepsa.com/psa-revision/prescribing-safely/ - How to revise for the PSA: https://passthepsa.com/psa-revision/how-to-revise/ - PSA format and marking: https://passthepsa.com/psa-revision/format-and-marking/ - Renal dosing for the PSA: https://passthepsa.com/psa-revision/renal-dosing/ Sources: BNF guidance on prescription writing; BNF. Exam-revision notes, not patient-specific advice; the BNF is the authority. More: https://passthepsa.com/psa-revision/ (all notes), https://passthepsa.com/psa/ (exam guide), https://passthepsa.com/ (practice questions).