This was a prospective observational study. Sample size of the study was determined from the variability of the suPAR in the Azam et al. study - the median suPAR level reported in the entire cohort was 5.61 ng/ml (IQR, 4.00–7.88), suggesting a SD of less than 1.5 [12]. Therefore, 18 patients were enrolled in the study.

The study was conducted in 2020 autumn-winter wave of COVID-19 patients in Lithuania, that correlates to the 2nd wave in Europe. Selection criteria were: COVID-19 diagnosis, admission to ICU, start of cytokine adsorption. The criteria of initiation of cytokine adsorption were based both on clinical state of the patients, evaluated by the senior physician (oxygen requirements, severity and progression speed, etc.) and by laboratory data indicative of acute systemic response (IL-6 concentration above 100 pg/ml and ferritin concentration above 1000 mcg/L). Exclusion criteria were age < 18y and chronic kidney disease. Study lasted 6-months until 18 consecutive patients were enrolled.

SuPAR was measured using the The suPARnostic® TurbiLatex (ViroGates A/S, Birkerød, Denmark) test, based on latex particle-enhanced turbidimetric immunoassay that quantitatively determines the suPAR level in human EDTA- or Heparin plasma samples.

The first series of suPAR measurements were performed 10 minutes after start of the perfusion, sampling both incoming and outgoing lines of the adsorber, via the special sampling ports. A second series of the measurements was performed before finishing the first hemoadsorption session with the same adsorber.

Hemoadsorption procedure was performed with a “Cytosorb®” (CytoSorbents Europe GmbH) adsorber, according to the recommendations of the manufacturer. The adsorber was integrated into the continuous renal replacement therapy machine (Fresenius Medical Care multiFiltratePRO Ci-Ca®) before the hemofilter, the modality chosen was continuous veno-venous hemodialysis (CVVHD, filter - Ultraflux AV1000S), with a dialysate flow of 2000 ml per hour, blood flow of 100-150 ml per minute and ultrafiltration according to the clinical state of the patient, ranging from 0 ml per hour to 200 ml per hour, generating a filtration fraction of less than 3%. The length of the procedure planned was 24 hours per one adsorber. The exposure to CRRT was weighted against the possible positive effect of the cytokine adsorption before the decision to initiate the therapy, keeping the best interest of the patient in mind.

Statistical analysis was carried out by the SPSS statistical software package version 26.0 (IBM/SPSS, Inc., Chicago, IL). Baseline characteristics were defined using descriptive statistics. Categorical variables were stated as an absolute number (n) and a relative frequency (%), and continuous variables were represented as a median (interquartile range) or as a mean (± SD), depending on the normality of the distribution. The normality of distribution was tested by one sample Kolmogorov-Smirnov test.

To compare related non-parametric variables Wilcoxon test was used. Linear regression analysis was used to determine the effect of hemadsorption hours on the fraction cleared during the session. Statistical significance level set was < 0.05.