148 Chapter 8 assessment schedule as the Intervention Group, except for Visit 2. These participants will be followed for 12 months in total to collect additional data on bleeding according to emicizumab concentrations. Participants with emicizumab Ctrough concentrations <25 µg/mL at Visit 1 will be monitored closely by their treating physician, and may receive increased emicizumab dosing at the discretion of their treating physician. Since treatment of these PwHA (emicizumab Ctrough <25 µg/mL) is outside the scope of this study (i.e., dose reduction intervention), these PwHA will not perform Visit 2 through Visit 4. However, to follow-up on safety, we will continue to collect available selective safety data, such as bleed assessment and the presence of anti-drug antibodies (ADA), for these PwHA for a period of 12 months. The study is designed with two age-based cohorts consisting of participants aged ≥16 years (Cohort 1) and aged <16 years (Cohort 2). After a total of 25 participants from Cohort 1 have completed six months of follow-up on PK-guided dosing, data on bleed control will be analysed and the power calculation will be repeated to provide a more precise estimate of the number of participants required for the study. Data of this interim analysis will be reviewed by the Scientific Advisory Committee (NVHB and Steering Committee). Enrolment of paediatric participants of Cohort 2 can start if bleed control in these 25 participants of Cohort 1 participants is good, according to definitions in the inclusion criteria, combined with a maximum of one additional bleed in six months. Discontinuation of study Criteria for discontinuation are withdrawal of consent (at any time for any reason), development of a medical condition that precludes participation and/or is associated with increased bleeding risk (e.g., other bleeding disorders), formation of neutralizing or clearing ADA against emicizumab, occurrence of a spontaneous joint- or muscle bleed or >2 treated bleeds during 6 months as determined by the investigator or haemophilia treating physician, persistent non-adherence to protocol requirements, or loss to followup. Withdrawn participants during the Bleeding Assessment Phase may be replaced to reach the required sample size. All efforts will be made to complete and report the protocol-defined study observations up to the time of the participant’s withdrawal as completely as possible. No further data will be collected after the moment of withdrawal, except for withdrawal due to ADAs after which selective safety data will be collected, and discontinuation of emicizumab treatment, after which one last contact moment will be arranged. Participant recruitment and retention As haemophilia is a lifelong condition, PwHA remain in care at their haemophilia treatment centers. Recruitment is based on both information provided in ongoing conversations with their haemophilia treating team and information provided by the patient society (NVHP) which is represented in the Steering Committee. No specific measures for patient retention during follow up are in place.