What does a blood urea nitrogen of 4 mg/dL and a BUN‑to‑creatinine ratio of 6 indicate in a patient with an estimated glomerular filtration rate of 119 mL/min/1.73 m²?

Interpretation of BUN 4 mg/dL and BUN/Creatinine Ratio of 6 with eGFR 119 mL/min/1.73 m²

Your laboratory values indicate excellent kidney function with a supranormal eGFR, but the unusually low BUN and markedly reduced BUN/creatinine ratio (normal is typically 10-20:1) most likely reflect either very low dietary protein intake, significant muscle wasting with malnutrition, or overhydration—not kidney disease. 1, 2

Understanding Your Laboratory Pattern

What These Numbers Mean

• eGFR of 119 mL/min/1.73 m²: This is above the normal range for young adults (approximately 120-130 mL/min/1.73 m²) and indicates your kidneys are filtering blood efficiently 3. This level represents normal or even hyperfiltration, not kidney dysfunction 3.

• BUN of 4 mg/dL: This is significantly below the typical range (7-20 mg/dL). BUN is produced from protein breakdown in the liver and filtered by the kidneys 2. A low BUN typically indicates either inadequate protein intake, severe liver disease, or dilution from overhydration 1, 2.

• BUN/Creatinine ratio of 6: This is markedly below the normal range of 10-20:1 1, 4. The low ratio suggests your creatinine is disproportionately elevated relative to BUN, or more likely, your BUN is inappropriately low 2.

Most Likely Explanations (in Order of Probability)

1. Low Protein Intake or Malnutrition 2

• Inadequate dietary protein reduces urea production in the liver, leading to low BUN
• Muscle wasting decreases creatinine production (creatinine comes from muscle metabolism), which can create a falsely reassuring creatinine level 3
• The National Kidney Foundation emphasizes that low creatinine from decreased muscle mass may not adequately reflect renal function, particularly in women, elderly, and malnourished patients 3, 2

2. Overhydration 1

• Excessive fluid intake dilutes BUN more than creatinine
• Volume expansion can lower both values but affects BUN disproportionately 1

3. Severe Liver Disease (less likely given isolated finding)

• Impaired hepatic urea synthesis reduces BUN production

Critical Next Steps

Immediate Assessments Required

Nutritional Evaluation 2

• Document your current edema-free body weight and any recent weight changes 2
• Obtain serum albumin level—target should be >3.5 g/dL or above your laboratory's lower limit of normal 2
• Assess dietary protein intake: normal adults require approximately 0.8 g/kg body weight per day 3
• If available, measure lean body mass (target ≥63%) 2

Complete Metabolic Panel 2, 5

• Check electrolytes (sodium, potassium, chloride, bicarbonate)
• Measure calcium, magnesium, and phosphate 2
• Obtain liver function tests to exclude hepatic dysfunction

Urinalysis 2, 5

• Rule out proteinuria (albumin-to-creatinine ratio should be <30 mg/g) 3
• Check for hematuria or abnormal sediment that would suggest intrinsic kidney disease 1

Clinical Pitfalls to Avoid

Do not assume normal kidney function based solely on a low BUN/creatinine ratio 2. While your eGFR is reassuring, the National Kidney Foundation warns that low creatinine from muscle wasting can mask significant kidney dysfunction, making the BUN/creatinine ratio unreliable for assessing renal status 2.

Do not ignore potential malnutrition 2. Severe muscle wasting or malnutrition is the most likely explanation for your laboratory pattern, particularly if serum creatinine is inappropriately low 2.

When to Seek Specialist Consultation

Consider nephrology referral if 2, 5:

• You develop symptoms of uremia (fatigue, nausea, confusion) despite these laboratory values
• Urinalysis reveals proteinuria or hematuria
• eGFR begins to decline on serial monitoring

Consider nutrition consultation if 2:

• Serum albumin is below the lower limit of normal for your laboratory
• Clinical signs of malnutrition are present with declining lean body mass
• Dietary protein intake is inadequate

Monitoring Recommendations

Serial Laboratory Monitoring 2, 5

• Recheck BUN, creatinine, and eGFR in 3-6 months to establish a trend 5
• Monitor serum albumin every 4 months if nutritional concerns exist 2
• Trend creatinine values rather than relying on absolute values when assessing kidney function 1

Nutritional Targets if Malnutrition Confirmed 2

• Aim for dietary protein intake of 1.2-1.3 g/kg body weight per day in stable patients 2
• Target serum albumin >3.5 g/dL 2
• If on dialysis (not applicable in your case), normalized protein nitrogen appearance should be ≥0.9 g/kg/day 2

Key Takeaway

Your eGFR of 119 mL/min/1.73 m² confirms excellent kidney filtration capacity 3. The low BUN and BUN/creatinine ratio do not indicate kidney disease but rather point toward nutritional status, hydration, or muscle mass issues that warrant evaluation 1, 2. Focus your attention on nutritional assessment and ensuring adequate protein intake rather than worrying about kidney dysfunction 2.