to the editor: I appreciated the excellent article by Drs. Munar and Singh on drug dosing in patients with chronic kidney disease in the May 15, 2007, issue of American Family Physician.¹ As an academic family physician with a certificate of added qualification in geriatrics, I encounter this issue nearly daily, and I would like to provide a geriatrician's perspective.

Although the Cockcroft-Gault equation and the Modification of Diet in Renal Disease (MDRD) equation are valuable and relatively accurate for approximating a patient's creatinine clearance, I find their utility limited in daily practice. The Cockcroft-Gault equation requires knowledge of ideal body weight, and the developers of the MDRD equation caution against using it in older patients and in those with stable kidney function.²

I use (and teach) the long-overlooked and less well-known Jelliffe equation, which is well validated.³ Although it is no more or less accurate than the Cockcroft-Gault or MDRD equations, its advantage is that it can be calculated in one's head. There are several versions of this equation; the simplest is:

For women, the resultant value is multiplied by 0.9. Consequently, an 80-year-old man with a creatinine level of 1.0 mg per dL (90 μmol per L) has a clearance of 50 mL per minute (0.83 mL per second), whereas a woman of similar age with the same creatinine level has a clearance of 45 mL per min (0.75 mL per second).

Once a patient's creatinine clearance is calculated, it is important to recognize certain medications that should be reduced or avoided. Drs. Munar and Singh laudably review antibiotic adjustments;¹ I routinely refer to the “Dosage in Renal Failure” table of my pocket Sanford Guide.⁴ Two antibiotics in particular deserve emphasis. The double-strength tablet of trimethoprim/sulfamethoxazole (Bactrim, Septra), which is commonly prescribed, should be avoided unless the patient's creatinine clearance is known to exceed 50 mL per minute; the single-strength tablet of trimethoprim/sulfamethoxazole is preferable. Nitrofurantoin (Furadantin) is contraindicated if a patient's creatinine clearance is less than 60 mL per min, a condition that occurs often in patients older than 65 years.

Thiazides, often prescribed to patients who are older, are ineffective in patients with a creatinine clearance less than 30 mL per min (0.50 mL per second). Finally, it is important to recognize that alendronate (Fosamax), risedronate (Actonel), and ibandronate (Boniva), bisphosphonates used to prevent hip fractures in patients with osteoporosis, are contraindicated in patients with creatinine clearances less than 35 mL per minute (0.58 mL per second), 30 mL per minute, and 30 mL per minute, respectively.

I echo Drs. Munar's and Singh's recommendations to adjust for renal clearance when prescribing medications. I prefer the Jelliffe formula for its bedside ease; but whichever method we use, as long as we calculate clearance we will prescribe more safely.