What stone‑preventive medication regimen is appropriate for a 78‑year‑old man with an estimated glomerular filtration rate of 88 mL/min/1.73 m²?

Medication for Kidney Stone Prevention in a 78-Year-Old with eGFR 88 mL/min/1.73 m²

Potassium citrate 30-80 mEq/day in divided doses (typically 20 mEq three times daily) is the appropriate stone-preventive medication for this patient, as it has demonstrated sustained reduction in stone formation rates across multiple stone types while being safe at this level of kidney function. 1

Stone Type Assessment Required First

Before initiating therapy, determine the stone composition through analysis of passed stones or 24-hour urine metabolic evaluation to identify:

• Calcium oxalate stones with hypocitraturia
• Uric acid stones
• Mixed calcium/uric acid stones
• Presence of renal tubular acidosis 1

Potassium Citrate Dosing and Efficacy

For calcium oxalate nephrolithiasis with hypocitraturia, potassium citrate 30-100 mEq/day (typically 20 mEq three times daily) achieved:

• Stone formation rate reduction from 13±27 stones/year to 1±2 stones/year over 2 years in patients with renal tubular acidosis 1
• 67% stone-passage remission rate in distal tubular acidosis patients 1
• Sustained increase in urinary citrate from subnormal to normal levels (400-700 mg/day) 1
• Sustained increase in urinary pH from 5.6-6.0 to approximately 6.5 1

For uric acid lithiasis, potassium citrate 30-80 mEq/day in 3-4 divided doses resulted in:

• Urinary pH increase from 5.3±0.3 to 6.2-6.5 1
• Only 1 stone formed across 18 patients during treatment 1
• Urinary citrate rising to high normal range 1

Safety at This eGFR Level

At eGFR 88 mL/min/1.73 m², this patient has Stage 2 CKD (GFR 60-89 mL/min/1.73 m²), where potassium citrate is safe without dose adjustment 1. Monitor serum potassium periodically, particularly if the patient is on ACE inhibitors, ARBs, or mineralocorticoid receptor antagonists 2.

Adjunctive Dietary Measures

Implement concurrent dietary modifications:

• Sodium restriction to <100 mEq/day (approximately 2.3 g/day) 1
• Reduce oxalate intake (limit nuts, dark leafy greens, chocolate, tea) 1
• Moderate calcium restriction (400-800 mg/day) if hypercalciuria is present 1
• Maintain adequate hydration 1

Monitoring Protocol

Monitor serum creatinine and potassium levels periodically during treatment, particularly in the first 3 months after initiation 2. At this eGFR level (88 mL/min/1.73 m²), annual monitoring of kidney function is appropriate unless other risk factors are present 3.

Critical Considerations for This Age Group

In this 78-year-old patient, age is the strongest predictor of kidney function decline in stone formers 4. Recurrent symptomatic nephrolithiasis is associated with loss of kidney function, with average GFR slope of -2.83 mL/min/1.73 m² per year in stone formers 4. Therefore, aggressive stone prevention is particularly important in elderly patients to preserve remaining kidney function.

The prevalence of CKD is 9.3% among recurrent stone formers compared to 1.3% in controls 5, and stone formers have significantly lower eGFR (87±20 vs 104±22 mL/min/1.73 m²) than matched controls 5.

Alternative Considerations Based on Stone Type

If the patient has calcium stones with hypercalciuria rather than hypocitraturia, thiazide diuretics may be added to potassium citrate 1. If hyperuricosuria or hyperuricemia is present, allopurinol may be combined with potassium citrate 1.

Common Pitfalls to Avoid

• Do not withhold potassium citrate based solely on age; the eGFR of 88 mL/min/1.73 m² indicates adequate kidney function for standard dosing 1
• Do not discontinue ACE inhibitors or ARBs if the patient is on them for blood pressure control, as they should not be stopped at eGFR >30 mL/min/1.73 m² 2, 6
• Do not assume all kidney stones require the same treatment; stone composition and metabolic abnormalities guide therapy 1