Easy formula for identifying normal 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.
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