Renal Dose Adjustment and a Potential Interaction in an Older Patient
An 82-year-old woman (weight 58 kg, serum creatinine 130 micromol/L) is admitted with a urinary tract infection and dehydration. Her regular medicines are ramipril 5 mg, furosemide 40 mg, metformin 1 g twice daily, and ibuprofen 400 mg three times daily as needed for knee pain, which she has taken regularly for the past two weeks. Admission bloods show eGFR 28 mL/min/1.73 m2 and potassium 5.6 mmol/L. The admitting doctor asks you to review her medicines and suggest changes.
1. Which of her current medicines are contributing to her current renal function and potassium, and what immediate changes would you recommend?
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The combination of ramipril, furosemide and regular ibuprofen in a dehydrated older patient is a classic 'triple whammy' for acute kidney injury: NSAIDs reduce afferent arteriolar dilation, ACE inhibitors reduce efferent constriction, and diuretics reduce intravascular volume. The result is a fall in glomerular filtration. Ibuprofen should be stopped immediately and not restarted, given her age, reduced renal function and hyperkalaemia. Ramipril contributes to hyperkalaemia and should be withheld during the acute illness while she is dehydrated, with a plan to review once she is rehydrated and renal function stabilises. Metformin should be withheld because of the acute kidney injury and risk of lactic acidosis, and restarted only when renal function recovers and is stable. Furosemide may be appropriate to continue if she is fluid overloaded, but should be reviewed in light of her volume status. Seek senior or pharmacist review before acting.
2. Calculate whether metformin should be restarted at her current renal function using the eGFR thresholds, and explain the reasoning.
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Metformin is contraindicated when eGFR is below 30 mL/min/1.73 m2, and the dose should be reviewed when eGFR falls below 45. Her eGFR of 28 is below the threshold, so metformin should not be restarted at this level. Even if her eGFR recovers to 35, she should not receive more than 1 g daily, and the dose should be titrated with monitoring of renal function. The reason is that metformin is renally cleared, and accumulation in renal impairment increases the risk of lactic acidosis, particularly during acute illness, dehydration or contrast exposure. Document the eGFR at which metformin was stopped and the plan for review. Advise her to hold metformin during any future acute illness with vomiting or diarrhoea, and to have renal function checked before restarting.
3. What monitoring and counselling would you arrange before discharge to reduce the risk of recurrence?
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Arrange repeat urea, electrolytes and creatinine before discharge and again within a week of any medicine change. Check potassium after stopping ramipril and ibuprofen; if it remains above 5.5, escalate. Counsel her to avoid over-the-counter NSAIDs, including ibuprofen and naproxen, and suggest paracetamol as the preferred analgesic for her knee pain, with a maximum of 3 g daily given her age and low weight. Explain the 'sick day rules': temporarily stop ramipril, furosemide and metformin during vomiting or diarrhoea and seek advice. Provide written information about her kidney function and the medicines to avoid. Arrange follow-up with her GP to review her antihypertensive and diuretic therapy once she has recovered, and consider whether her renal function should be monitored more frequently given her age and polypharmacy.