The adverse event (AE) severity grading scale used was the Common Terminology Criteria for Adverse Events (CTCAE) version 3.0 grading scale, where Grade 1 = Mild AE; Grade 2 = Moderate AE; Grade 3 = Severe AE; Grade 4 = Life-threatening or disabling AE; Grade 5 = Death related to AE. A serious adverse event was defined as an adverse event that met at least one of the following serious criteria: * fatal * life threatening (places the subject at immediate risk of death) * required in-patient hospitalization or prolongation of existing hospitalization * resulted in persistent or significant disability/incapacity * congenital anomaly/birth defect * other medically important serious event. The investigator assessed whether each adverse event was possibly related to the investigational product.

Exposure adjusted rate was defined as the total number of events divided by the duration of time participants were under observation. The adverse event (AE) severity grading scale used was the Common Terminology Criteria for Adverse Events (CTCAE) version 3.0 grading scale, where Grade 1 = Mild AE; Grade 2 = Moderate AE; Grade 3 = Severe AE; Grade 4 = Life-threatening or disabling AE; Grade 5 = Death related to AE. A serious adverse event was defined as an adverse event that met at least one of the following serious criteria: * fatal * life threatening (places the subject at immediate risk of death) * required in-patient hospitalization or prolongation of existing hospitalization * resulted in persistent or significant disability/incapacity * congenital anomaly/birth defect * other medically important serious event. The investigator assessed whether each adverse event was possibly related to the study drug.

Two validated assays were used to test for antibodies to romiplostim / the thrombopoietin-mimetic peptide component of romiplostim (TMP). The first was an immunoassay to confirm the presence of antibodies. The second was a cell-based bioassay to detect neutralizing or inhibitory effects in vitro. If a sample was positive in both assays, a participant was defined as positive for neutralizing antibodies. Transient antibodies are those positive post-baseline but negative at the last time point tested. Persistent antibodies were those positive at the last time point tested.

Two validated assays were used to test for antibodies to endogenous thrombopoietin (TPO). The first was an immunoassay to confirm the presence of antibodies. The second was a cell-based bioassay to detect neutralizing or inhibitory effects in vitro. If a sample was positive in both assays, a participant was defined as positive for neutralizing antibodies. Transient antibodies are those positive post-baseline but negative at the last time point tested. Persistent antibodies were those positive at the last time point tested.