Anti-romiplostim binding and neutralizing antibodies among patients enrolled in clinical trials. Pediatric patients (age <18 years at screening) with ITP who received 1 or more doses of romiplostim in any of the ITP studies (NCT00515203, NCT00116688 [extension], NCT01444417, NCT01071954 [extension], and NCT02279173) are included in the analysis. Patient samples taken at baseline and at scheduled intervals, typically before and after treatment in shorter trials and every 12 to 24 weeks in longer trials, were screened for anti-romiplostim binding antibodies first and then those that tested positive for binding antibodies were tested for anti-romiplostim neutralizing antibodies. *Negative result at the patient’s last time point tested. ^†Positive result at the patient’s last time point tested.