I've encountered several times in practice where I see first time diagnostic marrow biopsies morphologically consistent with PMF with marked reticulin fibrosis, but if CBC demonstrates cytopenias, and if splenomegaly is absent, the clinician questions whether this might be MDS with fibrosis. Although studies have shown there is interobserver variability in the evaluation of megakaryocyte atypia, I think the morphologic features of atypical megas in PMF or MPNs is specific enough that it can be reasonably distinguishable from MDS dysmegakaryopoiesis. I don't believe there is much of a basis to consider MDS with fibrosis as a significant differential diagnostic consideration when you have a marrow biopsy that looks like advanced fibrotic phase PMF with cytopenias. I was wondering if anyone else might have any opinions on this? Basically, if JAK2, CALR, MPL are negative, I will still top line as, "MPN, morphologically compatible with PMF" but lately I've felt the need to include a sentence in the comment, "MDS with fibrosis could be considered, although the cytologic features identified in this case would be unusual." Obviously I'm referring to cases where cytogenetic studies are negative for MDS defining abnormalties.