Post-traumatic stress disorder (PTSD) is a potentially debilitating response to life-threatening events that occurs in a significant minority of adults and children. Established guidelines prevent doctors from diagnosing PTSD until 30 days after a traumatic event. However, some people develop a delayed form of the disorder and don\u2019t display any symptoms for a number of months after the opening of this diagnostic window. In a study published in August 2013 in the journal JAMA Psychiatry, an Australian research team investigated the potential causes for delayed PTSD. This team concluded that people with delayed symptoms have certain backgrounds that distinguish them from people who experience the effects of post-traumatic stress relatively rapidly. PTSD Basics Post-traumatic stress disorder produces its damaging effects because it overrides the built-in stress tolerances of the human brain and body. When these tolerances are exceeded, an affected individual can continue to experience extreme stress reactions long after a moment of fear or danger passes by. When the debilitating or dysfunctional after effects of a traumatic event appear in the 30-day time period following that event, doctors have the freedom to diagnose a condition called acute stress disorder (ASD). If ASD continues for more than 30 days, doctors can modify their original diagnoses and identify the presence of PTSD. They can also diagnose PTSD in people who never experienced the effects of acute stress disorder. In 2013, the American Psychiatric Association altered some of the criteria used for diagnosing PTSD. To qualify for a diagnosis, an affected individual must experience, witness or learn indirectly about a life-threatening event that endangered the self or others. In addition, he or she must mentally\/emotionally relive that event, make ongoing efforts to avoid reminders of that event, experience heavily negative changes in thought processes or emotional status, and develop signs of an unusually heightened sensitivity to changes in his or her local environment. The symptoms of PTSD must continue for over 30 days and must meaningfully damage an individual\u2019s ability to function in public or in his or her private life. Delayed PTSD Delayed PTSD is also known as delayed-onset PTSD or PTSD with delayed expression. Doctors can officially identify this subtype of post-traumatic stress disorder in people who meet the criteria for a PTSD diagnosis only half a year or longer following a traumatic event. However, this does not mean that an affected person has no PTSD symptoms prior to six months. It merely means that he or she does not have enough symptoms to qualify for a diagnosis. Still, some people with delayed PTSD truly don\u2019t experience any of the disorder\u2019s symptoms in the months following trauma. Roughly one out of four people diagnosed with post-traumatic stress disorder has delayed PTSD. New Findings In the study published in JAMA Psychiatry, researchers from several Australian institutions used an examination of 1,084 people with traumatic physical injuries to identify the factors that could potentially contribute to the chances for developing delayed PTSD. Specific factors under consideration included a prior history of mental illness, the presence of a mild traumatic brain injury (MTBI), the severity of PTSD-associated symptoms, exposure to significant stress after the originating traumatic event and the amount of hospital time needed to heal the patients\u2019 physical injuries. Assessments were made three months, one year and two years after the traumatic injuries occurred. After reviewing their assessments, the researchers found that 44 percent of the study participants who had diagnosable cases of PTSD two years after a traumatic event did not have any PTSD symptoms three months after trauma exposure. (In other words, these participants eventually developed delayed PTSD.) The remaining 66 percent of the affected participants had either diagnosable PTSD or isolated PTSD symptoms after the same three-month time period. After examining the potential contributing factors, the researchers concluded that people diagnosed with delayed PTSD tend to experience some form of mild traumatic brain injury during the events that trigger their post-traumatic stress. They also concluded that the experience of additional high-stress situations in the aftermath of a traumatic event can increase the impact of post-traumatic stress enough to push a person over the line into diagnosable delayed PTSD. The experience of additional high-stress situations can also potentially worsen the severity of symptoms found in people who receive a delayed PTSD diagnosis. Other factors that can worsen the impact of delayed PTSD include a lengthy hospital stay for the treatment of physical trauma and the development of intense, isolated symptoms prior to an official diagnosis.