The need for previously untreated patient population studies in understanding the development of factor VIII inhibitors.

The presence of inhibitory antibodies to factor VIII (FVIII) remains one of the most serious complications of haemophilia therapy. Accordingly, understanding risk factors that may contribute to inhibitor developments in young patients with haemophilia A continues to be an area of great interest. Previously untreated patient (PUP) population studies have been instrumental in understanding the aetiology of inhibitor development. These studies have revealed the importance of risk factors such as clotting factor exposure history, ethnicity, and FVIII genotype in the development of inhibitors, while also providing insights into potential risk factors that may be related to therapeutic practice. However, due to differences in study designs and patient populations among previous PUP studies, there are limitations to the value of these studies in deciphering the role of potential risk factors. Therefore, future PUP studies should be prospective, consistent in their study designs and consider all established parameters and also those that possibly may influence inhibitor formation, thereby facilitating a better understanding of the aetiology of inhibitor formation in haemophilia A patients.