A new blood test can tell doctors whether a person has suffered a concussion up to a week after he or she was injured. A study describing the test was published in the journal JAMA Neurology. People who suffer a head injury should rest soon after they are hurt. Doctors say rest is important in helping the brain to heal after the injury. Individuals who have suffered a concussion do not always show symptoms. Experts say those warning signs can be delayed, especially in children.