I never had abnormal results in standard cognitive testing done by neurologists, only in a neuro rehab clinic where I was assessed for working ability.
The testing was 3.5 hours, with two short breaks (too late and too short for me), so in sum this was sufficient time to show a clear decline in alertness and other cognitive function (and also in my handwriting) but not the typical abnormalities you would see due to other cognitive impairment in established neurological or psychiatric disease.
When you say standard cognitive testing done by neurologists, do you mean the brief questions they ask during their clinics, such as asking you to state the day, month, year and what hand you use?
When I underwent neuropsychological testing during the initial interview, I explained that some of my cognitive difficulties seem to fluctuate. The assessor asked me if fatigue/tiredness (I can't remember the exact wording) can make the difficulties worse. The way it was explained to me is that post-exertional functioning decline is not unheard of and although neuropsychological tests are not designed to pick that up, they can be used to inform other brain testing.
I think that this could give false clues when applied in ME/CFS.
If it takes sufficient time to induce relevant cognitive fatigue, but that isn't taken into account by the assessor, you will then score better on the tests done earlier in the assessment session and worse later, not related to the kind of task but just to being more fatigued.
If it doesn't induce relevant cognitive fatigue, it will probably show no abnormalities.
It may still make sense for monitoring long-term changes.
What I found was that, as you said, I performed very well in many parts of the test due to increased energy. I had severe brain fog earlier in the day, so I was sure I was going to score poorly overall, but with a stroke of typical luck, my symptoms fluctuated, and I had a lot of increased energy, and it felt like all my usual impairments disappeared until I was asked to perform actions that required the use of multiple cognitive abilities altogether.
My difficulties came to light when I was asked specific style questions, which made no difference to my fatigue levels or how much I exerted myself. I'm guessing the questions I struggled with are probably the same styled questions that neuropsychologists experienced with ME/CFS would concentrate on. My brain would be fried had I undergone 2 hours of joint multi-cog style questioning. So although the test is not ME/CFS or Long Covid specific, it can be used to highlight abnormalities that warrant further investigation that better assessment tools are geared towards. The added benefit is you are also screened for primary mental health conditions so the assessor can determine what the likely cause of the abnormalities is.
In theory, it's still not an actual objective assessment as it could be manipulated but I think most people don't want to (or perhaps even can't) pretend in such an assessment setting that they are less intelligent /cognitively fit than they actually are.
So I still would count that as a 'pretty objective' assessment, but would need to be adjusted to what's relevant to measure in ME/CFS.
I agree, I don't think the test, in theory, is objective either, but as it stands, it's still a gold-standard brain test used to objectively evaluate injury/disorder/mental health. Until that changes, it counts as objective.
My cognitive assessment would show huge differences from a lying down and upright position.
A form of cognitive postural testing is performed during Tilt Table testing.
