  The N2 index ([N II]6584/Ha) is used to determine emission line galaxy
metallicities at all redshifts, including high redshift, where galaxies tend to
be metal-poor. The initial aim of the work was to improve the calibrations used
to infer oxygen abundance from N2 employing updated low-metallicity galaxy
databases. We compare N2 and the metallicity determined using the direct method
for the set of extremely metal-poor galaxies compiled by Morales-Luis et al.
(2011). To our surprise, the oxygen abundance presents a tendency to be
constant with N2, with a very large scatter. Consequently, we find that the
existing N2 calibrators overstimate the oxygen abundance for most low
metallicity galaxies, and then they can be used only to set upper limits to the
true metallicity in low-metallicity galaxies. An explicit expression for this
limit is given. In addition, we try to explain the observed scatter using
photoionization models. It is mostly due to the different evolutionary state of
the HII regions producing the emission lines, but it also arises due to
differences of N/O among the galaxies.
