What are the urinalysis findings in IgA (Immunoglobulin A) vasculitis, especially red blood cell (RBC) morphology?

Urinalysis Findings in IgA Vasculitis

In IgA vasculitis with renal involvement, urinalysis typically shows microscopic hematuria with dysmorphic red blood cells and red cell casts, along with proteinuria that is usually moderate (1-3 g/day). 1

Red Blood Cell Morphology

Dysmorphic RBCs Indicate Glomerular Origin

• Dysmorphic RBCs are the hallmark finding in IgA vasculitis with renal involvement, indicating glomerular bleeding. 1
• Dysmorphic urinary RBCs show variation in size and shape with irregular or distorted outlines, distinguishing them from normal doughnut-shaped RBCs that originate from lower urinary tract bleeding. 1
• Red cell casts are virtually pathognomonic for glomerular bleeding, though they are relatively insensitive markers and may not always be present. 1
• Accurate determination of RBC morphology requires inverted phase contrast microscopy for optimal visualization. 1

Diagnostic Thresholds for RBC Morphology

• Greater than 75% dysmorphic RBCs confidently indicates renal (glomerular) hematuria, while less than 17% dysmorphic cells suggests non-renal hematuria. 2
• At the threshold of ≥25% dysmorphic RBCs, specificity is 96.3% and positive predictive value is 94.6% for glomerular disease, though sensitivity is only 20.4%. 3
• The presence of ≥25% dysmorphic RBCs is specific but not sensitive for glomerulonephritis. 3

Isomorphic RBCs May Indicate Severity

• Predominant isomorphic (non-dysmorphic) RBCs (≥70%) in ANCA-associated vasculitis correlates with more severe disease, including lower eGFR, higher disease activity scores, and increased risk of progression to end-stage kidney disease. 4
• Patients with ≥70% isomorphic RBCs had significantly higher rates of glomerular basement membrane fracture on biopsy (46.3% vs 22.9%). 4
• While this data is from ANCA vasculitis, it suggests that RBC morphology patterns may provide prognostic information beyond simple glomerular localization. 4

Additional Urinalysis Findings

Proteinuria

• Moderate proteinuria (1-3 g/day) is typical in IgA vasculitis with renal involvement. 1, 5
• Significant proteinuria is defined as total protein excretion greater than 1,000 mg per 24 hours, which should prompt thorough evaluation or nephrology referral. 1
• Proteinuria of 1+ or greater on dipstick should prompt 24-hour urine collection for quantitation. 1

Hematuria Characteristics

• Microscopic hematuria is the most common finding, often accompanied by dysmorphic RBCs and red cell casts. 1, 5
• Urine dipstick will be positive for blood in the setting of glomerular hematuria. 1
• The combined presence of hematuria (>10 RBCs/high-power field) and proteinuria performs well in predicting underlying glomerulonephritis. 3

Clinical Pitfalls and Caveats

Persistence Does Not Always Mean Active Disease

• While hematuria and proteinuria are present during active disease and can resolve completely, their persistence does not necessarily imply ongoing active disease. 1
• This is critical when assessing for relapse versus residual findings from prior injury. 1

When to Suspect Primary Renal Disease

• The presence of significant proteinuria (>1,000 mg/24 hours), red cell casts, renal insufficiency, or predominance of dysmorphic RBCs should prompt evaluation for renal parenchymal disease or nephrology referral. 1
• Elevated serum creatinine based on normal reference ranges for sex also warrants further evaluation. 1

Laboratory Requirements

• Proper specimen collection is essential—the recommended definition of microscopic hematuria requires three or more RBCs per high-power field on microscopic evaluation of two of three properly collected specimens. 1
• Benign causes must be excluded, including menstruation, vigorous exercise, sexual activity, viral illness, trauma, and infection. 1