I'm 70, ET-CALR x 17 years, HU x 6 years. My platelets run up and down in 400s with occasional spikes over 500. Hemo considers that "very stable." Bet your levels will go down again next test. My guess is that you'd have to show at least three consecutive tests over 450 before docs felt meds needed adjustment. Like Ginger said, they're looking at trends, not one-time spikes. To be honest, with your numbers I'd be asking the doc if I could reduce my HU by 500 mg a few days per week.