In this study the investigators will evaluate the long-term renal function in children treated with continuous renal replacement therapy (CRRT) due to acute kidney injury (AKI) and multiple organ failure (MOF) in the pediatric intensive care unit (PICU). These children are not always referred for nephrology follow up after their ICU stay and it is unclear to what extent the patients suffer from chronic renal disease. The primary aim is to establish the frequency of chronic kidney disease (CKD) in children treated with CRRT due to AKI. Secondary outcomes will include mortality, frequency of end stage-renal disease (ESRD) and need for hemodialysis and/or renal transplantation.

-Background: Children receiving CRRT due to AKI are a group of severely ill patients. The mortality is high, varying from 16-58 % in different studies (1). Degree of fluid overload at CRRT start is an independent risk factor for PICU mortality (2). MOF and low weight also contribute to a significantly increased mortality rate (3).

Previous studies on adults have shown that CRRT in the ICU is an independent risk factor for both in-hospital and long-term mortality, as well as the development of CKD and end-stage renal disease. Patients with continued impaired renal function at ICU discharge are at higher risk, but patients with complete recovery of their AKI are also at risk of adverse long-term outcomes (4). Data describing the incidence of chronic renal disease after pediatric CRRT are scarce.

Many pediatric ICU patients with AKI don´t have clinical signs of renal damage after ICU discharge (5-8). However, the kidneys do have a capacity to compensate for renal damage and subclinical decreased renal function is not always detected. Subclinical renal damage in children can over the years develop into more severe renal damage and significant symptoms may appear later in life. Recent data show that AKI in the pediatric ICU leads to increased long-term mortality as well as renal damage (5, 9, 10). However, these studies mainly include older children and also covers milder degrees of AKI. Studies investigating long term outcomes after pediatric CRRT are needed. Moreover, it is important to identify risk factors for the development of chronic renal disease in these children in order to initiate adequate follow up and preventive treatment.

-Methods: This is a combined retrospective and prospective single center observational cohort study. All pediatric patients treated with CRRT from 2008 to 2021 at Karolinska University Hospital in Stockholm will be evaluated for enrollment. Data regarding patient characteristics in the PICU and CRRT data will retrospectively be collected from their medical chart. These data include age and weight at CRRT initiation and hospital discharge, comorbidities, reason for ICU admission, mechanical ventilation and ECMO treatment. Data regarding renal function include CRRT indication, KDIGO and PELOD 2 stage at CRRT initiation, serum-creatinine, serum-chloride at CRRT initiation, serum-creatinine at PICU discharge, urine dipstick and urine-albumin/creatinine ratio.

For patients with no nephrology follow-up after CRRT treatment, the renal function will be evaluated by a pediatric nephrologist. Due to practical matters however, this is only possible for patients from the Stockholm area and patients with ongoing care at Karolinska University Hospital. Renal function will be evaluated using serum Creatinine and Cystatin C. Follow-up will also include urine dipstick, urine albumin/creatinine ratio and blood pressure. Further investigations, such as renal ultrasonography or scintigraphy will be performed if deemed clinically relevant. For patients that have already had nephrology follow-up after CRRT, data will be collected from patient charts.

A substantial number of patients will be lost to prospective follow-up due to the high mortality in this group of patients and the fact that many patients are referred to Karolinska University Hospital from other regions in Sweden.

In order to describe the mortality and the risk of symptomatic chronic renal disease after pediatric CRRT the investigators will also conduct a retrospective register-based study of all children who received CRRT due to AKI and/or CRRT at Karolinska University Hospital from 2008-2021. Data regarding mortality, cause of death and diagnosis of chronic renal disease will be collected from the Swedish National Patient Register and National Cause of Death Register. Patients referred from outside Sweden will be excluded from this part of the study.

Inclusion Criteria:

- Age newborn to 18 when admitted to ICU. CRRT treatment 2008-2021 due to AKI stage ≥ stage 1 (according to KDIGO) and/or ≥ 2 organ failures

Exclusion Criteria:

• CRRT due to metabolic and/or endocrinologic comorbidity.
• Chronic renal failure before CRRT.
• Previous renal transplantation.

Prospective part:

- Not a resident of Stockholm County and no planned follow up at Karolinska University Hospital.

Retrospective part:

- Patient referred to Karolinska University Hospital from a hospital outside Sweden.

Children treated with CRRT in the intensive care unit.