An invariant for symplectic involutions on central simple algebras of degree divisible by 4 over fields of characteristic different from 2 is defined on the basis of Rost's cohomological invariant of degree 3 for torsors under symplectic groups. We relate this invariant to trace forms and show how its triviality yields a decomposability criterion for algebras of degree 8 with symplectic involution.
Berhuy, G., Monsurrò, M., & Tignol, J.-P. (2003). The discriminant of a symplectic involution. Pacific Journal of Mathematics, 209(2), 201-218. https://doi.org/10.2140/pjm.2003.209.201 (Original work published 2003)