Biologics such as risankizumab are currently the most effective treatment option for patients with moderate to severe psoriasis. But they are costly for healthcare systems and still described according to a 'one dose fits all' dosing regimen, leading to potential over-and undertreatment. In this study the investigators aim to investigate the predictive value of early serum trough levels of risankizumab and determine the therapeutic window of risankizumab in psoriasis patients.