Let ρA : = trB(ρAB) and ρB : = trA(ρAB). Then the reduction criterion states that for any separable ρAB, two derived operators must be positive semidefinite:
ρA ⊗ IB − ρAB ≥ 0
IA ⊗ ρB − ρAB ≥ 0.
The criterion comes from applying the positive but not completely positive (PnCP) map
L(ρ) = I tr(ρ) − ρ to one part of a bipartite system.
In general it is weaker than the PPT criterion, but any state violating it is always distillable. For states of dimension 2 × 2 or 2 × 3 it is equivalent to the PPT criterion and therefore also necessary and sufficient.