It’s unfortunately quite challenging to diagnose cognitive impairment based on neuropsychological assessment at a single point in time post-injury. That’s because we don’t know what the person’s level of cognitive function was before the injury. If a person had cognitive function 1 standard deviation above the mean pre-injury (equivalent to an IQ of 115, speaking loosely) and suffered a 1 standard deviation loss as a result of the injury, they would then test in the normal range - but still clearly have suffered damage.

The response articles calling for a 5th percentile measure to diagnose impairment are in a sense statistically appropriate - you’d like to see very significant impairment to diagnose it from a single measure - but not sensitive - they will miss a lot of true cases of mild impairment. For example, that threshold would miss virtually all cases of concussion, chemobrain, cognitive impairment in MS, etc.