Red Blood Cells in Urinalysis: Diagnostic Significance and Evaluation
The presence of red blood cells (RBCs) in urinalysis indicates hematuria, which requires risk stratification and appropriate evaluation to rule out potentially serious underlying conditions including urinary tract infection, urological malignancy, or glomerular disease. 1
Definition and Classification of Hematuria
- Microhematuria: Defined by the American Urological Association (AUA) as ≥3 RBCs per high-power field (RBC/HPF) in a properly collected urine sample 1
- Gross hematuria: Visible blood in urine, which carries a higher risk of underlying pathology
Clinical Significance and Risk Stratification
The AUA recommends classifying patients with microhematuria into risk categories:
| Risk Level | Criteria | Cancer Risk |
|---|---|---|
| Low | 3-10 RBC/HPF + Age <60 years (women) or <40 years (men) + Non-smoker or <10 pack-years | 0-0.4% |
| Intermediate | 11-25 RBC/HPF or Age 60+ (women)/40-59 (men) or 10-30 pack-years smoking | 0.2-3.1% |
| High | >25 RBC/HPF or Age 60+ (men) or >30 pack-years smoking | 1.3-6.3% |
Diagnostic Value of RBC Morphology
RBC morphology provides crucial information about the source of bleeding:
Isomorphic RBCs: Indicate non-glomerular origin (lower urinary tract)
- Uniform in size and shape
- Associated with conditions like urinary tract infections, kidney stones, or urological malignancies
- Present in 100% of bladder cancer cases with hematuria 2
Dysmorphic RBCs: Indicate glomerular origin (kidney disease)
- Irregular shape and size
- Found in 94% of samples from patients with chronic glomerulonephritis 3
- Associated with glomerular diseases like glomerulonephritis or vasculitis
Recommended Evaluation Approach
Initial Assessment:
- Confirm true hematuria by ruling out contamination (menstrual blood, exercise-induced hematuria)
- Urine culture to rule out infection 1
- If infection is present, treat and repeat urinalysis after 6 weeks
If hematuria persists:
- Complete blood count
- Serum creatinine and BUN
- Comprehensive urinalysis with microscopic examination 1
Further evaluation based on risk stratification:
Important Clinical Considerations
- Even with a positive urine culture, urologic malignancy may still be present 1
- Anticoagulant/antiplatelet therapy should not be assumed as the sole cause of hematuria 1
- Nephrology consultation is warranted if there are signs of glomerular disease (significant proteinuria >1,000 mg/24 hours, red cell casts, dysmorphic RBCs) 1
- The level of RBCs in urinary sediment can reflect disease activity in certain conditions like Wegener's granulomatosis 4
Follow-up Recommendations
For patients with persistent asymptomatic microscopic hematuria after negative initial evaluation:
- Low-risk patients: Annual urinalysis 1
- Intermediate/high-risk patients: Consider urine cytology and repeat urinalysis at 6,12,24, and 36 months 1
Common Pitfalls to Avoid
- Failing to distinguish between glomerular and non-glomerular hematuria
- Assuming anticoagulation therapy is the cause without proper evaluation
- Neglecting nephrology evaluation when there are signs of glomerular disease
- Overlooking the possibility of malignancy in patients with UTI and hematuria
- Relying solely on urine dipstick without microscopic confirmation of RBCs
Remember that RBC morphology analysis using automated urine flow cytometry has shown high sensitivity (100%) and specificity (91.7%) for detecting bladder cancer when isomorphic RBCs are present 2, making it a valuable diagnostic tool.