In a preoperative patient with a blood urea nitrogen of 5 mg/dL and a BUN/creatinine ratio of 5, should I be concerned about proceeding with surgery?

Low BUN and BUN/Creatinine Ratio in Preoperative Setting

A BUN of 5 mg/dL with a BUN/creatinine ratio of 5 is not a contraindication to surgery and does not indicate renal dysfunction requiring delay of the procedure. This pattern suggests low protein intake, overhydration, or low muscle mass rather than kidney disease, and perioperative guidelines focus on elevated—not low—BUN as a risk factor.

Understanding Your Laboratory Values

Your laboratory results show the opposite pattern from what increases surgical risk:

• Perioperative renal risk is defined by elevated creatinine ≥2 mg/dL, not low BUN, according to ACC/AHA guidelines for noncardiac surgery 1
• A creatinine of 1.0 mg/dL (calculated from BUN 5 ÷ ratio 5) indicates normal kidney function, as values <2 mg/dL are not associated with increased perioperative complications 1
• The normal BUN/creatinine ratio is 10-15:1, and your ratio of 5:1 is below normal, not elevated 2, 3

Why Low BUN Occurs

Low BUN typically results from benign conditions rather than kidney disease:

• Decreased protein intake or malnutrition commonly lowers BUN production, as urea is the end product of protein metabolism 2
• Overhydration or aggressive IV fluid administration dilutes BUN concentration disproportionately compared to creatinine 2
• Low muscle mass in elderly or debilitated patients reduces both creatinine production and protein turnover 3
• Liver disease can impair urea synthesis, though this would typically present with other clinical signs 2

Perioperative Risk Assessment

The ACC/AHA guidelines identify specific renal parameters that increase surgical risk—none of which apply to your case:

• Creatinine ≥2 mg/dL is the threshold for increased cardiac and renal complications after major noncardiac surgery 1
• Estimated GFR <60 mL/min/1.73 m² correlates with major adverse cardiovascular events, but your normal creatinine suggests GFR is adequate 4
• In patients >70 years undergoing cardiac surgery, creatinine >2.6 mg/dL dramatically increases dialysis risk, but this does not apply to normal creatinine values 1

What Actually Matters for Surgical Clearance

Focus your preoperative assessment on established risk factors rather than low BUN:

• Calculate creatinine clearance using the CKD-EPI or Cockcroft-Gault equation to confirm adequate renal function, as this provides more accurate assessment than creatinine alone 1, 4
• Assess nutritional status if BUN remains low, as serum albumin <2.5 g/dL and malnutrition increase perioperative complications 3
• Review volume status to determine if overhydration is contributing to dilutional low BUN, though this alone does not contraindicate surgery 2
• Evaluate for anemia with hematocrit, as values <28% are associated with increased perioperative ischemia and complications 1

Clinical Context Considerations

The significance of low BUN depends on your overall clinical picture:

• If you are elderly or have low muscle mass, a BUN of 5 mg/dL may simply reflect reduced protein turnover and is not concerning 3
• If you have been NPO or on clear liquids preoperatively, reduced protein intake naturally lowers BUN 2
• If you have received IV hydration, dilutional effects can lower BUN to 5 mg/dL without clinical significance 2

When Low BUN Would Be Concerning

Low BUN rarely indicates serious pathology, but consider these scenarios:

• Severe liver disease with impaired urea synthesis would present with jaundice, coagulopathy, or encephalopathy—not isolated low BUN 2
• Severe malnutrition with albumin <2.5 g/dL increases surgical risk independent of BUN level 3
• Syndrome of inappropriate antidiuretic hormone (SIADH) causes dilutional hyponatremia and low BUN, but requires hyponatremia for diagnosis 2

Proceed with Surgery

Your laboratory values do not warrant surgical delay. The perioperative literature consistently identifies elevated—not low—BUN and creatinine as risk factors for adverse outcomes 1, 4. Your normal creatinine indicates adequate kidney function for safe anesthesia and surgery. Address any underlying nutritional deficiencies or volume status issues as part of routine perioperative optimization, but do not postpone the procedure based on these values alone.