What is the interpretation of a patient's kidney function with a Blood Urea Nitrogen (BUN) of 6, Glomerular Filtration Rate (GFR) of 88, and creatinine of 0.86?

Kidney Function Interpretation: Mildly Reduced GFR in Context of Low BUN

These laboratory values indicate mildly reduced kidney function for a young adult, with the notably low BUN of 6 mg/dL suggesting either decreased protein intake, overhydration, or reduced muscle mass rather than kidney disease. 1

GFR Assessment

The GFR of 88 mL/min/1.73 m² represents approximately 30% below the expected normal range for young adults (120-130 mL/min/1.73 m²), which is clinically significant and warrants further evaluation. 1

• A GFR of 88 mL/min/1.73 m² falls below the threshold (90 mL/min/1.73 m²) used for kidney donor acceptance, indicating this is not considered optimal kidney function for younger individuals 1
• This level does not meet criteria for CKD stage 3 (GFR <60 mL/min/1.73 m²) but may represent CKD stage 2 if markers of kidney damage are present 2
• The creatinine of 0.86 mg/dL appears normal but can mask reduced GFR, particularly in individuals with lower muscle mass 2, 3

BUN Interpretation

The BUN of 6 mg/dL is abnormally low and suggests factors unrelated to kidney dysfunction:

• Low BUN typically indicates decreased protein intake, liver disease, overhydration, or reduced muscle mass rather than kidney pathology 4
• The BUN/creatinine ratio of approximately 7:1 (normal 10:1 to 20:1) is markedly low, supporting non-renal causes for the low BUN 5
• Higher BUN levels are associated with adverse renal outcomes in CKD patients, but low BUN does not confer kidney protection 4

Required Follow-Up Actions

Do not rely on a single GFR calculation - confirmation with repeat testing and additional markers is essential: 1

1. Repeat GFR assessment in 3 months to determine if this represents persistent kidney dysfunction versus laboratory variability 1

2. Measure urine albumin-to-creatinine ratio (ACR) in a random urine specimen, as persistent proteinuria is the principal marker of kidney damage and defines CKD stage 2 when GFR is 60-89 mL/min/1.73 m² 1, 2

3. Assess for causes of low BUN: evaluate dietary protein intake, hydration status, liver function, and muscle mass 4, 5

4. Do not use serum creatinine alone to assess kidney function, as it is insensitive to mild-to-moderate GFR decreases and affected by muscle mass, age, and other non-renal factors 2, 6

Clinical Decision Points

If repeat GFR remains <90 mL/min/1.73 m² AND proteinuria is present:

• This confirms CKD stage 2 and requires nephrology referral for comprehensive evaluation 1
• Initiate blood pressure control targeting <130/80 mmHg with ACE inhibitor or ARB if proteinuria present 2
• Evaluate and control cardiovascular risk factors including dyslipidemia 2

If repeat GFR normalizes or proteinuria is absent:

• The initial low GFR may represent measurement variability, dehydration, or medication effects 2
• Continue annual monitoring if risk factors for CKD are present (diabetes, hypertension, family history) 2

Important Caveats

• GFR estimates >60 mL/min/1.73 m² have reduced accuracy, and values just below 90 mL/min/1.73 m² require clinical context for interpretation 2, 7
• The MDRD equation used for GFR estimation has not been fully validated in healthy individuals or those with near-normal kidney function 2, 3
• Medication dosing adjustments are generally not required until GFR falls below 60 mL/min/1.73 m² unless using nephrotoxic agents 2