What are the possible causes of Coombs‑negative hemolytic anemia in a young man with proteinuria?

Differential Diagnosis of Coombs-Negative Hemolytic Anemia with Proteinuria in a Young Male

Primary Consideration: Atypical Hemolytic Uremic Syndrome (aHUS)

The combination of Coombs-negative hemolytic anemia and proteinuria in a young male should immediately raise suspicion for atypical hemolytic uremic syndrome (aHUS), a complement-mediated thrombotic microangiopathy that requires urgent diagnostic evaluation and potentially life-saving treatment. 1

Diagnostic Criteria for aHUS

The diagnosis requires the simultaneous presence of three elements:

• Microangiopathic hemolytic anemia: Elevated LDH, reduced or absent haptoglobin, negative direct and indirect Coombs tests, and schistocytes on peripheral blood smear 1, 2
• Thrombocytopenia: Platelet count <150,000/mm³ or ≥25% drop from baseline 1, 2
• Acute renal injury: Hematuria, proteinuria, or elevated creatinine (≥1.5 mg/dL in individuals ≥13 years or ≥50% increase over baseline) 2

Critical caveat: Up to 50% of aHUS cases may not present with the complete triad at onset, but at least one laboratory abnormality is always detectable. 1, 2

Immediate Laboratory Workup Required

• Complete blood count with differential and peripheral blood smear to identify thrombocytopenia and schistocytes 2, 3
• Hemolysis markers: LDH (elevated), haptoglobin (reduced/absent), indirect bilirubin (elevated), reticulocyte count 1, 2, 3
• Direct and indirect Coombs tests (must be negative to confirm non-immune hemolysis) 1, 2
• Renal function: Creatinine, BUN, urinalysis for hematuria and proteinuria 1, 2
• ADAMTS13 activity (urgent): Must be obtained to exclude thrombotic thrombocytopenic purpura (TTP); activity <10% indicates TTP requiring immediate plasma exchange 4, 2, 3
• Complement testing: C3, C4, CH50, and AP50 to assess complement pathway activation 1, 2

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Secondary Differential Diagnoses

Thrombotic Thrombocytopenic Purpura (TTP)

Clinical presentation: Fever, microangiopathic hemolytic anemia, renal dysfunction, and fluctuating neurological signs characterize TTP. 1

• Distinguishing feature: ADAMTS13 activity <10% confirms TTP and mandates immediate plasma exchange and high-dose glucocorticoids 4, 3
• PLASMIC score: In adults, a score ≥5 indicates moderate/high risk for TTP and warrants immediate treatment while awaiting ADAMTS13 results 4
• Neurological manifestations: Confusion, coma, seizures, aphasia, hemiplegia may occur 1

Typical (STEC-Associated) Hemolytic Uremic Syndrome

Clinical presentation: Onset within 3 weeks after acute or bloody diarrhea 2

• Etiologic testing: Stool culture and Shiga toxin testing for E. coli O157 (most common serotype) 2
• Distinguishing feature: Diarrhea typically precedes HUS by 4-5 days; simultaneous onset of diarrhea and HUS suggests aHUS instead 1, 2
• Management: Supportive care only; plasma exchange offers no benefit and adds risk 4

Paroxysmal Nocturnal Hemoglobinuria (PNH)

Clinical presentation: Complement-mediated intravascular hemolysis, thrombophilia, and bone marrow failure 5

• Mechanism: Acquired mutation in PIG-A gene impairs expression of complement regulators CD55 and CD59 on blood cells 5
• Distinguishing features: Chronic hemoglobinuria, hemosiderinuria, thrombotic events (especially cerebral venous thrombosis), and pancytopenia 1, 5
• Diagnostic test: Flow cytometry to detect absence of GPI-anchored proteins on blood cells 6, 5

Hereditary Red Cell Enzyme Deficiencies

Pyruvate Kinase (PK) Deficiency:

• Clinical features: Chronic hemolysis with variable reticulocytosis, unremarkable red cell morphology (some anisocytosis/poikilocytosis), elevated unconjugated bilirubin, and disproportionately elevated serum ferritin 1
• Distinguishing feature: Red cell osmotic fragility can be normal or altered (not informative); diagnosis requires demonstration of decreased PK enzyme activity or PKLR gene mutations 1
• Proteinuria consideration: Not a typical feature; if present, consider concomitant renal disease 1

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency:

• Clinical presentation: Hemolysis triggered by oxidative stress (infections, certain drugs, fava beans) 7
• Distinguishing feature: Episodic rather than chronic hemolysis; bite cells and blister cells on smear 7

Autoimmune Hemolytic Anemia (AIHA) with False-Negative Coombs

Rare scenario: Flow cytometry may detect antierythrocytic autoantibodies when routine Coombs testing and gel filtration are negative 6

• Clinical context: More common in transplant patients or those with underlying autoimmune disorders 6
• Proteinuria consideration: May indicate concurrent glomerulonephritis from systemic lupus erythematosus or other autoimmune disease 1

Homocystinuria

Clinical presentation: Marfanoid appearance, ectopia lentis, mental retardation, seizures, and rapidly progressive arteriosclerotic vascular disease 1

• Mechanism: Enzyme deficiencies (cystathionine β-synthetase, homocysteine methyltransferase, or MTHFR) cause both arterial and venous thrombosis 1
• Proteinuria: May result from renal vascular thrombosis 1
• Diagnostic test: Elevated plasma homocysteine level 1

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Algorithmic Approach to Diagnosis

Step 1: Confirm Microangiopathic Hemolysis

• Obtain peripheral blood smear for schistocytes, LDH, haptoglobin, indirect bilirubin, and direct/indirect Coombs tests 2, 3
• If Coombs-positive: Consider AIHA; treat with prednisone 0.5-1 mg/kg/day 3
• If Coombs-negative with schistocytes: Proceed to Step 2

Step 2: Assess for Thrombotic Microangiopathy (TMA)

• Check platelet count and ADAMTS13 activity urgently 4, 2, 3
• If ADAMTS13 <10%: Diagnose TTP; initiate plasma exchange and high-dose glucocorticoids immediately 4, 3
• If ADAMTS13 normal: Proceed to Step 3

Step 3: Differentiate HUS Subtypes

• Obtain stool culture and Shiga toxin testing 2
• Assess timing of diarrhea relative to HUS onset 1, 2
• If recent bloody diarrhea (4-5 days prior): Likely STEC-HUS; provide supportive care only 4, 2
• If no diarrhea or simultaneous onset: Suspect aHUS; proceed to Step 4

Step 4: Evaluate for aHUS and Initiate Treatment

• If full TMA triad present (hemolysis + thrombocytopenia + renal injury): Start eculizumab or ravulizumab immediately without waiting for genetic confirmation 1, 3
• Send complement testing (C3, C4, CH50, AP50) and genetic testing (next-generation sequencing of CFH, CFHR1-5, C3, CD46, CFI, THBD, DGKE, CFB) 1, 2
• Critical pitfall: Do not delay complement inhibition while awaiting genetic results; 40-50% of aHUS patients have no identifiable mutation 1, 2

Step 5: Consider Alternative Diagnoses if TMA Excluded

• If hemolysis without thrombocytopenia or schistocytes: Evaluate for PNH (flow cytometry), hereditary enzyme deficiencies (PK or G6PD enzyme assays), or homocystinuria (plasma homocysteine level) 1, 7, 5
• If proteinuria disproportionate to hemolysis: Consider primary glomerular disease with secondary hemolysis or systemic disease (lupus, antiphospholipid syndrome) 1

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Critical Management Considerations

Urgent Treatment Thresholds

• aHUS with confirmed TMA triad: Initiate eculizumab within 4-8 hours of diagnosis; delays increase morbidity and mortality 1
• TTP with PLASMIC ≥5 or ADAMTS13 <10%: Start plasma exchange immediately; each hour of delay increases mortality 4

Vaccination and Prophylaxis for Complement Inhibition

• Before eculizumab/ravulizumab: Administer quadrivalent meningococcal conjugate vaccine (A, C, W, Y) and serogroup B vaccine 1
• Long-term prophylaxis: Penicillin (or macrolides for penicillin-allergic patients) for the duration of complement inhibitor treatment 1

Monitoring Protocol

• Daily monitoring of hemoglobin, platelets, LDH, creatinine, and electrolytes during acute phase 2
• Neurological assessment: Obtain neurology consultation, EEG, and brain MRI if neurological symptoms develop (occurs in 10-20% of aHUS cases) 1, 3

Common Pitfalls to Avoid

• Do not start eculizumab without laboratory-confirmed active TMA: Complement inhibition carries serious infection risk (meningococcal disease) and requires lifelong vaccination; initiating therapy based solely on clinical suspicion lacks proven benefit 1, 2
• Do not use plasma exchange for STEC-HUS or drug-induced TMA: It offers no benefit and adds procedural risk 4
• Do not assume normal complement levels exclude aHUS: Baseline C3, C4, CH50, and AP50 are typically normal regardless of disease activity; they are useful for monitoring but not acute diagnosis 2
• Do not delay ADAMTS13 testing: It must be obtained urgently to distinguish TTP (which requires immediate plasma exchange) from HUS 4, 2, 3