  It has been known for over three decades that the monochromatic X-ray and UV
luminosities in quasars are correlated, though non-linearly. This offers the
possibility of using high-z quasars as standard candles for cosmological
testing. In this paper, we use a recently assembled, high-quality catalog of
1598 quasars extending all the way to redshift ~6, to compare the predictions
of the R_h=ct and LCDM cosmologies. In so doing, we affirm that the parameters
characterizing the correlation depend only weakly on the chosen cosmology, and
that both models account very well for the data. Unlike LCDM, however, the
R_h=ct model has no free parameters for this work, so the Bayesian Information
Criterion favours it over LCDM with a relative likelihood of ~88 % versus ~10
%. This result is consistent with the outcome of other comparative tests, many
of which have shown that R_h=ct is favoured over the standard model based on a
diverse range of observations.
