Increases in exhausted immune T cells and lower levels of cortisol.
Also, patient reported outcomes alone are sufficient to identify Long COVID patients with ~90% accuracy.
https://twitter.com/VirusesImmunity/status/15573941922251571...
https://www.medrxiv.org/content/10.1101/2022.08.09.22278592v...
On the other hand, there was this paper [1][2] from the NIH, which was a well-controlled, pre-registered cohort study, did pretty much every clinical test imaginable (about 150 different measurements per person), and found no measurable differences between "long covid" people and controls. This is a much better study, and rebuts the claim.
[1] https://www.medpagetoday.com/infectiousdisease/covid19/98885...
https://www.scientificamerican.com/article/people-with-long-...