  Einstein-Podolsky-Rosen steering refers to the quantum phenomenon whereby the
state of a system held by one party can be "steered" into different states at
the will of another, distant, party by performing different local measurements.
Although steering has been demonstrated in a number of experiments involving
qubits, the question of which two-qubit states are steerable remains an open
theoretical problem. Here, we derive a necessary condition for any two-qubit
state to be steerable when the steering party suffers from a given probability
of qubit loss. Our main result finds application in one-way steering
demonstrations that rely upon loss. Specifically, we apply it to a recent
experiment on one-way steering with projective measurements and POVMs, reported
by Wollmann et. al. [Phys. Rev. Lett., 116, 160403 (2016)].
