PT1 is the largest ever study involving patients with the myeloproliferative disorder essential or primary thrombocythaemia (ET or PT). Well over a thousand patients have been enrolled to date and the study is recruiting throughout the UK, Northern Ireland and sites in Australia, New Zealand and France. The original protocol included high, intermediate and low risk arms. The high risk arm was a randomisation between hydroxyurea (also known as hydroxycarbamide) plus aspirin versus anagrelide plus aspirin. It closed in 2004 and the results, demonstrating the superiority of hydroxyurea plus aspirin, were published. The intermediate risk arm was a randomisation between hydroxyurea plus aspirin and aspirin alone. This arm closed in July 2012, with patients being followed up for a further year, when the results will be analysed and published. The low risk arm of the trial is still open to recruitment until April 2014. We are also collecting data for pregnancy in ET, and a link to further information is provided here: http://www.haem.cam.ac.uk/primary-thrombocythaemia/pregnancy-substudy/.

As well as guiding therapy we have been able to use clinical and biological samples (blood and bone marrow) collected from the study to make important observations about the impact of JAK2 V617F and MPL mutations; prognostic factors in ET; and lastly the difficulty in applying the current WHO histological criteria to ascertain the correct diagnosis.