What a concussion baseline can tell you, and where it runs out

It's the week before the season. A line of athletes sits in a gym, each filling in the same symptom checklist: 22 symptoms, each rated from 0 to 6, a severity total out of 132. The forms get scored, filed and forgotten.

In October one of those athletes takes a hit, and the clinician pulls the file. Now the August number has to do real work. It helps to know in advance how much work it can do.

The people who wrote the test are careful about it

The SCAT6, the current Sport Concussion Assessment Tool from the Concussion in Sport Group, says on its first page that preseason baseline testing can be helpful for interpreting post-injury scores, and that it is not required for that purpose.

The group's statement on baseline screening goes further. A baseline may be useful for comparison, it says, but comparison with age and gender specific normative data may be possible instead where those norms exist. Baseline screening is only useful when a healthcare professional runs it, or a trained designee they supervise. Parents should not use the SCAT6 to screen their own children, and coaches should not use it to screen athletes. For children aged 8 to 12, the group does not recommend baseline screening with the Child SCAT6, citing the rate of neurodevelopment at that age.

Then it sets the priority. More important than any baseline, the statement says, is a thorough post-injury clinical assessment by a healthcare professional.

A symptom score moves on its own

The symptom scale is self-report, and the two sittings ask different questions. At baseline the athlete rates how they typically feel. After an injury they rate how they feel now. Answers drift even when nothing has happened.

Much of the published reliability data comes from a closely related checklist, the Post-Concussion Symptom Scale used in ImPACT, which also rates 22 symptoms from 0 to 6. In a 2018 study, 38 university students completed it twice, about six weeks apart. The commonly used total symptom score had a test-retest correlation of 0.67 (intraclass correlation 0.62), lower than several of the scale's other indices.

The minimal detectable change for that total, the smallest shift that can be told apart from measurement noise, has been put at 12.3 points. The minimal clinically important difference, estimated in 2022 against patients who rated themselves greatly improved, was 26.5 points out of 132.

Put those numbers next to a real rule. Take an athlete whose baseline total is 10. A rule written as a percentage, say within 20 percent of baseline, allows a band of 2 points. The noise alone is about 12.

This is why the research on change works in points. Iverson, Lovell and Collins set out reliable change confidence intervals for ImPACT in 2003, and the 2018 study published reliable change indices for the symptom scale at 80, 90 and 95 percent confidence. A percentage of a small number is a small number. A reliable change index is built from the instrument's own noise.

The baseline that gets used every day

The SCAT6 has a second baseline in it, and it gets used far more often. In the return-to-sport strategy, an athlete can keep going through a mild and brief rise in symptoms. The instructions define that as no more than 2 points on a 0 to 10 scale, for less than an hour, compared with the value reported before that day's activity. More than that during the early steps, and the athlete stops and tries again the next day. Each step typically takes at least 24 hours.

That baseline is taken fresh every session. It only helps if someone records it before the activity and again after, every time, somewhere it can be found a week later.

What a preseason baseline is good for, and what it isn't

It is a reference for one person. An athlete who typically reports headaches or poor sleep starts from a different place than a teammate who reports nothing, and their own baseline records that. It is also the first point in a series. After an injury, the scores mean most against each other: day two against day one, this visit against the last.

It is not a clearance line. The SCAT6 says its scoring should not be used on its own to diagnose concussion, measure recovery, or decide whether an athlete is ready to return to sport. It adds that an athlete can score within normal limits and still have a concussion. And for 8 to 12 year olds, the tool's authors recommend against the preseason baseline altogether.

What a program can do with this

• Give baselines an owner. A healthcare professional, or a trained designee they supervise, and the CISG position on younger children written into the program.
• Read change in points. Know the measurement noise of the instrument before writing a rule around it.
• Keep the whole series together. The baseline, the first post-injury score and every visit after it, in one place, read as one line.
• Log the day-of number. The pre-activity score and the post-activity score are the evidence behind each step of the return.

That last stretch is where software earns its keep. Oculabs puts each new symptom score beside the baseline and every earlier visit, so the clinician reads a trajectory instead of a single number, and its triage suggests who may need a closer look. The clinician decides.

A baseline is a useful starting point. The recovery is what happens after it, and that is the part worth measuring carefully.

This article is general information for clinicians and program leads. It is not medical advice about any individual athlete.