Hi Steve, Welcome to Connect.
ET, or essential thrombocythemia is one of a group of blood disorders where there’s an overproduction of one or more types of blood cells called Myeloproliferative neoplasms. (MPN). Just flows off the tongue, right?! In your wife’s case, she has too many platelets.
Her hematologist ordered the testing to check for the potential cause for the over production. As you discovered in your research, the JAK2 acquired mutation accounts for more than 50% of the cases, though there are other possible mutations. Being positive for the JAK2 mutation is enough to diagnose the blood disorder.
Her hematologist/oncologist has her on the first line treatment for ET, hydroxyurea, to help control the proliferation of the cells and keep them at a more normal level. She’ll be having labs on a routine basis which will let you and her doctor know if her platelet level is going back down and then holding in a comfortable zone. If her doctor (and you, by watching her results) starts seeing trends elevating in spite of the meds, then her doctor may want to do a bone marrow biopsy to check on the health of her bone marrow. It’s not always necessary to diagnose a MPN.

There are so many sites online that offer information but they can lead you down some rabbit holes to articles that won’t apply to your wife’s condition. They tend to cause unwarranted anxiety. So here are a couple sites that have great information and are credible sources:

Very Well Health.com
https://www.verywellhealth.com/essential-thrombocythemia-2860907.
Leukemia & Lymphoma Society
https://www.lls.org/myeloproliferative-neoplasms/essential-thrombocythemia
Was your wife able to go ahead with her knee replacement?