Study ID	Pretorius 2021	Pretorius 2022	Kruger 2022	Laubscher 2023	Turner 2023
Domain 1: Collection and handling of samples	Have patients been clinically evaluated to ensure that they fulfil the criteria for PCC?	No: source not described	No: self‐identified participants	No: self‐identified participants	No: no reference to clinical diagnostic criteria	No: self‐identified participants
	Are samples contemporaneous?	No: samples from healthy participants and those with diabetes were obtained prior to the SARS‐CoV‐2 outbreak	No: samples collected from March 2021 onwards	Yes: samples were collected over a 2‐week period	No: samples collected from March 2021 onwards	Yes: samples were collected over a 2‐week period
	Have patients been matched on age, sex, or health status?	No: no statistical comparison of study groups	N/A	No: no statistical comparison of study groups	N/A	Yes
	Have samples from both groups been collected and prepared in the same way?	Yes: stored at −80 °C and prepared in the same way	N/A	Yes	N/A	Yes
	Risk of bias for domain 1	High risk of bias	High risk of bias	High risk of bias	High risk of bias	Some concerns
Domain 2: Experimental methods	Is a validated methodology used? If not, is the methodology well described?	No: the staining methodology is not well described or validated	No: the staining methodology is not well described or validated	No: the staining methodology is not well described or validated	No: the staining methodology is not well described or validated	Yes
	Has a normal range been calculated for the control group of the study or referenced?	No	No	No	No	Yes: values for a control group with median [Q1‐Q3] are presented
	Has a statistical test been used to compare values of patients with controls?	No	N/A	No	N/A	Yes: data were analysed using a one‐tailed t‐test with Welch’s correction, or a one‐tailed Mann‐Whitney test
	Are the people evaluating the data blind to the source of the samples?	No	N/A	No	N/A	No
	Is an internal control used?	Yes	Yes	Yes	Yes	Yes
	Risk of bias for domain 2	High risk of bias	High risk of bias	High risk of bias	High risk of bias	Some concerns
Domain 3: Reporting of the results	Are the results quantified and with statistical analysis? (e.g. mean or median, range or SD)	No: results are narratively described and no statistical analysis	No: the scoring system used is not described, and individual sample data are not provided	No: results are narratively described and no statistical analysis	No: results are narratively described and no statistical analysis	Yes
	Are all, or nearly all, data available? If a subset of data is presented in the article, is this randomly selected?	No: a subset of data is presented in photographs. Does not seem to be randomly selected	No: results are only presented for 30/80 participants	No: a subset of data is presented in photographs. Does not seem to be randomly selected	No: a subset of data is presented in photographs. Does not seem to be randomly selected	Yes: blood samples from PCC and control populations were randomly selected
	Are appropriate statistical tests used to test for differences?	No: no quantification of data and no statistical tests	No: uncontrolled study, so no tests for differences	No: no quantification of data and no statistical tests described	No: no quantification of data and no statistical tests described	Yes
	Are the data available on a data repository and available to others?	Yes: microscopic images available online but without a scale bar. Online statistics and proteomics files inaccessible	No: authors state available on request	No: not stated	No: not stated	No: not stated
	Risk of bias for domain 3	High risk of bias	High risk of bias	High risk of bias	High risk of bias	Low risk of bias
Overall risk of bias		High risk of bias	High risk of bias	High risk of bias	High risk of bias	Some concerns
Abbreviations: N/A: not applicable; PCC: post‐COVID‐19 condition; SD: standard deviation