All variables were collected from hospital records (and stratified as follows).

* Oxygen supplement (strata: mask, mask with bag reserve, non-invasive ventilation, intubation);

* Degree of severity of COVID-19.

⚬ Mild COVID-19 (26):

a) Any signs and symptoms of COVID-19 (fever, cough, sore throat, malaise, headache, muscle pain, nausea, vomiting, diarrhea, loss of taste and smell), BUT

b) No shortness of breath, dyspnea, or abnormal chest imaging.

⚬ Moderate COVID-19 (26):

a) ≥94% O₂ saturation without mask, AND

b) <50% lung involvement at imaging.

⚬ Severe COVID-19 (26):

a) SpO₂ <94%; OR

b) Respiratory rate ≥30, OR

c) PaO₂/FiO₂ <300 mmHg; OR

d) Signs of pulmonary involvement at imaging ≥50%.

* Sex (female and male);

* Age (years: mean ± SD; classes: <47; 47–56; 57–65; 66+ years);

* Timeline of AKI onset (days since hospital admission: mean ± SD; classes: <2; 2–3; 4–6; and 7+ days);

* Length of hospital stay (LoS: mean ± SD; classes: <7; 7–9; 10–13; 14+ days);

* ICU admission (no, yes)

* Timeline of ICU (days since hospital admission: mean ± SD; classes: <2; 3–5; 6+ days);

* Mortality (no, yes);

* Comorbidities: diabetes mellitus (no, yes); hypertension (no, yes); ischemic heart disease (IHD: no, yes); chronic heart failure (CHF: no, yes); end-stage liver disease (ESLD: no, yes); chronic obstructive pulmonary disease—interstitial lung disease (COPD-ILD: no, yes); chronic kidney disease (CKD: no, yes if eGFRr <45); [CKD: no, yes if Estimated Glomerular Filtration Rate (eGFR) <45]

* AKI stage (stage 1, stage 2, stage 3).

The International Classification of Diseases (ICD10) was used to evaluate mortality and comorbidities. AKI and stage of renal failure were identified using Kidney Disease: Improving Global Outcomes (KDIGO) guidelines (27) (Figure 3).

* eGFR (mL/min/1.73 m²): mean ± SD. The following formula for modification of diet in renal disease (MDRD) was used to estimate the glomerular filtration rate:

eGFR = 186 × (Cr)^−1.154 × (Age)^−0.203× (1.212 if Black) × (0.742 if Female)

* Electrolyte imbalance: natremia (hypo Na⁺ <135 mEq/l; Hyper Na⁺ >145 mEq/l); kalemia (Hypo K⁺ <3.5 mEq/l; hyper K⁺ > 5 mEq/l); magnesemia (hypo Mg²⁺ <1.46 mg/dl; hyper Mg²⁺ >2.68 mg/dl); calcemia (hypo Ca²⁺ <8.8 mg/dl; hyper Ca²⁺ >10.5 mg/dl).

* White blood cells, WBC (× 10⁹/l): mean ± SD;

* Platelets (× 10⁹/l): mean ± SD;

* Hemoglobin (g/dl): mean ± SD;

* Lactate de-hydrogenase, LDH (U/L); mean ± SD;

* Blood creatinine (mg/dL): mean ± SD;

* Blood urea nitrogen (BUN; mg/dL): mean ± SD.

All clinical information but ICU referral was collected at hospital admission. Blood creatinine and BUN were also measured at end of follow-up (hospital discharge or death).

The national guidelines of the Iran Ministry of Health (MoH) and medical education as well as WHO guidelines on management of COVID-19 (26, 28–30) were followed for the hospital management of COVID-19 patients and decision making on ICU admissions.

After the initial admission, all patients were evaluated, monitored, and treated for volume depletion and high blood sugar. All patients were stabilized in A&E department before being transferred to COVID-19 wards and constantly monitored for their hemodynamic status during hospitalization. Treatment of patients was mainly supportive and based on WHO guidelines on COVID-19 patient management at the time of the study. The only non-steroid-anti-inflammatory drug (NSAID) used was naproxen, administered routinely but to patients with low eGFR and with other contradictions. Enoxaparin or heparin was used as anticoagulants. In patients with mild/moderate COVID-19, dexamethasone was administered, whereas in those affected by severe disease methylprednisolone pulse was employed. Antibiotic therapy was administered only in case of secondary bacterial infection.

Treatment drugs were adjusted based upon eGFR and administration of diuretics; ACE inhibitors and angiotensin II receptor blockers were refrained in patients at risk of AKI. Furthermore, remdesivir was not administered in patients with eGFR<30 mg/dl/1.73 m², because of its debated nephrotoxicity risk (31).

Distribution of variables by AKI status (yes vs. no) was estimated by chi-squared test in case of categorical terms, whereas ANOVA was employed for comparison of continuous terms by AKI. The distribution of timeline of ICU referral since hospital admission was contrasted by the mean length of hospital stay (LoS, in days), AKI onset (yes vs. no), timeline of AKI onset (days since hospital admission), and vital status at end of follow-up (death or discharge). The mean and median concentration of BUN and blood creatinine were contrasted between hospital admission and end of hospital dischargefollow-up by Wilcoxon test.

This prospective study was conducted over a short period of time collecting both clinical information and several dates (of: birth; hospital admission; AKI onset; ICU admission; hospital discharge; death). In order to examine how the above risk factors influenced mortality risk and AKI incidence during hospitalization in a particular point in time, survival analysis using the Cox proportional-hazard models was adopted. Two separate multiple Cox regression models were fitted for each outcome (AKI and death). The two multivariable models were built up only including variables significant at univariable analysis.

Statistical interaction was modeled by including a product of ICU and severity of COVID-19 in the regression model to evaluate whether COVID-19 severity modified the association between ICU and in-hospital mortality. Similarly, we assessed the interaction between ICU and AKI and between COVID-19 severity and AKI. Results were expressed as hazard ratio (HR) with 95% confidence interval (95% CI). Non-significant terms at multivariable analysis were omitted from the respective tables. The level of significance for each test was set at 0.05. All terms and interactions not being significant were dropped out, and the corresponding results were not shown in the tables.

Statistical analysis was conducted using Stata 14.2 (Stata Corporation, College Station, Texas, USA).

The prevalence of AKI in the present study was 28.5%, a figure fairly in line with reports from different settings and time periods (32). The risk of AKI increased with diabetes mellitus as well as hyperkalemia, higher WBC count, and increasing level of BUN measured at hospital admission. On the other hand, the overall mortality risk among COVID-19 patients was 19.3%. Factors associated with a higher risk of death were ICU admission for severe COVID-19 as well as for mild/moderate COVID-19, hypokalemia, higher level of BUN, increasing WBC, and increasing LDH measured at hospital admission.

In a systematic review and meta-analysis on 14,415 COVID-19 patients from different countries, the prevalence of AKI was 11% (95% CI: 0.07–0.15; p < 0.01), hence a figure much lower than the present study. Moreover, in the latter meta-analysis AKI was significantly associated with death (OR = 8.45; 95% CI: 5.56–12.56; p < 0.001) and severe COVID-19 (OR = 13.52; 95% CI: 5.43–33.67; p < 0.001) among hospitalized patients (33). In our prospective study—where a survival analysis by Cox proportional-hazard models was employed to examine risk factors influencing AKI incidence and mortality risk in a particular point in time during hospitalization—although 28.5% COVID-19 patients developed AKI and the crude death rate was higher among patients developing it (67.7%), AKI did not increase the adjusted mortality risk at multivariable analysis, a rather unexpected finding. The majority of COVID-19 patients developed AKI after ICU referral; therefore, the risk of death appeared to rise following ICU admission rather than AKI. Almost 40% patients were referred to the ICU within 2 days since hospital admission, suggesting critical clinical conditions of these patients even with less severe form of COVID-19.

Kidney involvement and AKI onset in COVID-19 patients have multiple risk factors, and several explanatory mechanisms have been proposed (Figure 5), including electrolyte imbalance, medication-induced injury, organ failure in late stages of the disease, impairment of gas exchange, hemodynamic alterations including right heart failure, systemic congestion due to fluid overload, and secondary infections/sepsis, among others (32, 34).

AKI is reportedly a frequent pattern in patients dying from COVID-19 (15, 35–38), with acute tubular injury being the most common morpho-pathologic finding in kidney autopsies (37–39). Furthermore, ACE-2 and TMPRSS2 are highly expressed in proximal tubules, where SARS-CoV-2 particles could be detected postmortem in the respective podocytes from COVID-19 patients (40), hinting that the novel coronavirus can directly target the kidneys (7, 10, 34, 41). Experimental and epidemiological studies evidenced that SARS-CoV-2 can infect and damage target renal epithelial cells expressing ACE-2 and TMPRSS2, triggering a cytokine storm (sustained especially by IL-6 and interferon) and directly causing AKI by increasing vascular permeability, shock, and multiorgan failure or aggravating/perpetuating a kidney injury already initiated by non-viral processes (37, 38, 42). Less frequent autoptic findings include micro-angiopathy and collapsing glomerulopathy (40), a frequent cause of proteinuria rapidly progressing to kidney failure (43–45), which predominantly affect patients with homozygous apolipoprotein L1 (APOL-1) high-risk alleles (45–46).

This single-center clinical study was conducted from October 22, 2020, to January 7, 2021, during the third pandemic wave in Iran (Figure 1), before large-scale vaccination campaigns against COVID-19 were deployed globally. The massive surge of COVID-19 cases did not allow to perform autopsies or biopsies on patients with AKI.

Although SARS-CoV-1 proved capable of infecting kidney cells in vitro (47, 48), the evidence supporting persistent infection of the kidney by SARS-CoV-2 is still unconvincing (45). An alternative plausible pathogenetic hypothesis is the “hit and run” model, where the renal injury persists after the clearance of an early direct kidney infection by SARS-CoV-2. However, AKI associated with COVID-19 is probably determined by multiple factors, including an indirect organ damage induced by the physiologic disturbances caused by COVID-19 and the therapies administered to treat the severe acute respiratory syndrome (9, 32, 34, 45). Contrast media, corticosteroids, NSAIDs, ACEs, Angiotensin receptor blockers (ARBs), and various antibiotics are reportedly associated with increased risk of AKI in COVID-19 patients (34, 40), although there is also some evidence that high daily doses (40 mg) of methylprednisolone are associated with increased mortality but lower risk of AKI in COVID-19 patients (49). In a meta-analysis on 23,655 hospitalized, critically ill COVID-19 patients, the incidence of AKI was not significantly different between COVID-19 patients (51%) and critically ill patients infected with ACE2-associated (56%) or non-ACE2-associated viruses (63%). The latter meta-analysis estimated a lower risk of renal replacement therapy in patients affected by COVID-19 or ACE2-associated viruses (featured by a lower risk of shock and use of vasopressors) as compared with patients infected with non-ACE2-binding viruses (50).

As already mentioned, considerable inconsistencies exist regarding the prevalence of AKI in hospitalized COVID-19 patients, with figures widely ranging from 0.5% to 80% (14). Explanatory factors for the inconsistency of the epidemiological evidence on AKI prevalence among COVID-19 patients include ethnicity, genetic polymorphism, type of SARS-CoV-2 variant, the underlying mechanism of kidney injury (either pre-renal, renal, or post-renal), and the methodology employed in various studies (19, 41, 45, 51).

Up until November 14, 2021 (before the spread of the Omicron variant), 56,900,000 cumulative COVID-19 infections were recorded in Iran, where the overall infection/hospitalization rate was 1%–3% and the mortality rate equaled to 277.5/100,000 (11). According to a systematic review and meta-analysis, in Iran the proportion of hospitalized COVID-19 patients developing AKI was 24% (95% CI: 17%–31%), slightly lower than figures from the present study (28.5%) (52). In an earlier study conducted in Iran during February–April 2020, the prevalence of AKI was lower (13.8%) [53]. A lower prevalence of AKI reported from China in the early stages of the COVID-19 pandemic could also be due to underestimation of signs and symptoms not involving the respiratory system (16, 17). Moreover, clinical management of COVID-19 has considerably evolved over time since the early days of the pandemic, and this may account for the inconsistency in AKI figures reported by different studies in diverse periods.

Despite the inconsistencies of prevalence among COVID-19 patients, common risk factors of AKI according to the open literature are advanced age, male gender, and comorbidities such as diabetes mellitus, hypertension, CKD, ischemic heart disease, electrolyte imbalance, and inflammatory markers (22, 23, 32, 36, 41, 54). Likewise, the role of diabetes, electrolyte imbalance, and inflammation in AKI was confirmed in the present study. These factors were already present at hospital admission, reflecting critical clinical conditions of patients entering the hospital independently from the severity of their viral lung disease, thereby supporting the hypothesis that treatment and alterations induced also by mild/moderate forms of COVID-19 may contribute to MOF, including AKI. The enhanced risk of AKI in males may reflect on one side a higher SARS-CoV-2 infection rate in males, on the other side their higher susceptibility to viral infections due to differences in natural immunity linked to sex chromosomes (41). The enhanced expression of ACE2 and TMPRSS2 receptors in males, regulated by androgens, might account for their higher susceptibility to severe COVID-19 (41, 55, 56). In contrast, estrogen may inhibit the cell invasion of SARS-CoV-2 by reducing the expression of TMPRSS2 (57).