Why would a 16-year-old female with congenital Pelvic Ureteric Junction (PUJ) obstruction and normal Diethylene Triamine Pentaacetate (DTPA) scan results have proteinuria?

Proteinuria in PUJ Obstruction with Normal DTPA Findings

Proteinuria in this 16-year-old with congenital PUJ obstruction and reassuring DTPA findings is unlikely to be directly caused by the PUJ obstruction itself, and you should investigate alternative causes of proteinuria including glomerular disease, tubular dysfunction from other etiologies, or unrelated renal pathology.

Understanding the Clinical Disconnect

The presence of proteinuria in this clinical scenario is paradoxical given the imaging findings:

• Equal split function (50/50) indicates preserved bilateral renal function, which argues against significant chronic obstructive nephropathy from the PUJ obstruction 1
• Preserved drainage with furosemide suggests the obstruction is not functionally significant at this time, as truly obstructed systems retain radiotracer and cannot increase clearance rate through the narrowed segment 1
• PUJ obstruction typically causes proteinuria only when there is significant functional impairment (differential renal function <40%) or progressive deterioration (>5% decline on serial scans), neither of which is present here 1

Mechanisms of Proteinuria in Obstructive Uropathy

When PUJ obstruction does cause proteinuria, it occurs through specific pathophysiologic mechanisms:

• Tubular dysfunction from chronic obstruction manifests as increased fractional excretion of sodium and chloride, which correlates with poor renal outcomes 2
• Glomerular injury from sustained increased intrapelvic pressure leads to glomerular hyperfiltration in the contralateral kidney and eventual proteinuria, but this requires significant obstruction 2
• The intermittent nature of vascular PUJ obstruction (from crossing vessels) typically preserves function despite hydronephrosis, as the obstruction is not continuous 3

Alternative Explanations to Investigate

Given the normal DTPA findings, you must pursue other causes:

• Primary glomerular disease (IgA nephropathy, focal segmental glomerulosclerosis, membranous nephropathy) should be considered, particularly in an adolescent female with isolated proteinuria
• Orthostatic proteinuria is common in adolescents and should be excluded with first-morning urine protein measurements
• Tubular proteinuria from unrelated causes including medications, infections, or metabolic disorders
• Occult vesicoureteral reflux can coexist with PUJ obstruction and cause tubular injury, though this would typically show on imaging 1

Diagnostic Approach

Your immediate workup should include:

• Quantify the proteinuria with 24-hour urine collection or spot urine protein-to-creatinine ratio to determine severity
• Characterize the proteinuria with urine protein electrophoresis to distinguish glomerular (albumin-predominant) from tubular (low molecular weight proteins) patterns
• Assess for glomerular disease with serum complement levels, ANA, anti-dsDNA, and ANCA if glomerular proteinuria is confirmed
• Evaluate the contralateral kidney carefully with ultrasound to exclude compensatory hyperfiltration injury or unrecognized pathology
• Consider renal biopsy if proteinuria is significant (>1 g/day) or if there are other features suggesting glomerular disease (hematuria, hypertension, declining GFR)

Critical Pitfall to Avoid

Do not assume the PUJ obstruction is causing the proteinuria simply because it is the known anatomic abnormality 3, 2. The DTPA findings argue strongly against functionally significant obstruction. Delaying investigation of alternative causes could miss treatable glomerular disease that requires immunosuppression.

When PUJ Obstruction Would Cause Proteinuria

For completeness, PUJ obstruction would be expected to cause proteinuria when:

• Differential renal function falls below 40% on the affected side 1, 2
• Serial scans show >5% functional decline indicating progressive injury 1
• T1/2 drainage time exceeds 20 minutes despite furosemide, confirming true obstruction 1, 4
• Bilateral disease or solitary kidney leads to overall renal functional compromise 4

None of these criteria are met in this patient based on the described DTPA findings.