Which result indicates impaired renal function?

Identifying Renal Dysfunction in Laboratory Values

Creatinine clearance of 44 mL/min (option C) is the result that indicates impaired renal function in this patient.

Understanding Normal vs. Abnormal Laboratory Values

When evaluating renal function, it's important to understand the normal ranges for common laboratory tests:

• Uric acid = 5 mg/dL (normal range: 3.5-7.2 mg/dL for men) 1
• BUN = 15 mg/dL (normal range: 7-20 mg/dL) 1
• Plasma creatinine = 1 mg/dL (normal range: 0.7-1.3 mg/dL for men) 1
• Creatinine clearance = 44 mL/min (normal range: >60 mL/min) 2

Classification of Renal Dysfunction

According to the National Kidney Foundation guidelines, chronic kidney disease is classified into stages based on GFR 2:

• G1: ≥90 mL/min/1.73m² (Normal or high)
• G2: 60-89 mL/min/1.73m² (Mildly decreased)
• G3a: 45-59 mL/min/1.73m² (Mildly to moderately decreased)
• G3b: 30-44 mL/min/1.73m² (Moderately to severely decreased)
• G4: 15-29 mL/min/1.73m² (Severely decreased)
• G5: <15 mL/min/1.73m² (Kidney failure)

The patient's creatinine clearance of 44 mL/min falls into stage G3b (moderately to severely decreased), indicating significant renal impairment 2.

Why Creatinine Clearance is More Sensitive Than Serum Creatinine

Serum creatinine alone is an inadequate screening test for renal dysfunction, especially in certain populations:

• Normal serum creatinine can mask significant renal impairment 3, 4
• Up to 46.4% of critically ill patients with normal serum creatinine can have creatinine clearance <80 mL/min/1.73m² 3
• In elderly patients, serum creatinine has a sensitivity of only 12.6% for detecting renal failure 5

In this case, the patient's serum creatinine (1 mg/dL) is within normal limits but doesn't reflect the actual reduction in kidney function shown by the creatinine clearance.

Clinical Implications for This Patient

This 39-year-old male with moderate renal impairment has several risk factors:

1. Acetaminophen use: Taking 4-10 extra-strength Tylenol daily (potentially 2000-5000 mg/day) can be nephrotoxic, especially with reduced kidney function 6

2. Alcohol consumption: 3-4 rum drinks daily with increased weekend intake adds additional nephrotoxic burden

3. Clinical presentation: Nausea, vomiting, and scleral icterus suggest liver dysfunction, which often accompanies kidney injury in acetaminophen toxicity

Management Considerations

Given the patient's reduced creatinine clearance:

• Acetaminophen should be discontinued immediately
• Medication dosages may need adjustment based on reduced renal function
• N-acetylcysteine may be considered if acetaminophen toxicity is suspected 6
• Alcohol cessation should be strongly recommended
• More frequent monitoring of renal function is indicated (every 3-4 months) 1

Common Pitfalls to Avoid

1. Relying solely on serum creatinine: Normal serum creatinine values can mask significant renal dysfunction 4, 5

2. Overlooking creatinine clearance: This is a more sensitive indicator of renal function than isolated creatinine measurements 3

3. Failing to recognize early kidney dysfunction: Early intervention can prevent progression to more severe kidney disease 2, 1

4. Ignoring alcohol and acetaminophen as nephrotoxic agents: Both can contribute to kidney injury, especially when combined

In conclusion, while the other laboratory values (uric acid, BUN, and plasma creatinine) are within normal ranges, the creatinine clearance of 44 mL/min clearly indicates moderate to severe renal impairment that requires clinical attention.