Urinalysis and Microscopy Findings in Interstitial Nephritis
In a patient with interstitial nephritis, urinalysis typically shows white blood cells (pyuria), white blood cell casts, and often hematuria with red blood cells, while microscopy may reveal eosinophiluria (though this is neither sensitive nor specific), and the sediment is generally characterized by the presence of inflammatory cells rather than the dysmorphic RBCs and red cell casts seen in glomerulonephritis. 1, 2
Key Urinalysis Findings
White Blood Cells and Casts
- Pyuria is a hallmark finding, with white blood cells present on microscopic examination 3, 2
- White blood cell casts are the most diagnostically significant finding when present, indicating tubular inflammation 1, 2
- The presence of WBCs helps distinguish interstitial nephritis from other causes of acute kidney injury 1
Red Blood Cells
- Hematuria is commonly present but typically shows normal-appearing (isomorphic) RBCs rather than dysmorphic RBCs 3, 1
- Dysmorphic RBCs (>80% dysmorphic) suggest glomerulonephritis rather than interstitial nephritis 1
- Hematuria was documented in 40% of interstitial nephritis cases in one series 3
Eosinophiluria
- Urine eosinophils may be present but provide only suggestive evidence and cannot reliably confirm or exclude the diagnosis 2
- This finding is neither sensitive nor specific for acute interstitial nephritis 2
Proteinuria Patterns
Typical Presentation
- Proteinuria is usually mild to moderate (<1 g/day in most cases) 3, 2
- When present, proteinuria reflects tubular dysfunction rather than glomerular pathology 4
Atypical Presentations
- Moderate-to-heavy proteinuria (>1 g/day) is uncommon but can occur in interstitial nephritis 3
- Nephrotic-range proteinuria may be seen in rare cases, likely due to cytokine-mediated increased permeability from inflammatory cells in the interstitium 3
- When heavy proteinuria occurs without associated glomerulonephritis, it represents an unusual presentation 3
Distinguishing Features from Other Conditions
Versus Glomerulonephritis
- Interstitial nephritis lacks dysmorphic RBCs and red blood cell casts that characterize glomerular disease 1
- Nephrologist-performed microscopy is 100% accurate in distinguishing acute tubular injury and glomerulonephritis when compared to biopsy 1
- The sediment in interstitial nephritis shows inflammatory cells rather than the glomerular bleeding pattern 1
Versus Acute Tubular Injury
- Interstitial nephritis shows more prominent WBCs and WBC casts compared to the granular casts and renal tubular epithelial cells typical of acute tubular injury 1
- The inflammatory component is more pronounced in interstitial nephritis 1, 2
Clinical Context and Associated Findings
Classic Triad (Often Absent)
- Fever, rash, and arthralgias may be absent in up to two-thirds of patients with acute interstitial nephritis 2
- The absence of these classic findings does not exclude the diagnosis 2
Renal Function
- Acute kidney injury is the typical presentation, with elevated serum creatinine 2, 5
- Oliguria or anuria may occur but is more common in reversible cases 5
Important Diagnostic Limitations
Laboratory Findings Are Nonspecific
- Apart from medication exposure history, there are no specific urinalysis findings that definitively distinguish interstitial nephritis from other causes of acute renal failure 2
- Complement levels are typically normal, and autoantibodies (ANA, anti-dsDNA, ANCA) are negative 3
Gold Standard Diagnosis
- Renal biopsy remains the gold standard for definitive diagnosis of acute interstitial nephritis 2
- Biopsy may show granulomatous interstitial nephritis, nongranulomatous inflammation, or associated small vessel vasculitis 3
- Histologic features including tubular atrophy, interstitial granuloma, and pronounced interstitial cell infiltration indicate chronicity and worse prognosis 5