Opioid Prescribing Renal Failure — FRCA Final MCQ

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ModeratePain MedicineOpioid Prescribing Renal FailureFRCA Final

A 72-year-old woman (ASA III, 55 kg) is on the ward 12 hours after dynamic hip screw fixation of a fractured neck of femur. She has stable stage 4 chronic kidney disease (eGFR 25 mL/min/1.73 m², not on dialysis). Her fascia iliaca block has regressed, NSAIDs are contraindicated and paracetamol alone is inadequate, so a regular opioid is required for the next few days; the route and dose will be titrated by the acute pain team. Which opioid is least likely to accumulate and cause opioid toxicity at this level of renal function?

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Correct answer: E — Fentanyl

Explanation lettering: B = shown as A · E = shown as B · A = shown as D · D = shown as E

Fentanyl (D) is hepatically metabolised to norfentanyl and other inactive, non-toxic metabolites, with only about 10% excreted unchanged in urine, so dose adjustment is not required even in CKD stages 4–5; titrated parenteral fentanyl or PCA is the standard UK choice when renal clearance is poor. Morphine (A) generates renally cleared morphine-6-glucuronide, causing delayed sedation and respiratory depression. Codeine (B) is a prodrug requiring variable CYP2D6 conversion to morphine, so it combines unpredictable efficacy with the same metabolite accumulation. Oxycodone (C) is the strongest distractor — acceptable with cautious titration in mild–moderate impairment — but UK guidance advises avoiding it at eGFR below 30 mL/min because parent drug and metabolites accumulate. Tramadol (E) and its active O-desmethyl metabolite have delayed elimination, with seizure and serotonergic risk in the elderly uraemic patient.

Reference: UK Medicines Information (NHS), Opioid use in renal impairment — sections on fentanyl and oxycodone; corroborated by Scottish Palliative Care Guidelines (NHS Scotland), 'Oxycodone' (avoid if eGFR <30 mL/min). https://www.cheshire-epaige.nhs.uk/wp-content/uploads/2018/11/Opioid-use-in-renal-impairment-UK-medicines-information.pdf