Is the Hubble diagram of quasars in tension with concordance cosmology?
  Recently Risaliti \& Lusso [Nature Astron. 3 (2019) 3 272] reported new
measurements of the expansion rate of the Universe by constructing the Hubble
diagram of 1598 quasars in the redshift range $0.5<z<5.5$. It is claimed a
$4\sigma$ tension with the standard concordance $\Lambda$CDM concerning both
the fractionary matter density $\Omega_{m0}$ and the dark energy equation of
state parameter $w_{de}$ standard values. In this work we promote an
independent analysis of the same data set using a model-independent estimator
for cosmic acceleration. Our results corroborate that the source of such
tension can be related to the $\Omega_{m0}$ value with a reasonable indication
of a higher $\Omega_{m0}$ value ($\Omega_{m0} \gtrsim 0.4$). On the other hand,
we find that the role played by $w_{de}$ on the claimed tension is weak. We
also discuss the use of this estimator as a "quality tool" to test the
robustness of Hubble diagrams. We conclude claiming that the Quasars data can
not yet be seen as a reliable cosmological tool since they can not even state
the universe experienced an accelerated expansion phase.
