The Consensus Statement on Concussion in Sport of the 3rd International Conference on Concussion in Sport in 2008  included a Sport Concussion Assessment Tool 2 (SCAT2)   for standardized assessment by medical and health professionals (team physicians, certified athletic trainers, neuropsychologists) of sports concussion in athletes ages 10 years and older.
In March 2013, the SCAT2 was replaced by the SCAT3   for athletes 13 years and old issued coincident with the Consensus Statement issued after the 4th International Conference on Concussion in Sport held in Zurich in November 2012 ("Zurich statement"),  and a modified version (Child SCAT3) was issued for children aged 5 to 12 years. 
The SCAT takes 15-20 minutes to complete and computes a composite score, comprised of the Glasgow Coma Scale , a Standardized Assessment of Concussion (SAC)  score (cognitive and physical evaluation, delayed recall), and a balance assessment score (modified Balanced Error Scoring System or BESS ).
Although the SCAT3 is designed for use by medical and health professionals, it includes a page of information to be given to the athlete and his parents when, after examination in an emergency room or doctor's office, no sign of any serious complications have been found, and he is allowed to return home, including:
When the SCAT2 was issued, superseding the original SCAT published in 2005, the authors recommended continued reliance on the SAC until prospective studies could be conducted to assess the SCAT2's sensitivity (how good the test is in identifying athletes with concussion; for example, a test which is very sensitive will have few false negatives, rarely missing those later found to have concussion) and specificity (a test with high specificity will have few false positives, rarely mis-classifying people without concussion as having concussion). In its 2010 clinical report on sport-related concussions in children and adolescents,  the American Academy of Pediatrics endorsed this note of caution.
As of March 2013, no large scale studies of concussed athletes to assess the SCAT2's sensitivity and specificity to concussion have been conducted. 
In the four years since it was issued, the SCAT2 became one of the most widely used tools for assessing concussions on the sports sidelines (8), but, as a 2013 meta-analysis notes, "a number of concerns have been expressed about" its design and scoring (7). Among its critics is MomsTEAM's sports concussion neuropsychologist, Rosemarie Scolaro Moser, PhD. Moser observes that the test seems to be given greater weight and importance internationally than in the U.S., where, she says, anecdotal evidence suggests that it is not in widespread use.
Moser shares the concern, noted above, that the SCAT2 has not been adequately validated as a diagnostic tool, and says that she has "personally found it to be of limited value in her clinical practice." In her professional opinion, "if sideline personnel suspect that an athlete has sustained a concussion, they should send him or her to see a concussion specialist. Period. Performing a SCAT2 isn't needed to document that suspicion. Indeed, many of the athletic trainers with whom I work find the tool cumbersome and duplicative," she says.
The SCAT3 itself appears to back up Dr. Moser's advice, stating that, in the event an athlete, after a direct or indirect blow to the head, displays any obvious potential signs of concussion (LOC, balance or motor incoordination, disorientation or confusion, loss of memory, blank vacant look, visible facial injury in combination with any of the other signs), the "athlete should stop participation, be evaluated by a medical professional and should not be permitted to return to sport the same day." (emphasis in original).
Studies after the SCAT2 was issued document other drawbacks:
1. Consensus Statement on Concussion in Sport: the 3rd International Conference on Concussion in Sport held in Zurich, November 2008.  Br J Sports Med 2009; 43:i76-i84.
2. SCAT2,  Br J Sports Med. 2009; 43; i85-i88
3. SCAT3 , Br J Sports Med 2013;47:259
4. McCrory P, et. al. Consensus statement on concussion in sport: the 4th International Conference on Concussion in Sport held in Zurich, November 2012.
5. Child - SCAT3 , Br J Sports Med 2013;47:263
6. Halstead, M, Walter, K. Clinical Report - Sport-Related Concussion in Children and Adolescents" Pediatrics. 2010;126(3):597-615.
7. McCrea M, Iverson G, Echemendia R, et a. Day of injury assessment of sports-related concussion. Br J Sports Med 2013;47:272-284.
8. Dziemianowicz M, Kirschen MP, Pukenas BA, Laudano E, Balcer LJ, Galetta SL. Sport-Related Concussion Testing. Curr Neurol Neurosci Rep 2012 (published online July 13, 2012)(DOI:10.1007/s11910-012-0299-y)
9. Schneiders AG, Sullivan SJ, Handcock P, et. al. Sports concussion assessment: the effect of exercise on dynamic and static balance. Scand J Med Sci Sports 2010, Epub ahead of print.
10. Hunt TN, Ferrara MS, Bornstein RA, et. al. The reliability of the modified balance error scoring system. Clin J Sport Med 2009;19:471-475.
11. McCrea M, Barr WB, Guskiewicz KM, et al. Standard regression-based methods for measuring recovery after sport-related concussion.J Int Neuropsychol Soc. 2005;11:58-69.
Revised and updated March 26, 2013