What does impaired renal function with an abnormal BUN/Creatinine ratio signify in a healthy 28-year-old woman?

Interpretation of eGFR 74 with Abnormal BUN/Creatinine Ratio in a Young Woman

The eGFR of 74 mL/min/1.73m² is within normal range, but the low BUN/creatinine ratio of 11.1 requires further evaluation for potential pre-renal causes despite normal overall kidney function.

Assessment of Kidney Function Parameters

Interpretation of Current Values

• eGFR: 74 mL/min/1.73m² - Within normal range (normal is ≥60 mL/min/1.73m²) 1
• BUN: 25 mg/dL - Slightly elevated (normal range typically 7-20 mg/dL)
• Creatinine: 1.05 mg/dL - Within normal range for women (0.6-1.0 mg/dL) 1
• BUN/Creatinine ratio: 11.1 - Lower than typical range (normal is 12-20)
• Urea/Creatinine ratio: 23.85 - Lower than expected

Clinical Significance

The patient presents with:

• Normal kidney filtration (eGFR >60)
• Low BUN/creatinine ratio
• Normal creatinine with slightly elevated BUN

Differential Diagnosis for Low BUN/Creatinine Ratio

Physiological Causes

• Increased protein catabolism
• Low protein intake or malnutrition
• Pregnancy (first or second trimester)
• Liver dysfunction affecting urea synthesis

Pathological Causes

• Rhabdomyolysis (increased creatinine production)
• Use of creatine supplements 2, 3
• Certain medications affecting creatinine secretion

Recommended Evaluation

Immediate Assessment

1. Dietary and supplement history:

   • Specifically ask about creatine supplement use, which can artificially elevate serum creatinine 2, 3
   • Assess protein intake patterns

2. Medication review:

   • Check for medications that affect tubular secretion of creatinine
   • Review any recent medication changes

3. Hydration status assessment:

   • Evaluate for signs of volume depletion or overhydration

Laboratory Testing

1. Repeat testing:

   • Confirm findings with repeat measurements as biological variability can affect results 1
   • Collect at least 2-3 specimens within 3-6 months before confirming abnormality 1

2. Additional tests:

   • Urinalysis to assess for proteinuria or hematuria
   • Urine albumin-to-creatinine ratio to detect early kidney damage 1
   • Consider cystatin C measurement (less affected by muscle mass than creatinine) 4
   • Liver function tests to rule out hepatic causes

Clinical Implications and Management

Interpretation

• An eGFR of 74 mL/min/1.73m² indicates normal kidney function (Stage 1 CKD if other evidence of kidney damage exists, otherwise normal) 1
• The low BUN/creatinine ratio is unusual and warrants investigation but does not necessarily indicate kidney disease

Management Approach

1. If creatine supplements are identified:

   • Discontinue supplements for 2-4 weeks
   • Repeat kidney function tests after discontinuation 2
   • Expect normalization of parameters if this was the cause

2. If no supplements identified:

   • Evaluate nutritional status and protein intake
   • Consider referral to dietitian if malnutrition suspected
   • Monitor kidney function every 3-6 months 4

3. When to refer to nephrology:

   • Not indicated based on current values alone
   • Consider if abnormalities persist after addressing potential causes
   • Immediate referral only if rapid deterioration in kidney function occurs 1

Important Considerations

Pitfalls to Avoid

• Don't overdiagnose kidney disease based on a single abnormal parameter when eGFR is normal 4
• Don't ignore the abnormal BUN/creatinine ratio despite normal eGFR, as it may indicate an underlying metabolic process
• Avoid unnecessary nephrotoxic medications until the cause is clarified

Patient Education

• Explain that current kidney function appears normal based on eGFR
• Discuss importance of follow-up testing to monitor for changes
• If supplements are involved, explain their effect on laboratory values

In this otherwise healthy 28-year-old woman, the most likely explanation is a non-pathological cause or laboratory variation rather than true kidney dysfunction. Close monitoring and identification of potential causes like supplement use should be the focus of management.