Currently there is no reliable diagnostic test or marker that can be used to definitely identify a concussion when its occurred, or similarly determine when a concussion has resolved. It is for this reason that there has been a growing emphasis on objective baseline testing protocols that can be used to track a patient's recovery and serve as a tangible measurement for return-to-play readiness. By measuring an athlete's “normal” level of functioning (through baseline), Healthcare Providers are better able to gauge the level of impairment that may exist post-injury by performing comparative testing.
Baseline testing, although lends value and objectivity, is never relied upon solely when determining return-to-play readiness. Healthcare Providers utilize an array of tools and several factors as part of their clinical decision-making process.
Update: Current Stance on Baseline Testing