Worldwide, the use of Extracorporeal Blood Purification (EBP) in everyday clinical practice is becoming increasingly common, particularly in critical care settings. The efficacy of most of these treatments on removal of inflammatory mediators is the main rationale behind the use of EBP in critically ill patients with multiorgan dysfunction. Nonetheless, there are still some doubts as to the clinical efficacy of bacterial toxins and cytokines removal and many clinical trials aiming at exploring the effect of EBP on long-term outcomes of septic patients have failed to demonstrate consistent results regarding 28 day- or hospital-mortality rates. The primary aim of this observational prospective web-based registry is to define the possible clusters of critically ill patients - treated with extracorporeal blood purification therapies worldwide - who are homogeneous regarding both clinical and treatment characteristics and seem to benefit the most from EBP.

Since May 2019, our research group has developed, implemented, and nationally spread the "ARRT registry" with the aim to describe the subpopulations of critically ill patients that benefit the most from EBP therapy with oXiris (6). The project is currently ongoing, involving more than 50 centers in Italy, with data from more than 65 patients treated with oXiris. Differently from other registries, the ARRT registry runs on a web-based platform easily accessible from internet-based technologies, including smartphones, that offers a user-friendly approach, facilitates data uploading, and enhances research collaboration. Furthermore, it adopts a proactive approach, in the sense that it includes several automatic calculators and decision support tools that might help clinicians to personalize treatments directly at the bedside (e.g. automatic calculation of clinical scoring systems, ideal body weight, mechanical ventilation setting, antibiotic adjustment according to renal function, etc.). All these tools can provide the clinician with real-time feedbacks. This web-based registry provides a clear example of translational medicine and translational research where data from clinical practice feed a database for clinical research and, contemporaneously, the database research tools improve clinical practice. Finally, this web-based registry allows each participating center to instantaneously evaluate its own data and obtain real-time basic statistics for each recorded variable (e.g. age at enrollment, main comorbidities, baseline serum creatinine, mortality rate, rate of multidrug-resistant bacteria, etc.), thus allowing for continuous monitoring of outcomes and local practices.

Overall, these features may prove particularly useful during treatment of critically ill septic patients with multiorgan dysfunction. Recently, the COVID-19 pandemic has been characterized by high prevalence of patients with severe multiorgan dysfunction, high mortality rate, lack of ICU resources, and the need for ICU discharge in a rapid, but safe, manner. The association between acute kidney injury and COVID-19 infection is well established. Organ crosstalk and systemic inflammation are the most accredited causes of Acute Kidney Injury (AKI) in these patients. Several EBP therapies have been proposed to attenuate systemic inflammation and/or support renal function in COVID-19 patients. Nevertheless, no data is currently available on application and feasibility of EBP therapies in COVID-19 patients or on their outcomes. Notably, the ARRT registry was able to effectively capture clinical data on systemic inflammation, organ dysfunction, and outcomes in patients treated with oXiris during the COVID-19 pandemic in Italy. In this regard, the implementation of a similar registry at the global level might prove effective for supporting clinicians involved in the treatment of patients with COVID-19 infection and multiorgan dysfunction worldwide.

Inclusion Criteria: patients who meet all the following inclusion criteria may be included in this study:

1. Admission to ICU

2. Indications for at least one of the following extracorporeal blood purification treatments:

   1. Continuous Renal Replacement Therapy (CRRT) / Intermittent Hemodialysis (IHD) / Hybrid therapies for renal support/replacement;
   2. Immunomodulation therapy in critically ill patients using hemodiafilters with larger pore sizes characterized by enhanced transmembrane clearance of larger molecules (such as cytokines), hemodiafilters with enhanced unselective absorption of cytokines and/or endotoxins, cartridges with enhanced absorption of cytokines and/or endotoxins, techniques aimed at improving extracorporeal removal of cytokines and/or endotoxins.

It should be underlined that the lack of established guidelines on the use of membranes for extracorporeal blood purification (and on RRTs in general) leads to variability in clinical practice and treatments are initiated in accordance with the judgement of the responsible physician. Under these circumstances, it is preferable to keep inclusion criteria as wide as possible so as to obtain a real picture of clinical practice worldwide.

Exclusion Criteria: besides contraindications to the use of the EBP adopted (as from the manual of instructions), there are no exclusion criteria.

All medical institutions that provide EBP treatments to critically ill patients are eligible for participation.