High platelet count: What does this mean?

Hello. My mom has suffered from ET for almost 20 years. She is currently taking HU (high dose) and aspirin. However, for some time she has stopped responding well to these drugs and her platelets do not drop below 1,200,000. What are the treatment methods for this disease in the US, what else can be done to help? In Poland, where we live, only HU and thromboreductin (anagrelide) are available, but my mother feels very bad after this (cardiovascular problems). What do genetic tests for mutations give? Does the presence of a mutation affect treatment?
I will be grateful for any answer.

It is important to go to hemotologist/oncologist. get a bone marrow biopsy/aspiration and many blood test to make sure what is cause of high platelets. Especially in older people high platelets can result in stroke or heart attack. Another treatment is Peginterferon alfa-2a (Pegasys®).......but most important to get a good diagnosis

My mom was well diagnosed almost 20 years ago and since then she has been under the constant care of a hematologist/oncologist. She also had a biopsy. She has blood tests done every 2 months and unfortunately they are very bad. She is also anemic and often has colds requiring antibiotic treatment.
Anyways thanks for the answer. My mom will ask the doctor about peginterferon.

It seems some people are taking both HU and a dose of Anagrelide at the same time. She could ask her hematologist about this possibility. How high a dose of HU does she take?
Best wishes, Eileen

Eileen, Tkanks for answer.
Yes, mum is considering this option (HU+anagrelid) - she will ask the doctor at the next visit, but she is afraid because anagrelide causes her heart palpitations and severe weakness. Currently she takes HU: 3 days/week: 1000 mg and 4 days/week: 1500 mg.
She also takes Milurit (allopurinol) - 100mg each day.