**1. Introduction**

Renal tubular epithelial cells are prone to hypoxia and metabolic stress, thus they become first target cells in the course of kidney injury. Contrast-induced nephropathy is a classic example of reversible acute kidney injury (AKI) with tubular involvement [1]. Animal and human studies showed that contrast administration triggers both systemic and renal cytotoxic e ffects [2,3]. However, if the conditions are unfavorable, further irreversible changes may lead to progression to chronic kidney disease [4].

Acute kidney injury is a well-documented phenomenon characteristic for HSCT [5–8]. However, most studies on AKI take into account the KDIGO classifications, focusing on the serum creatinine values and diuresis [9]. Such criteria do not ease the AKI diagnosis. In order to secure the patient with positive fluid balance and prevent oliguria, additional hydration and forced diuresis are implemented. These conditions may bias the values of estimated glomerular filtration rate (eGFR) and urine output.

Recent classifications have expanded the definition of AKI beyond the functional criteria. They distinguish four options, based on the combined evaluation of function and damage markers [10]. This new approach defines normal renal function as an absence of any index alteration, subclinical AKI as an isolated increase of any damage marker, functional AKI when solely function markers are modified and combined AKI with both function and damage markers altered [10,11].

The search for new markers was conditioned by the failure of serum creatinine as an early marker of renal function decrement. The indices of cellular damage—especially of tubular injury—are of particular interest as new markers of the so-called "subclinical AKI" [10]. The preliminary studies concerning children after cardiosurgery or hematopoietic stem cell transplantation (HSCT) proved that combination of the biomarkers of renal function and tubular damage may be of added value in the early diagnosis of AKI [12,13].

Indeed, the risk of AKI is increased in children undergoing hematopoietic stem cell transplantation (HSCT), mainly due to the nephrotoxicity of drugs. Additionally, renal hypoperfusion, infections and immune complications (including graft versus host disease) count. Moreover, patients with allogeneic HSCT su ffer from AKI more often than those undergoing the autologous transplantation [14,15]. The assessment of renal function in the early (up to 28 days) post-transplantation period seems of paramount importance, because it may reveal the potential direction of future changes into either full renal recovery or acute kidney disease or chronic kidney disease [16,17]. First promising results in the population of children undergoing HSCT should urge further search for reliable early markers of kidney injury [13].

#### **2. Aim of Study**

Therefore, the objective of the study was to assess the usefulness of serum and urinary clusterin as new indices of kidney injury in the early post-HSCT period in relation to other renal damage (KIM-1) and functional (cystatin C) markers and to estimate their potential value as factors di fferentiating between children transplanted because of oncological and non-oncological reasons.

#### **3. Material and Methods**

#### *3.1. Study Design and Settings*

This observational pilot study concerned 27 children (15 girls, 12 boys) undergoing first alloHSCT in the Department of Bone Marrow Transplantation, Pediatric Oncology and Hematology, in 2019 (patient flow is shown in Figure 1). The observation period started before introducing conditioning therapy, then parameter examinations were performed 24 h, 1, 2, 3 and 4 weeks after HSCT.

The exclusion criteria for the patients were the age below 2 years and over 18 years, autologous HSCT and retransplantation. The whole alloHSCT group contained 27 patients (median age 4.5 years, interquartile range 3.1–8.0 years). The subdivision into two groups was carried out depending on the indications for allotransplantation. Seventeen patients (median age 6.6 years, interquartile range 4.0–9.8 years) were qualified for transplantation due to oncological reasons, 10 (median age 4.5 years, interquartile range 3.1–7.0 years) underwent HSCT due to non-oncological indications (mainly severe aplastic anemia). In 79% of cases the donor was unrelated, in 18%-related and in 3%-haploidentical.

The conditioning therapy concerned myeloablative (busulfan, cyclophosphamide and fludarabine or fludarabine, treosulfan, thiotepa) or non-myeloablative (cyclophosphamide, fludarabine) regimens. In most patients graft versus host disease (GvHD) protocol contained pre-HSCT ATG, cyclosporine A since 1 day before HSCT and methotrexate given in the 1st, 3rd and 6th day after transplantation. Nineteen out of 27 patients developed GvHD. None of the patients died in the observation period.

**Figure 1.** Patient flow.

The control group contained 18 age-matched children (9 girls, 9 boys; median age 7.8 years, interquartile range 7.0–9.8 years) with monosymptomatic nocturnal enuresis and normal kidney function. Informedconsentwasobtainedfromthepatientsover16andtheirparents,ifnecessary.
