A middle aged man found down presenting w/ AMS, ARF, and microangiopathic hemolytic anemia. He was initially presumed to have TTP and emergently received plasmapheresis; several days later, the ADAMSTS13 test actually returned normal, and the diagnosis of atypical HUS was made. He was ultimately discharged on eculizumab.
*Take away points 1) TTP is the most life-threatening, don’t miss dx among the MAHAs; 2) Very high creatinine elevations above 5 or so are more consistent with HUS than TTP, however most of these patients will receive plasmapheresis until diagnostic tests can rule out TTP; 3) Atypical HUS is a complement-mediated MAHA, and treatment with eculizumab impairs the complement cascade, hence when starting treatment these patients are at a VERY high risk for meningococcal disease and should be vaccinated + receive abx prophylaxis*
Diagnostic algorithm in differentiating TTP and HUS:
1) If initial presentation is consistent with TTP (MAHA including elevated LDH, schistos + renal failure, AMS) –> emergent plasmapheresis
2) If ADAMSTS13 returns normal, dx is likely HUS –> stop plasmapheresis + supportive care
3) If no history of preceding GI illness, dx is likely atypical HUS –> eculizumab