Dear Renee,

The 2016 WHO classification and diagnostic criteria for myeloproliferative neoplasms always includes a bone marrow biopsy to distinguish between ET/PV and PMF.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC58...

There is no other way to know other than a bone marrow biopsy, because PMF has a pre-fibrotic stage that can mimic ET, from what I could read.
"Prefibrotic PMF (Pre-PMF), a disease state that mimics ET but shows additional abnormalities in the granulocytic lineage, has a risk of progressing to overt PMF or AML in 15.2% or 4.7%, respectively [16,17]."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC87...

Best regards,
Tatiana