Diagnosis: Pregnancy-Associated Atypical Hemolytic Uremic Syndrome (aHUS)
This clinical presentation is most consistent with pregnancy-associated atypical hemolytic uremic syndrome (aHUS), a complement-mediated thrombotic microangiopathy that likely occurred during the obstetric event and is now manifesting with persistent subclinical complement activation. 1, 2, 3
Key Diagnostic Features Supporting aHUS
The constellation of findings strongly points toward complement-mediated TMA rather than other etiologies:
Low-normal C3 (89) with normal C4 (21) indicates alternative complement pathway activation, which is the hallmark of aHUS and distinguishes it from classical pathway activation seen in immune complex disease 4, 5
Intermittent hemoglobinuria with normal LDH and haptoglobin represents subclinical, chronic complement-mediated hemolysis rather than acute intravascular hemolysis, consistent with ongoing low-grade complement activation 1, 6
Negative DAT excludes immune-mediated hemolysis, making autoimmune hemolytic anemia unlikely and supporting a non-immune microangiopathic process 1, 6
Low inflammatory markers (ESR 5, CRP 0.3) exclude active systemic inflammation, making secondary causes like autoimmune disease or infection less likely 5
Low-titer ANA (1:40) is not clinically significant and does not suggest lupus or other autoimmune TMA triggers 5
The obstetric history suggesting a missed major TMA event is critical, as pregnancy is a well-recognized trigger for unmasking aHUS, particularly in the postpartum period 2, 3, 7
Essential Immediate Workup
Order these tests urgently to confirm diagnosis and guide treatment:
ADAMTS13 activity level and inhibitor titer to definitively exclude TTP (ADAMTS13 <10% indicates TTP; >10% supports aHUS) 1, 6
Complete blood count with platelet count and peripheral blood smear to assess for thrombocytopenia and schistocytes, even if intermittent 1, 6
Lactate dehydrogenase, haptoglobin, and reticulocyte count to quantify hemolysis during symptomatic episodes 1, 8
Comprehensive metabolic panel with creatinine and urinalysis to assess renal involvement and proteinuria, as kidney injury is the primary organ manifestation in aHUS 1, 6
Complement testing: C3, C4, CH50, and alternative pathway functional assays to document complement dysregulation 1, 5
Genetic testing for complement regulatory protein mutations (CFH, CFI, CFB, CD46, C3, CFHR1-5) as up to 60% of aHUS patients have identifiable variants 3, 5
Anti-complement factor H antibodies to identify acquired complement dysregulation 4, 5
Critical Diagnostic Pitfall
Do not dismiss this diagnosis based on currently normal LDH and haptoglobin alone—these markers reflect acute intravascular hemolysis, but aHUS can present with chronic, low-grade complement activation causing intermittent hemoglobinuria without florid hemolysis markers 1, 6. The presence of 1+ hemoglobinuria between episodes indicates ongoing subclinical disease activity.
Management Algorithm
If ADAMTS13 >10% (Confirming aHUS):
Initiate eculizumab therapy immediately with dosing of 900 mg IV weekly for 4 weeks, then 1200 mg at week 5, followed by 1200 mg every 2 weeks 1, 9
Administer meningococcal vaccination (quadrivalent and serogroup B) before or concurrent with first eculizumab dose 1, 9
Start prophylactic penicillin or equivalent antibiotic and continue for at least 2 weeks after vaccination, then consider long-term prophylaxis 1, 9
Monitor daily CBC, platelet count, LDH, haptoglobin, and creatinine during acute phase to assess treatment response 1, 8
Avoid plasma exchange as it has no role in genetically-mediated aHUS and may delay appropriate complement-targeted therapy 6, 5
If ADAMTS13 <10% (Indicating TTP):
Immediately initiate therapeutic plasma exchange without waiting for confirmatory testing if clinical suspicion is high 1
Administer methylprednisolone 1g IV daily for 3 days starting after first plasma exchange 1
Platelet transfusion is contraindicated unless life-threatening bleeding occurs 1
Pregnancy-Specific Considerations
Pregnancy-associated aHUS typically manifests postpartum (as suggested by the obstetric history), with up to 60% of cases occurring in the postpartum period 2, 3
Genetic variants in complement regulatory proteins are found in approximately 60% of pregnancy-associated aHUS cases, making genetic testing essential for long-term management and family counseling 3, 5
Future pregnancies carry extremely high risk for recurrence (up to 80% in subsequent pregnancies), requiring preemptive eculizumab therapy throughout pregnancy and postpartum period 2, 3
Eculizumab is safe during pregnancy and should be continued or initiated if aHUS is diagnosed during gestation 3, 7
Monitoring and Long-Term Management
Continue eculizumab indefinitely in most cases, as discontinuation carries high risk of relapse, particularly in patients with identified genetic variants 9, 3
Monitor complement levels (C3, C4, CH50) every 3-6 months to assess disease activity 1, 5
Assess renal function regularly with eGFR and urinalysis as chronic kidney disease is a common sequela even with treatment 9, 6
Coordinate with nephrology and hematology for ongoing management and consideration of kidney transplantation if end-stage renal disease develops 6, 5