Would you offer BM biopsy as the next step for progressive thrombocytosis when peripheral blood is negative for JAK2, CALR, and MPL mutations for MPN diagnosis?

Yes, definitely. Always need bone marrow morphology to diagnose MPNs. Triple-negative ET or PMF comprises 5-10% of all ET and PMF and lacks the 3 canonical driver mutations, i.e., in the JAK2, CALR, and MPL genes.