  We analyze the validity of optical diagnostic diagrams relying on
emission-lines ratios and in the context of classifying Active Galactic Nuclei
(AGNs) according to the cosmic metallicity evolution in the redshift range 0 <
z < 11.2. In this regard, we fit the results of chemical evolution models
(CEMs) to the radial gradients of the N/O abundances ratio derived through
direct estimates of electron temperatures (Te-method) in a sample of four local
spiral galaxies. This approach allows us to select representative CEMs and
extrapolate the radial gradients to the nuclear regions of the galaxies in our
sample, inferring in this way the central N/O and O/H abundances. The nuclear
abundance predictions for theoretical galaxies from the selected CEMs, at
distinct evolutionary stages, are used as input parameters in AGN
photoionization models built with the Cloudy code. We found that standard BPT
diagnostic diagrams are able to classify AGNs with oxygen abundances 12+logO/H
> 8.0 [(Z/Zsolar) > 0.2) preferably found at redshift z > 4. On the other hand,
the HeII4685/Hbeta versus [N II]6584/Halpha diagram produces a reliable AGN
classification independent of the evolutionary stage of these objects.
