I apologize for the delay…Amnesia surrounding the event or injury is a common finding in traumatic brain injury. The length of post-traumatic amnesia (PTA), or the period of time after an injury during which the individual is typically confused and unable to form new memories, is an indicator of the severity of the brain injury. Concussion is a mild form of traumatic brain injury, so amnesia following concussion is typically very brief, if it is present at all. For a severe traumatic brain injury, the length of PTA is typically much longer, often weeks. As you describe, in severe TBI, there is often a period immediately before the injury that the individual does not recall as well.
There is no blood or physiologic test for amnesia. Rather, amnesia is determined by asking questions of the individual to determine the period of time before or after the injury during which they were unable to form new memories.