How can a Glomerular Filtration Rate (GFR) be 228?

Understanding an Abnormally High GFR of 228

A glomerular filtration rate (GFR) of 228 mL/min/1.73m² is abnormally high and likely represents either a measurement error, a calculation error, or a physiological state of hyperfiltration.

Possible Explanations for an Elevated GFR of 228

1. Measurement or Calculation Errors

• Laboratory or calculation errors are the most common explanation for an abnormally high GFR 1
• Errors in the Cockcroft-Gault equation inputs (incorrect weight, age, or serum creatinine) 2
• Errors in timed urine collection (incomplete collection or excessive collection) 2
• Analytical variations between different creatinine assays 1

2. Physiological Hyperfiltration States

• Pregnancy - GFR can increase by 40-50% during pregnancy
• High protein diet - can temporarily increase GFR
• Early stages of diabetes - hyperfiltration often precedes diabetic nephropathy 2
• Obesity - associated with increased renal plasma flow and hyperfiltration
• Unilateral kidney disease - compensatory hyperfiltration in the remaining healthy kidney

3. Measurement Method Considerations

• Direct measurement methods using exogenous markers (inulin, iothalamate, iohexol) can yield different results than estimated GFR 2
• Different pharmacokinetic modeling approaches can affect measured GFR results 2
• Plasma clearance methods may overestimate GFR compared to urinary clearance methods 3

Verification Steps for Abnormal GFR Values

1. Verify calculation inputs:

   • Check patient demographics (age, weight, height) 2
   • Confirm serum creatinine value and units (mg/dL vs. μmol/L) 1
   • Review the equation used (Cockcroft-Gault, MDRD, CKD-EPI) 2, 1

2. Consider physiological factors affecting creatinine:

   • Muscle mass (athletes, bodybuilders may have higher creatinine generation) 1
   • Dietary factors (high meat consumption can increase creatinine) 1
   • Medications that affect tubular secretion of creatinine 1

3. Confirm with alternative measurement methods:

   • Use cystatin C-based GFR estimation 2, 4
   • Consider direct measurement using exogenous filtration markers 2, 3
   • Average creatinine and urea clearances for more accurate estimation 2

Normal GFR Reference Ranges

• Normal adult GFR: 90-120 mL/min/1.73m² 1
• Mildly reduced: 60-89 mL/min/1.73m² 1
• Moderately reduced: 30-59 mL/min/1.73m² 1
• Severely reduced: 15-29 mL/min/1.73m² 1
• Kidney failure: <15 mL/min/1.73m² 1

Clinical Implications and Next Steps

1. Repeat the measurement using the same method to check for consistency
2. Verify using an alternative method - if eGFR was used, consider measured GFR or cystatin C-based estimation 4
3. Evaluate for hyperfiltration causes - assess for diabetes, obesity, pregnancy, or high protein diet
4. Monitor over time - a single abnormal value should be interpreted with caution; trend is more important
5. Consider nephrology consultation for persistent unexplained hyperfiltration

Common Pitfalls in GFR Interpretation

• Relying on a single measurement - GFR naturally fluctuates and should be confirmed 1
• Using inappropriate equations for certain populations (very elderly, extremes of body size, amputees) 1
• Misinterpreting creatinine clearance as GFR - creatinine clearance typically overestimates true GFR due to tubular secretion 2
• Failing to adjust for body surface area when comparing to reference ranges 1
• Not considering the limitations of the specific measurement method used 2, 5

Remember that while GFR is an important indicator of kidney function, it should be interpreted in the clinical context and confirmed with repeat testing when abnormal values are encountered.