  In this Letter we model the chemistry of DCO$^{+}$ in protoplanetary disks.
We find that the overall distribution of the DCO$^{+}$ abundance is
qualitatively similar to that of CO but is dominated by thin layer located at
the inner disk surface. To understand its distribution, we investigate the
different key gas-phase deuteration pathways that can lead to the formation of
DCO$^{+}$. Our analysis shows that the recent update in the exothermicity of
the reaction involving CH$_2$D$^{+}$ as a parent molecule of DCO$^{+}$ favors
deuterium fractionation in warmer conditions. As a result the formation of
DCO$^{+}$ is enhanced in the inner warm surface layers of the disk where X-ray
ionization occurs. Our analysis points out that DCO$^{+}$ is not a reliable
tracer of the CO snow line as previously suggested. We thus predict that
DCO$^{+}$ is a tracer of active deuterium and in particular X-ray ionization of
the inner disk.
