Early kidney injury predicts disease progression in patients with COVID-19: a cohort study

Since December 2019, Coronavirus Disease 2019 (COVID-19) outbreak, caused by severe respiratory syndrome coronavirus 2 (SARS-CoV-2), has become a worldwide pandemic and public health emergency [1]. Respiratory system injury is the principal clinical manifestation of COVID-19, and acute respiratory distress syndrome is one of life-threatening complications [2]. Most studies have focused on the clinical characteristics of pneumonia caused by COVID-19. However, a significant fraction of patients with COVID-19 display abnormalities in kidney function [3].

Existing studies have shown that SARS-CoV-2 virus may specifically attack the kidney, causing acute kidney injury (AKI) [4]. However, the evidence of kidney injury in COVID-19 patients remained controversial. Studies reported that acute kidney injury was a frequent condition in critically ill patients that has been associated with substantial morbidity and mortality [2, 5]. However, a study with an analysis of 116 hospitalized patients with COVID-19 in a single hospital showed that SARS-CoV-2 infection does not significantly cause AKI [6]. Indeed, the significance of AKI in patients with COVID-19 remained uncertain due to a lack of clear and operable diagnostic criteria for AKI in most medical reports. Previous studies mainly focused on COVID-19 patients superimposed acute kidney injury. However, the early decline of renal function, usually evaluated by abnormal estimated glomerular filtration rate (eGFR) and endogenous creatinine clearance rate (Ccr), could occur prior to acute kidney injury [7]. At present, few studies have explored the relationship between early kidney function indexes and disease progression of COVID-19, especially in the aspects of the dynamic changes of the early kidney function indexes. Thus, we conducted this large hospital-based cohort study to address the association of early kidney injury and COVID-19 disease progression.

In this study, we found there was a high proportion of early kidney function injury in COVID-19 patients on admission. eGFR and Ccr decreased in 22.0% and 24.0% of patients, respectively. This study also demonstrated a significant association between early kidney function injury and poor outcomes in patients with COVID-19 (Additional file 3: Table S3).

Because the SARS-CoV-2 spread mainly through the respiratory airway with viral pneumonia symptoms as the first onset and most typical symptoms, clinicians have paid considerable attention on the pneumonia treatment. However, the expression level of ACE2 in kidney, especially in the renal tubules, was no less than that in lung, suggesting that kidney might also be another important target organ for SARS-CoV-2 virus [11]. Evidence on Middle East respiratory syndrome coronavirus (MERS-CoV) indicated that SARS-CoV-2 may also attack directly at renal tubular epithelial cells [12]. The granules of SARS-CoV-2 have been isolated in patients' urine. Viral particles were also detected in electron micrographs of kidneys by autopsy, supporting the direct viral attack theory in kidney [13]. Although, the mechanism of kidney damage by SARS-CoV-2 has not yet been fully elucidated and further research is warranted to investigate these issues.

Our study focused on the association of early kidney injury with clinical outcomes of COVID-19. We revealed that patients with normal baseline serum creatinine have showed the decrease of eGFR and Ccr on early stage. Our results were consistent with findings from a previous study that none of 12 COVID-19 patients have abnormal BUN and Scr levels, but the abnormal reduction rates of eGFR and Ccr was 66.7% and 41.7%, respectively [14]. We found that 22.0% and 24.0% of patients had decreasing eGFR and Ccr on admission respectively, and these two ratios were markedly less than those reported by the previous study. The possible reason may be that our study had a larger sample and included more representative patients. Our finding indicated that early kidney dysfunctions was found in a large number of COVID-19 patients on admission, even in mild/moderate patients. Accumulated evidence supported that it was more likely for those patients who died from COVID-19 to have experienced acute kidney injury (25%) during hospitalization [15], and acute kidney injury was associated with death risk [16]. However, previous studies have mainly focused on acute kidney injury, but neglected the early kidney function dysfunction decrease (such as decrease of eGFR or Ccr). Early detection and intervention of renal dysfunction might effectively prevent the development of severe acute kidney injury. Our study demonstrated that the early kidney injury indicator, eGFR, was an independent risk factor for the poor outcomes in patients with COVID-19. In addition, it should be pointed out that patients with a normal range level of BUN and Scr on admission had an abnormal decrease eGFR. Thus, eGFR could be a more sensitive index than BUN, Scr, and acute kidney injury in early warning of adverse prognosis.

Hematuria and proteinuria are common indicators of abnormal urinary analysis and have been increasingly recognized as important markers in attack glomerular and podocytes [17]. A study on 710 COVID-19 patients reported that 44% of patients have proteinuria and 26.9% hematuria on admission [5]. Researchers also found that urinalysis can detect the early renal-impairment in patients with COVID-19 [18]. But their clinical values and effects on disease progression were not yet known. In our analysis, both hematuria and proteinuria on admission were independent risk factors for the progression of COVID-19.

A study based on 138 hospitalized COVID-19 patients revealed the incidence of elevated Scr and BUN was 15.5% and 14.1%, respectively [5, 19]. Kidney function dynamic monitoring data showed that 31%, 22% and 20% patients with increased BUN, Scr and uric acid levels, respectively [20]. Another study revealed that the median levels of Scr and BUN were 77 μmol/L and 5.7 mmol/L, with the elevated in 14.4% and 13.1% COVID-19 patients, respectively. Our findings showed that 7.1% and 4.4% patients had elevated Scr (median level 64.1 μmol/L) and BUN (4.37 mmol/L), respectively. We also found that BUN and Scr were both risk factors for disease progression, which was consistent with the result of Cheng [3]. In addition, the dynamic changes of Scr and uric acid reflected the changes of kidney injury in the clinical process. Our generalized linear mixed model analysis indicated that the dynamic trajectories of uric acid was significantly related to disease progression and the variation of Scr deserved continuous exploration. Therefore, dynamic monitoring of uric acid levels will be helpful to predict the prognosis of patients and take timely treatment measures.

Uric acid, BUN, and Scr are the blood biochemistry indexes directly reflecting kidney function. These indicators are convenient to obtain and are helpful for clinicians to monitor the kidney function of patients with COVID-19. Though the decline of eGFR on admission is related to poor prognosis, the evaluation of eGFR needs to be carried out according to the characteristics of each patient. The Uric acid variation seem more suitable to predict disease progression during hospitalization. Considering that these indicators may play a different role in various disease stages of COVID-19, therefore all of them were analyzed in our study.

Currently, most clinicians adopted the definition of AKI recommended by the Organization for Improving the Prognosis of Kidney Diseases Worldwide (KDIGO) as one of the following: (1) an increase in serum creatinine by ≥ 0.3 mg/dL (≥ 26.5 µmol/L) within 48 h, (2) an increase in serum creatinine to ≥ 1.5 times baseline within the previous 7 days, (3) urine volume ≤ 0.5 mL/kg/h for 6 h [10]. Due to the special nature of the Huoshenshan hospital and the urgency of the epidemic in Wuhan at that time, we cannot obtain the baseline levels of the patient's blood creatinine and the patient's urine output. Therefore, we identified the AKI patients on admission according to the special situation of AKI diagnosis issued by KDIGO: endogenous creatinine clearance < 60 mL/min or serum creatinine > 133umol/L, BUN > 20 mmol/L [10]. We then found 26 patients with acute kidney injury. According to this criterion, we probably missed some patients with a large increase in serum creatinine in a 48 h after admission or a significant decrease in urine volume, because some of the AKI occurred in a short period and recovered with the start of treatment [21]. This may be the main reason why we only observed early AKI in 26 patients at admission, which could lead to underestimate of the AKI incidence in our study. However, our primary purpose was to explore the association of early kidney function indexes and their dynamic changes with disease progression of COVID-19. Therefore, we did not analyze the association of AKI with disease progression.

Even though this cohort study enrolled 2630 hospitalized patients, there were several limitations. First, the clinical data came from a single center, and our findings need to be validated by external cohorts. Second, our study was a real-world study with dataset from clinical practice, it was difficult to avoid information bias caused by missing and inaccurate clinical information. Third, although we attempted to adjust for potential confounders in multivariate analysis, there might be unmeasured or unknown confounding bias in our study.

There was a high proportion of early kidney function injury in COVID-19 patients on admission. The early injury in kidney function predicted the poor prognosis of COVID-19. The dynamic changes blood uric acid were associated the poor prognosis. Our findings could help clinicians prevent worse outcome and promote the recovery of patients.