There were no differences in circulating concentrations of MMP-9 (p = 0.697), MPO (p = 0.544), TIMP-1 (p = 0.252), nor TIMP-2 (p = 0.971) between PTSD participants with (n = 13) and without (n = 12) a history of mTBI. Here, MPO is linked to post-traumatic stress disorder.