When do you consider plasma exchange for multiorgan failure in a patient with sickle cell disease?  

The first choice for treating multiorgan failure in sickle cell disease is exchange transfusion that should be started as soon as possible. Sometimes the clinical and hematologic features of sickle cell multiorgan failure resemble and overlap those of thrombotic microangiopathies like TTP. Also, tru...

I agree and plasma exchange in addition to erythrocytapheresis and RBC exchange has been advocated by numerous scholars (see for instance Ballas, PMID 28470719) for cases with features of thrombotic microangiopathy.