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Lawrence Jackson remembers the moment, and everything that followed, down to its finest detail. As a Detroit Lions defensive end in 2010, Jackson was chasing Dallas Cowboys quarterback Jon Kitna. When Kitna sidestepped him, Jackson collided with teammate Kyle Vanden Bosch, spun in a helicopter motion and felt himself go limp for a few seconds.

"When I got to the sideline," Jackson said, "I knew I was concussed. But I wasn't disoriented. It wasn't like I didn't know where I was. I just had a headache and I could tell. But at that time, the test was pretty easy if you don't have a significant concussion. It still is. They asked me what day it was, approximately what time it was, what quarter we were in. They wanted me to say the months backwards and a few other things. I got those right. I didn't look disoriented, so they really didn't have any choice but to let me go back in the game."

"Unless you're really screwed up," Lawrence Jackson said, "anybody can pass a sideline [concussion] test." Timothy T. Ludwig/USA TODAY Sports

Jackson returned, sacked Kitna on his third play back and went on to one of his better games in a Lions uniform. Taking care to avoid further contact with his head, and playing through what he felt was an "out-of-body experience," he totaled 1.5 sacks in 32 snaps. He reported his symptoms the following day and was held out of the Lions' next game, a sequence of events that Jackson recalled via phone this week amid continuing discussion about the condition of New England Patriots receiver Julian Edelman in Super Bowl XLIX.

Edelman, as we noted at the time, displayed concussion symptoms based on the NFL's current protocol after a hit from Seattle Seahawks safety Kam Chancellor. Edelman did not leave the game -- although he was administered a sideline concussion assessment between possessions, according to The Associated Press -- and he ultimately scored the winning touchdown. Edelman has refused to discuss the hit, most recently during an interview with the New York Times last weekend, citing the Patriots' rules about discussing injuries publicly.

To Jackson, speaking not from a medical sense but from personal experience, the episode is fairly simple. Edelman, Jackson guesses, endured what players refer to as "getting your bell rung," but what is in reality a sub-concussive injury. It leaves players altered and vulnerable but still aware enough to pass a typical sideline test if they don't disclose their condition verbally.

Of course, we don't know the details of Edelman's circumstance and might never find out. But the issue, along with Jackson's story, illuminates what is currently an unavoidable gap in the NFL's concussion-related efforts. Whether or not it happened with Edelman, players still can and do beat the sideline test despite the best efforts of medical science and a protocol the league enhanced in 2013.

"Unless you're really screwed up," Jackson said, "anybody can pass a sideline test."

This mindset doesn't come as a surprise to the NFL's medical community. I spoke this week with Dr. Javier Cardenas, the medical director of the Barrow Concussion and Brain Injury Center in Phoenix. Cardenas also serves on the NFL's head, neck and spine committee and was one of the league's two independent sideline neurologists at the Super Bowl. Although he couldn't speak specifically about Edelman's case -- other than to say: "If there is a question about evaluations being made, I can assure you they were made" -- he readily acknowledged the shortcomings of the current sideline protocol.

When I told him Jackson's theory about beating the test, Cardenas said: "It's true. It's true. The test is as good as we have today. We do our best. The truth of the matter is, this is a two-way street. Of course, not always are the athletes aware of their injuries. Some of them don't recognize they have a concussion, but when they do recognize, the truth is they have a responsibility to their team, to themselves, to their loved ones of declaring that they don't feel right. The tests are only as sensitive as they can be. They're imperfect."

Cardenas estimated that only 10 percent of concussions result in an easily diagnosed loss of consciousness or obvious disorientation. The rest lead to varying stages of what is termed "altered consciousness," the type Jackson said he suffered in Dallas.

The NFL's enhanced protocol put an independent neurologist such as Cardenas on each sideline; they stand at catty-corner 25-yard lines to maximize coverage of the field. An independent athletic trainer is stationed in the press box, and the three scan the field throughout the game for possible concussions, using a video assembly on the sideline to review hits when necessary.

Doctors hope to one day be able to test for concussions chemically through saliva or blood. Ken Blaze/USA TODAY Sports

The NFL has distributed guidelines for the initial sideline test of a player displaying symptoms, viewable here. Cardenas said he first asks a player to describe "what happened," knowing that any gap in the story is usually a sign of a concussion. Players must recite facts such as the opponent, what quarter the game is, who scored last, who won the previous week and then repeat a series of numbers and words. If the player's responses are deemed unsatisfactory, he is taken to the locker room for a full assessment.

But it's the initial test that a slightly altered player, or one whose symptoms could be delayed, can reasonably pass. To combat this shortcoming, Cardenas' research group is among those searching for a more objective assessment tool that would identify the release of certain chemicals from the brain associated with concussions.

The "holy grail," Cardenas said, is a custom mouth guard that would change colors when it detects the concussion-related chemicals in saliva. Research to that end is underway, but reliable tests are probably years away if at all.

Until then, Cardenas said, the best path to overcoming limitations of the sideline test is a culture change that he says is already underway. It's worth noting, of course, that Seahawks defensive end Cliff Avril was ruled out of the Super Bowl because of a concussion.

"In the time I've been doing this," Cardenas said, "I've had more players coming off the field and saying, 'I don't feel right. I've got to sit.' I actually see more athletes self-report at the professional level than in the college and youth levels. To me, that an athlete [Avril] was taken out of the biggest game in the world because of a concussion was an important message."

Jackson isn't so sure. Despite the risks of playing with a concussion, Jackson said he has no regrets and thinks many players would feel the same way. At the time, he said, he was fighting an internal impression among Lions officials that he spent too much time in the trainers' room with mild injuries anyway.

"My family and friends, they expressed a little concern," Jackson said, "but I felt proud of the fact that I was concussed and went back into the game and was able to play. It was almost like you're on a battlefield and get an arrow in the chest, and you break it off and you go back to battle. I felt like it was good for me to be there for my teammates. I had the option to pull myself in that situation but I didn't want to."

The NFL announced last month that documented concussions dropped by 25 percent in 2014 compared to 2013, a real and important number. But the difficult truth is that concussion diagnosis remains a subjective matter, especially in real time during games, and the solutions remain incremental.