Two main studies in adults and a third one in children found Nplate effective for the treatment of long-standing ITP. All studies compared Nplate with placebo (a dummy treatment). Patients were treated for 24 weeks and the main measure of effectiveness was the increase in platelet count above a threshold of 50 million platelets per millilitre of blood during at least 6 of the last 8 weeks of treatment. A platelet count below 30 million per millilitre puts patients at risk of bleeding while the normal count is 150 to 400 million per millilitre.

The first study involved 63 patients whose disease was not controlled despite removal of their spleen. Platelet count rose above the threshold in 38% of patients who received Nplate (16 out of 42) compared with none of the 21 patients receiving placebo.

The second study involved 62 patients whose ITP had been treated previously (but who did not have their spleen removed). Platelet count rose above the threshold in 61% of patients who received Nplate (25 out of 41) compared with 5% receiving placebo (1 out of 21).

The study in children involved 62 patients aged 1 to less than 18 years whose ITP had been treated previously (including some whose spleen had been removed). Platelet count rose above the threshold in 52% of patients who received Nplate (22 out of 42) compared with 10% receiving placebo (2 out of 20).

Long-term studies involving over 1,000 patients, some treated for longer than 5 years, confirmed that Nplate remained effective both in patients whose spleen had been removed and those who had their spleen.