  Quasars (QSOs) are extremely luminous active galatic nuclei currently
observed up to redshift $z=7.642$. As such, they have the potential to be the
next rung of the cosmic distance ladder beyond SNe Ia, if they can reliably be
used as cosmological probes. The main issue in adopting QSOs as standard
candles (similarly to Gamma-Ray Bursts) is the large intrinsic scatter in the
relations between their observed properties. This could be overcome by finding
correlations among their observables that are intrinsic to the physics of QSOs
and not artifacts of selection biases and/or redshift evolution. The
reliability of these correlations should be verified through well-established
statistical tests. The correlation between the ultraviolet (UV) and X-ray
fluxes developed by Risaliti \& Lusso is one of the most promising relations.
We apply a statistical method to correct this relation for redshift evolution
and selection biases. \textbf{Remarkably, we recover the the same parameters of
the slope and the normalization as Risaliti \& Lusso. Our results establish the
reliability of this relation, which is intrinsic to the QSO properties and not
merely an effect of selection biases or redshift evolution. Hence,} the
possibility to standardize QSOs as cosmological candles, thereby extending the
Hubble diagram up to $z=7.54$.
