According to the DSM 5, the criteria used to diagnose PTSD is:

 
Criterion A: stressor (one required)
The person was exposed to: death, threatened death, actual or threatened serious injury, or actual or threatened sexual violence, in the following way(s):
Direct exposure
Witnessing the trauma
Learning that a relative or close friend was exposed to a trauma
Indirect exposure to aversive details of the trauma, usually in the course of professional duties (e.g., first responders, medics)

Criterion B: intrusion symptoms (one required)
The traumatic event is persistently re-experienced in the following way(s):
Unwanted upsetting memories
Nightmares
Flashbacks
Emotional distress after exposure to traumatic reminders
Physical reactivity after exposure to traumatic reminders

Criterion C: avoidance (one required)
Avoidance of trauma-related stimuli after the trauma, in the following way(s):
Trauma-related thoughts or feelings
Trauma-related external reminders

Criterion D: negative alterations in cognitions and mood (two required)
Negative thoughts or feelings that began or worsened after the trauma, in the following way(s):
Inability to recall key features of the trauma
Overly negative thoughts and assumptions about oneself or the world
Exaggerated blame of self or others for causing the trauma
Negative affect
Decreased interest in activities
Feeling isolated
Difficulty experiencing positive affect

Criterion E: alterations in arousal and reactivity
Trauma-related arousal and reactivity that began or worsened after the trauma, in the following way(s):
Irritability or aggression
Risky or destructive behavior
Hypervigilance
Heightened startle reaction
Difficulty concentrating
Difficulty sleeping

Criterion F: duration (required)
Symptoms last for more than 1 month.

Criterion G: functional significance (required)
Symptoms create distress or functional impairment (e.g., social, occupational).

Criterion H: exclusion (required)
Symptoms are not due to medication, substance use, or other illness.
Two specifications:
Dissociative Specification In addition to meeting criteria for diagnosis, an individual experiences high levels of either of the following in reaction to trauma-related stimuli:
Depersonalization. Experience of being an outside observer of or detached from oneself (e.g., feeling as if "this is not happening to me" or one were in a dream).
Derealization. Experience of unreality, distance, or distortion (e.g., "things are not real").
Delayed Specification. Full diagnostic criteria are not met until at least six months after the trauma(s), although onset of symptoms may occur immediately.