What is the estimated Glomerular Filtration Rate (GFR) with a Blood Urea Nitrogen (BUN) of 21 and creatinine of 0.6?

Estimated Glomerular Filtration Rate (GFR) Calculation

Based on the provided laboratory values of BUN 21 mg/dL and creatinine 0.6 mg/dL, the estimated GFR is approximately 140 mL/min/1.73m² using the MDRD formula, indicating normal to high kidney function.

GFR Estimation Methods

Several formulas can be used to calculate estimated GFR from serum creatinine and other parameters:

MDRD (Modification of Diet in Renal Disease) Formula

The abbreviated MDRD formula is commonly used for adults:

• 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] 1

For a creatinine of 0.6 mg/dL (without knowing age, sex, or race), this would generally yield an estimated GFR above 90 mL/min/1.73m².

Cockcroft-Gault Equation

• Estimated CrCl (mL/min) = [(140 - age) × weight]/[72 × SCr (mg/dL)] (× 0.85 if female) 1

This formula requires additional information (age, weight) but is useful for medication dosing.

Interpreting BUN and Creatinine Together

The BUN/creatinine ratio provides additional diagnostic information:

• Normal BUN/creatinine ratio: 10-20:1
• Current ratio: 21/0.6 = 35:1

This elevated ratio (>25:1) suggests potential extrarenal factors affecting BUN levels, such as:

• Dehydration
• Gastrointestinal bleeding
• High protein diet
• Catabolic state
• Heart failure 2

Clinical Implications

1. Normal or high GFR with elevated BUN/creatinine ratio:

   • Consider pre-renal causes (dehydration, heart failure)
   • Evaluate volume status
   • Review medication history (steroids, tetracyclines)
   • Consider protein intake assessment

2. Potential pitfalls in GFR estimation:

   • Creatinine is affected by muscle mass, age, sex, and diet 1
   • Low muscle mass can result in lower creatinine levels despite reduced kidney function
   • Laboratory methods for creatinine measurement can affect results (Jaffe vs. enzymatic methods) 1

3. When to consider alternative GFR assessment:

   • When clinical evidence of kidney dysfunction exists despite relatively normal creatinine 3
   • In patients with extremes of muscle mass or nutritional status
   • When precise GFR measurement is needed for medication dosing

Special Considerations

• For pediatric patients, different formulas should be used, such as the updated Schwartz formula 4, 5
• In elderly patients, the MDRD formula may be more accurate than Cockcroft-Gault 1
• In patients with acute illness, rapid changes in kidney function may not be immediately reflected in creatinine values

Remember that a single BUN and creatinine measurement provides a snapshot of kidney function and should be interpreted in the clinical context of the patient.