Easy formula for identifying normal glomerular filtration rate

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A nurse practitioner uses a formula for glomerular filtration rate
A nurse practitioner uses a formula for glomerular filtration rate

I recently took a call from a new practitioner. She had an 85-year-old diabetic, non-insulin-dependent female with a glomerular filtration rate (GFR) of 45 mL/min/1.73 m 2 . The practitioner asked if referral to nephrology was needed.

While the kidney guidelines (Kidney Disease Outcomes Quality Initiative, or KDOQI) recommend referral to nephrology at stage 3 (30–60 mL/min/1.73 m 2 ), with the split of stage 3 into stage 3A (45–59 mL/min/1.73 m 2 ) and stage 3B (30–44 mL/min/1.73 m 2 ), there is no consensus on the exact GFR for referral. 

However, we do know that everyone loses 0.8% kidney function/year after the age of 30. If you round this to a loss of 1% per year (for simplicity), you can calculate that this 85-year-old patient should have already lost 55 mL/min/1.73 m 2 (85–30=55). Thus, a GFR of 45 mL/min/1.73 m 2 is actually normal for her age (original GFR 100–55) (age-related loss).

Of course, the most important predictor of continued loss of kidney function is albumin. So, every person with diabetes should have a spot urine albumin-to-creatinine ratio (UACR) test done each year. (191-2)

Kim Zuber, PA-C, oversees patients in 7 dialysis centers for Metropolitan Nephrology Associates, Clinton, Md.

The Clinical Advisor asked our Advisor Forum consultants what sorts of clinical questions they have fielded from their colleagues lately and what puzzling cases they have encountered themselves. We invite you to participate. If you have a clinical pearl or clinical question, submit it here.

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