Depending on your point of view psychiatric diagnosis is either a logical scientific process, an art form or (not-so) inspired guesswork. People with severe mental-health problems are relatively easy to diagnose but some people can fall below the threshold of a diagnosable illness but still have a number of problems. A team of researchers from the University of Illinois-Chicago and the Oregon Research Institute carried out a 15-year study of 1,505 young adults in the community. They found that 'subthreshold' depression, anxiety, alcohol and substance abuse all escalated into their corresponding syndrome. 'Subthreshold' mental health problems also made it more likely that people would go on to develop other mental-health problems as well.
Shankman, Stewart A. ... [et al] - Subthreshold conditions as precursors for full syndrome disorders: a 15-year longitudinal study of multiple diagnostic classes Journal of Child Psychology and Psychiatry December 2009, 50(12), 1485-1494