When is a urea (blood urea nitrogen) plasma test ordered?

When to Order Blood Urea Nitrogen (BUN) Plasma Tests

Blood urea nitrogen (BUN) plasma tests should be ordered at least monthly in patients on hemodialysis, every 2-4 months in peritoneal dialysis patients, and whenever kidney function assessment is clinically indicated in other patients. 1

Indications for BUN Testing

Routine Monitoring in Dialysis Patients

• Hemodialysis patients:

  • Order monthly to assess dialysis adequacy 1
  • Paired pre-dialysis and post-dialysis BUN samples must be drawn at the same hemodialysis session 1
  • Used to calculate Kt/V and urea reduction ratio (URR) to measure delivered dose of hemodialysis 1

• Peritoneal dialysis patients:

  • Order every 4 months to assess total solute clearance 1
  • More frequent if residual kidney function is being considered as part of weekly clearance goals 1
  • If patient has >100 mL/d of residual kidney volume, collect 24-hour urine every 2 months 1

Clinical Indications in Non-Dialysis Patients

• Suspected kidney dysfunction:

  • Signs of volume overload or depletion
  • Medication use that may affect kidney function (ACE inhibitors, NSAIDs, diuretics) 2
  • Unexplained fatigue, nausea, or decreased appetite 1

• Monitoring known kidney disease:

  • To track progression of chronic kidney disease
  • To assess response to treatment interventions 2

• Dehydration assessment:

  • Elevated BUN/creatinine ratio >20:1 suggests dehydration 2
  • Direct measurement when clinical signs of dehydration are present

• Medication monitoring:

  • When using nephrotoxic medications
  • During diuretic therapy 2

Special Testing Considerations

Proper Collection Technique

• Hemodialysis patients:

  • Pre-dialysis: Draw immediately before dialysis without dilution 1
  • Post-dialysis: Use slow flow/stop pump technique to prevent sample dilution 1
  • Avoid the blood reinfusion sampling technique as it's less reproducible 1

• Timing considerations:

  • For peritoneal dialysis patients, collect samples when clinically stable and at least 1 month after peritonitis resolution 1
  • For non-dialysis patients, fasting samples are preferred for consistency

Clinical Interpretation Factors

• BUN should be interpreted alongside creatinine, as the BUN/creatinine ratio provides additional diagnostic information 2
• Consider factors that can elevate BUN independent of kidney function:
  • High protein diet
  • Gastrointestinal bleeding
  • Catabolic states
  • Certain medications

When to Increase Testing Frequency

• Dialysis patients:

  • After peritonitis episodes in peritoneal dialysis patients 1
  • When prescription changes are made 1
  • With unexplained clinical deterioration or failure to thrive 1
  • When wide variability in urea kinetic modeling results is observed 1

• Non-dialysis patients:

  • During acute illness affecting kidney function
  • After starting or adjusting medications affecting kidney function
  • With worsening symptoms of uremia 2

Pitfalls to Avoid

• Don't rely solely on BUN for assessing kidney function; always interpret with creatinine
• Avoid improper sampling techniques that can lead to inaccurate results, especially in dialysis patients 1
• Don't overlook non-renal causes of BUN elevation (high protein diet, GI bleeding, etc.)
• Remember that clinical signs and symptoms alone are not reliable indicators of dialysis adequacy 1

BUN testing is a fundamental component of kidney function assessment and dialysis adequacy monitoring, with established protocols for different patient populations and clinical scenarios.