Determining GFR with a BUN/Creatinine Ratio of 35
A BUN/creatinine ratio of 35 alone is insufficient to determine the GFR, as it only indicates a likely prerenal condition rather than providing a direct measurement of glomerular filtration rate.
Understanding BUN/Creatinine Ratio
The BUN/creatinine ratio is a diagnostic tool that helps differentiate between prerenal, intrarenal, and postrenal causes of kidney dysfunction:
- A ratio >20 (or >35 in your case) typically suggests prerenal azotemia 1
- Normal ratio is approximately 10-15:1
- The elevated ratio indicates increased urea reabsorption relative to creatinine, often due to:
- Volume depletion
- Decreased renal perfusion
- Increased protein catabolism
- Gastrointestinal bleeding
- High protein diet
- Corticosteroid use
Calculating GFR
To properly determine GFR, you need additional information beyond just the BUN/creatinine ratio. According to current guidelines, GFR should be calculated using validated formulas 2:
Required Information:
- Serum creatinine level (mg/dL or μmol/L)
- Age
- Sex
- Race (for some equations)
- Weight (for some equations)
Recommended Formulas:
MDRD (Modification of Diet in Renal Disease) Formula:
- eGFR (mL/min/1.73 m²) = 186 × [SCr (mg/dL)]^-1.154 × [age (years)]^-0.203 × [0.742 if female] × [1.21 if African American] 2
Cockcroft-Gault Formula:
- Estimated CrCl (mL/min) = [(140 - age) × weight]/[72 × SCr (mg/dL)] (× 0.85 if female) 2
Clinical Implications
The elevated BUN/creatinine ratio of 35 suggests:
- Possible prerenal azotemia 3
- Higher mortality risk compared to patients with lower ratios 3
- Need for prompt evaluation of volume status and renal perfusion
Steps to Determine GFR
- Measure serum creatinine level
- Collect necessary demographic information (age, sex, race, weight)
- Apply appropriate GFR estimation formula (MDRD or CKD-EPI preferred)
- Consider cystatin C measurement for more accurate assessment in specific populations 4
Staging and Management
Once GFR is calculated, kidney function can be staged according to KDIGO guidelines 2:
- Stage 1: GFR ≥90 mL/min/1.73 m² (with kidney damage)
- Stage 2: GFR 60-89 mL/min/1.73 m² (mild decrease)
- Stage 3a: GFR 45-59 mL/min/1.73 m² (mild to moderate decrease)
- Stage 3b: GFR 30-44 mL/min/1.73 m² (moderate to severe decrease)
- Stage 4: GFR 15-29 mL/min/1.73 m² (severe decrease)
- Stage 5: GFR <15 mL/min/1.73 m² (kidney failure)
Important Caveats
- BUN/creatinine ratio alone cannot determine GFR
- Serum creatinine has limitations in certain populations (elderly, reduced muscle mass)
- Consider using cystatin C in patients where creatinine may be unreliable
- Acute changes in kidney function may not be immediately reflected in GFR estimates
In summary, while the BUN/creatinine ratio of 35 suggests prerenal azotemia, you must obtain serum creatinine and apply an appropriate estimation formula to determine the actual GFR value.