Do quasar X-ray and UV flux measurements provide a useful test of
  cosmological models?
  The recent compilation of quasar (QSO) X-ray and UV flux measurements include
QSOs that appear to not be standardizable via the X-ray luminosity and UV
luminosity ($L_X-L_{UV}$) relation and so should not be used to constrain
cosmological model parameters. Here we show that the largest of seven
sub-samples in this compilation, the SDSS-4XMM QSOs that contribute about 2/3
of the total QSOs, have $L_X-L_{UV}$ relations that depend on the cosmological
model assumed and also on redshift, and is the main cause of the similar
problem discovered earlier for the full QSO compilation. The second and third
biggest sub-samples, the SDSS-Chandra and XXL QSOs that together contribute
about 30% of the total QSOs, appear standardizable, but provide only weak
constraints on cosmological parameters that are not inconsistent with the
standard spatially-flat $\Lambda$CDM model or with constraints from
better-established cosmological probes.
