X-ray luminosity versus bolometric luminosity for optically selected QSOs at z < 6 (collected from Lusso et al. 2012; Martocchia et al. 2017; Nanni et al. 2017; Salvestrini et al. 2019), and z > 6 QSOs (Connor et al. 2019, 2020; Vito et al. 2019, 2021; Pons et al. 2020; Wang et al. 2021a). Downward-pointing triangles represent upper limits for undetected objects. We used the updated value of L_bol for the 2021 epoch. The dashed black line represents the best-fitting relation of Duras et al. (2020). The location of J1641+3755 in 2018 and the upper limit in 2021 are marked in red.