PTSD causes biochemical changes in the brain and body, that differ from other psychiatric disorders such as major depression.
Individuals diagnosed with PTSD respond more strongly to a dexamethasone suppression test than individuals diagnosed with clinical depression.
Research has also found that PTSD shares many genetic influences common to other psychiatric disorders.
Panic and generalized anxiety disorders and PTSD share 60% of the same genetic variance.
While it is common to have symptoms after any traumatic event, these must persist to a sufficient degree (i.e., causing dysfunction in life or clinical levels of distress) for longer than one month after the trauma to be classified as PTSD (clinically significant dysfunction or distress for less than one month after the trauma may be acute stress disorder).