Patients with PNH and thrombosis usually have a relatively large PNH clone. Therefore, most PNH patients with thrombosis will have clinically apparent evidence of intravascular hemolysis. Patients with primarily or exclusively PNH type II cells, however, may represent an exception. In this case, patients could have a large PNH clone with only subtle clinical evidence of spontaneous hemolysis. Arterial thrombosis has been observed in patients with PNH but is uncommon compared with venous thrombosis.

Patients with aplastic anemia should be screened even in the absence of evidence of intravascular hemolysis to identify those with subclinical PNH (PNH-sc/aplastic anemia). Screening of patients with refractory anemia-MDS is also recommended even in the absence of clinical evidence of hemolysis. Routine screening of patients with other forms of MDS or with myeloproliferative disease that have no evidence of intravascular hemolysis is not recommended outside of a research setting.