Diagnostic approach to hypocellular marrow failure based on severity. (A) Diagnostic approach to severe hypocellular marrow failure requiring therapy—time-limited. (B) Diagnostic approach to nonsevere hypocellular marrow failure not requiring therapy—not time-limited. Time-limited indicates the urgency to get the workup completed before starting therapy. BM, bone marrow aspirate; PB, peripheral blood; PNH, paroxysmal nocturnal hemoglobinuria.