Thanks for commenting. You can download the full text here: https://www.researchgate.net/publication/367499752_Inflammatory_and_vascular_biomarkers_in_post-COVID-19_syndrome_A_systematic_review_and_meta-analysis_of_over_20_biomarkers

And no, I have not looked into kynurenine biomarker in detail, unfortunately. But after some reading, kynurenine may also be a highly promising biomarker based on this study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9537769/