  Recently [Phys. Rev. Lett. {\bf 106}, 093902 (2011)] it has been shown that
$\mathcal{PT}$-symmetric scattering systems with balanced gain and loss,
undergo a transition from $\mathcal{PT}$-symmetric scattering eigenstates,
which are norm preserving, to symmetry broken pairs of eigenstates exhibiting
net amplification and loss. In the present work we derive the existence of an
invariant non-local current which can be directly associated with the observed
transition playing the role of an "order parameter". The use of this current
for the description of the $\mathcal{PT}$-symmetry breaking allows the
extension of the known phase diagram to higher dimensions incorporating
scattering states which are not eigenstates of the scattering matrix.
