For patients with essential thrombocythemia already on prophylactic dose DOACs, do you defer starting aspirin?

I usually defer aspirin in these situations. There is evidence from PV that aspirin plus anticoagulation increases bleeding risk (Zwicker et al., PMID 34162181). If an ET patient has a high-risk disease with prior arterial thrombosis, then I would favor adding aspirin.

I agree with Dr. @Dr. First Last. Occasionally, patients have microcirculatory symptoms off aspirin and for those patients, both DOAC and aspirin are given, but as Dr. Tremblay mentioned, that is associated with increased bleeding risk.