This study found utilization of mental health services increases significantly with severity of patients' major depressive disorder (MDD).  The study also found that treatment costs were substantially higher for severe MDD than for mild MDD and that severely and moderately depressed workers missed work more than non-depressed workers.  Monthly salary equivalent lost for severely depressed ($199) and moderately depressed ($188) was also significantly higher than for non-depressed workers.  The authors project that for the US workforce, "monthly depression-related worker productivity losses had human capital costs of nearly $2 billion" and conclude that "MDD severity is significantly associated with increased treatment usage/costs, treatment adequacy, unemployment, and disability and with reduced work performance."

Birnbaum, H. G., R. C. Kessler, Kelley, D., Ben-Hamadi, R., Joish, V. N., & Greenberg, P.E. (2010). Employer burden of mild, moderate, and severe major depressive disorder: mental health services utilization and costs, and work performance. Depress Anxiety, 27(1): 78-89. http://www.ncbi.nlm.nih.gov/pubmed/19569060 

Authors: Howard G. Birnbaum, Ronald C. Kessler, David Kelley, Rym Ben-Hamadi, Vijay N. Joish, and Paul E. Greenberg