Future studies could utilize a randomized controlled trial design and measure variables such as: self reported levels of pain, disability, anxiety, depression, as well as objective blood and urine based measures of stress including: proinflammatory cytokines (TNF-alpha, IL-1, IL-6, IL-8, IL-18) [121,164,165] and anti inflammatory cytokines such as IL-4 and IL-10 [122,165,166]. This evidence concerns the gene CXCL8 and major depressive disorder.