The aim of this study is to evaluate the efficacy of romiplostim administered at every other week in ITP patients who attained stable platelet counts ≥ 50 x 109/L for 4 consecutive weeks after weekly doses of romiplostim.

Regarding the determination of appropriate dose, it is well known that romiplostim resulted in dose-dependent increases in platelet counts, but required dose for raising platelet count is variable between individuals according to each patient's ability to eliminate serum romiplostim. Because of this variability, after administration of initial 1mcg/kg of romiplostim, dose adjustment is recommended based on the platelet response to given dose of romiplostim. However, the optimal dose interval of romiplostim has rarely been studied. In pivotal studies, romiplostim was administered on a weekly schedule, and based on these studies, one dose at every week is being widely used. In one study in which ITP patients received weekly subcutaneous doses of romiplostim at a range from 3 to 15mcg/kg, half life of romiplostim was estimated from 1 to 34 days, suggesting that lengthening the interval between doses more than a week can be potentially possible in some patients.

Inclusion Criteria:

1. Subject has an ability to provide written informed consent prior to participating to the study
2. Subject is male or female ≥ 18 years of age
3. Subject has a diagnosis of immune thrombocytopenia (ITP) per the American Society of Hematology guidelines.
4. Subject received at least 1 prior therapy for ITP including corticosteroids, immunoglobulin, or splenectomy, but had an insufficient response to any of these treatment (PLT count should be < 30x109/L before romiplostim initiation.)
5. If subject had ever received TPO receptor agonist before, the patient can participate this study only after 8 weeks' wash out period

Exclusion Criteria:

1. Subject has a history of hematological malignancy, myeloproliferative disorder, myelodysplastic syndrome (MDS), or bone marrow stem cell disorder
2. Subject has participated in any study evaluating PEG-rHuMGDF, recombinant human thrombopoietin (rHuTPO), or related platelet product within 8 weeks
3. Subject has a known hypersensitivity to any recombinant E coli-derived product
4. Subject has received any therapeutic drug or device that is not approved by the local regulatory health agency for any indication within 4 weeks of Screening
5. Subject is of reproductive potential and is not using adequate contraceptive precautions, in the judgment of the investigator
6. Subject is pregnant or breast feeding