Top panel: comparison between the spectroscopic and photometric 2 keV monochromatic fluxes for 30 quasars randomly extracted from the SDSS–4XMM sample and the quasars of the z ≃ 3 sample (Nardini et al. 2019, marked with open star symbols). Bottom panel: difference between the spectroscopic and photometric 2 keV monochromatic fluxes as a function of redshift. The mean Δ log F_X = log F_X, spectro − log F_X, photo value and its 1σ dispersion are shown with the red and dashed lines, respectively. Points are colour-coded by the number of net counts. The Δ log F_X distribution is scattered around Δ log F_X ∼ 0 with no clear trend with redshift.