Accelerated platelet generation leads to shorter duration of aspirin effect and aspirin insensitivity in ET. In healthy individuals, suppression of serum TXB₂ levels (a marker of platelet activation) in response to a single dose of aspirin lasts for ∼3 days. However, platelet generation is increased in ET, which results in accelerated renewal of COX-1, the target for aspirin. Thus TXB₂ levels recover more rapidly. This effect can be overcome by shortening the dosing interval to once every 12 hours. bid, twice per day; qd, once per day.