ET PT-1 risk stratification

Essential thrombocythaemia risk groups, as defined in MRC PT-1 trial

Risk groupAge (years)High risk features*
Low< 40Absent
Intermediate40 - 60Absent
High> 60, OR at least one high risk featureAt least one, OR age > 60
* High risk features:

  • Platelet count ≥ 1500 x109/L (current or previous)**
  • History of ischaemia, thrombosis or embolic events (including erythromelalgia)
  • Haemorrhage considered to be related to essential thrombocythemia
  • Presence of hypertension or diabetes requiring treatment

** if a borderline platelet count is the sole reason for allocation to a high-risk group, requires at least three separate blood count results taken over at least 2 months

This risk stratification system for essential thrombocythemia (ET or PT; also known as primary thrombocythemia, or primary or essential thrombocytosis) is that used in the Medical Research Council PT-1 clinical trial.1 The criteria for allocation of patients to the high-risk group is similar to that proposed by other groups, but there is less consensus on the criteria for low- and intermediate-risk groups.2

In the PT-1 trial, low-risk patients received aspirin alone. High-risk patients received aspirin and cytoreductive therapy. Intermediate-risk patients were randomised between aspirin alone or aspirin with cytoreductive therapy.3