What is the clinical significance of eosinophils (white blood cells) in urine?

Clinical Significance of Eosinophils in Urine

The presence of eosinophils in urine has poor specificity and sensitivity for diagnosing acute interstitial nephritis, with a positive predictive value of only 38% at best, and should not be relied upon as a standalone diagnostic test.

Diagnostic Value of Urinary Eosinophils

Test Characteristics

• Sensitivity: Studies show low sensitivity ranging from 30.8% to 63% depending on the staining method used 1, 2
• Specificity: Varies from 68.2% to 96% across studies 3, 1
• Positive predictive value: Only 15.6-50% depending on the cutoff and staining method 1, 2
• Negative predictive value: 83.7% at the 1% cutoff level 1

Clinical Conditions Associated with Eosinophiluria

Eosinophils in urine can be found in various conditions:

1. Acute interstitial nephritis (AIN) - particularly drug-induced (25% of eosinophiluria cases) 2
2. Urinary tract infections (both upper and lower) - account for approximately 45% of cases 4
3. Rapidly progressive glomerulonephritis 5
4. Acute prostatitis 5
5. Occasionally in acute cystitis 5
6. Postinfectious glomerulonephritis 5

Technical Considerations

Staining Methods

• Hansel's stain: Superior for detecting urinary eosinophils compared to Wright's stain 5, 2
  • Increases sensitivity for AIN detection (63% vs 25%)
  • Improves positive predictive value (50% vs 25%)
  • Eosinophils appear with bright red granules, making identification easier

Cutoff Values

• 1% cutoff: Higher sensitivity (30.8%) but lower specificity (68.2%) 1
• 5% cutoff: Lower sensitivity but improved specificity 1
  • When eosinophils are >5% of total urinary WBCs, they may be more valuable as a predictor of AIN 4

Interpretation Guidelines

1. Quantify properly: Express eosinophils as a percentage of total urinary white blood cells 4

   • 85% of patients with eosinophiluria have <5% urine eosinophils
   • 45% have <1% urine eosinophils

2. Consider pretest probability: Urinary eosinophils at the 1% cutoff do not meaningfully shift the pretest probability of AIN in either direction (likelihood ratios near 1.0) 1

3. Avoid overreliance: Do not use eosinophiluria as the sole criterion for diagnosing AIN 3, 1

4. Consider confirmatory testing: Kidney biopsy remains the gold standard for diagnosing AIN when clinically indicated 1

Common Pitfalls

1. Misinterpreting low-level eosinophiluria: The presence of <5% eosinophils has poor predictive value for AIN 4

2. Using Wright's stain alone: This method has variable uptake by urinary eosinophils and may miss their presence 5

3. Failing to consider other diagnoses: Multiple conditions can cause eosinophiluria, not just AIN 4, 5

4. Overestimating diagnostic accuracy: The false positive and negative rates are too high to confirm an AIN diagnosis based solely on eosinophiluria 3

In summary, while the finding of eosinophils in urine may raise suspicion for certain conditions like AIN, this test should be interpreted cautiously within the broader clinical context and not used as a definitive diagnostic criterion.