How would you approach an asymptomatic older female patient with eosinophilia to 17,000, present for years, and normal eosinophilia workup including marrow and negative FLIP1?

Interesting case. Eos have been in the 17K range for years? Was it incidentally noted? Could just be idiopathic HES. I would worry about cardiac infiltration in an older patient, but if there have never been cardiac issues and no evidence of a myeloid variant, I would probably defer to the patient about the risk/benefit of not treating vs. empiric Nucala to treat a # per se. I consider Nucala to be a low-side-effect drug, so I would probably err on the side of treating if the patient is amenable...

Define years?

If really present and stable for a decade with no organ infiltration. I would consider leaving it alone.