We enrolled 68 patients with COVID-19 who were hospitalized at Chungnam National University Hospital (Daejeon, Korea) from February 1 to April 24, 2020. All patients were tested for serum creatinine (SCr) and underwent routine urinalysis. The inclusion criteria were: 1) Age of at least 18 years, 2) Having undergone urine protein to creatinine ratio (PCR) and urine albumin to creatinine ratio (ACR) testing, and 3) Estimated glomerular filtration rate (eGFR) > 60 mL/min/1.73 m² at the time of visit. Of 68 patients, one was pediatric, one was excluded due to a SCr level of 4.3 mg/dL (eGFR, 14.8 mL/min/1.73 m²), and 66 were retained for the study.

The patients' medical records were reviewed. Data were collected, including age, gender, initial and follow-up SCr and eGFR (chronic kidney disease [CKD]-epidemiology collaboration), routine urinalysis with microscopy, urine PCR, urine ACR, underlying disease (diabetes mellitus [DM], hypertension, CKD, and cardiovascular disease), and whether mechanical ventilation, extracorporeal membrane oxygenation (ECMO), or renal replacement therapy was implemented.

Both nasopharyngeal (using a swab) and sputum (secretion) samples were collected from all patients and tested by real-time reverse transcription polymerase chain reaction (RT-PCR) using the PowerChek 2019-nCoV real-time polymerase chain reaction kit (KogeneBiotech Co., Ltd., Seoul, Korea). When positive was found in real-time RT-PCR, COVID-19 was diagnosed.

Acute kidney injury (AKI) was identified according to the guidelines of Kidney Disease: Improving Global Outcomes (KDIGO). It is defined as any of the following: an increase in SCr of ≥ 0.3 mg/dL within 48 hours; an increase in SCr of ≥ 1.5 times the baseline, which is known or presumed to have occurred within the past 7 days; or urine volume < 0.5 mL/kg/hr for 6 hours. AKI is staged for severity according to the criteria presented in Table 1.11

The degree of urine PCR and ACR was classified according to the KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of CKD (Table 2).12

Continuous variables were analyzed using Student's t-test and categorical covariates using Pearson's χ² test and Fisher's exact test (used with limited data). Continuous variables are expressed as means and standard deviation and discrete variables as percentages (%). The differences in the duration of follow-up according to group were evaluated using the Kruskal-Wallis test. All analyses were conducted using SPSS Statistics version 20.0 (IBM Corp., Armonk, NY, USA), and P values of less than 0.05 were considered statistically significant (SPSS version 20.0; SPSS Inc., Chicago, IL, USA).

The Institutional Review Board (IRB) of Chungnam National University Hospital (IRB No.2020-04-030) approved this study. The informed consent was waived. We conducted this study in compliance with the principles of the Declaration of Helsinki.

A total of 66 patients, 35 (53.0%) of whom were male, were analyzed, and the mean age was 45.6 years. The mean initial SCr level was 0.65 ± 0.18 mg/dL, and the mean initial eGFR was 113.1 ± 17.4 mL/min/1.73 m². The prevalence of DM, hypertension, and cardiovascular disease was 15.2%, 18.2%, and 3.0%, respectively (Table 3).

During the observation period, three (4.5%) patients were found to have AKI, according to the criteria defined in this study. Three patients had AKI stage 1, AKI stage 2, and AKI stage 3 each. Hemodialysis was performed in the patient with AKI stage 3 due to renal function deterioration. However, renal function improved, hemodialysis was discontinued, and renal function returned to normal. Two patients with AKI stages 1 and 2 each regained normal renal function.

For the three AKI patients, it was confirmed that nephrotoxic agents were more likely to have been the cause than other causes of AKI. All three patients used vancomycin, and AKI developed after using vancomycin.

The first patient, a 62-year-old-man, had no other underlying disease than schizophrenia, and his SCr was 0.81 mg/dL at the first examination after hospitalization. Chest computed tomography (non-contrast) examination during the first visit showed multifocal consolidation with ground-glass opacity and reticular opacity in all 5 lobes of both lungs. The patient took lopinavir/ritonavir for 19 days starting from the day of hospitalization. Mechanical ventilation was performed the next day, and ECMO was commenced 4 days later and maintained for 17 days. Blood cultures were performed when fever was identified, and vancomycin was added on the 10th day of hospitalization. Before using vancomycin, his SCr level was 1.0 mg/dL, but his renal function worsened after using vancomycin. His SCr gradually deteriorated to 1.19 mg/dL after 2days, 2.26 mg/dL after 3 days, and 3.35 mg/dL (peak) after 4 days of using vancomycin. The urine output decreased too (< 400 mL/day), so vancomycin was discontinued and continuous renal replacement therapy (CRRT) was started on day 14 of hospitalization and maintained for 18 days. The patient's laboratory data were closely followed up after CRRT was discontinued. The urine output was well maintained, but his SCr was continuously elevated (peak SCr: 5.14 mg/dL) and hemodialysis was performed twice. Afterwards, the SCr values improved and remained in the normal range without hemodialysis. The last recorded SCr value for this patient was 1.09 mg/dL.

The Second patient, a 78-year-old-woman, had hypertension as underlying disease and was on medication. Although she had no history of DM, her serum glucose was over 200 mg/dL at the time of visit. The HbA1c level measured after admission was 6.2%. On the first day of hospitalization, she was intubated and mechanical ventilation was maintained. An inotropic agent (norepinephrine) was used as blood pressure decreased. ECMO was commenced 1 day after the admission. Initial SCr was 0.53 mg/dL upon hospitalization. Piperacillin/tazobactam and lopinavir/ritonavir were used from the first day of hospitalization. Piperacillin/tazobactam was maintained for 4 weeks, and lopinavir/ritonavir was used for 9 days. Trimethoprim/sulfamethoxazole was used after 17 days of admission, and vancomycin was used after 29 days of admission. The SCr level before using vancomycin was 0.60 mg/dL, SCr 0.61 mg/dL 1 day after, SCr 1.10 2 days after, and SCr 1.13 mg/dL (peak) after 3 days of vancomycin use indicated AKI (stage 1). Since then, though vancomycin was used continuously, SCr improved to baseline Cr level. The last recorded SCr value was 0.54 mg/dL.

The third patient, a 64-year-old-woman, had surgery for thyroid cancer 8 years ago and had no other underlying disease. Her SCr was 0.41 mg/dL at the time of admission. Immediately after admission, the patient was intubated, after which mechanical ventilation and ECMO were performed. Mechanical ventilation was maintained for 12 days, and ECMO was maintained for 11 days. During hospitalization, the patient took lopinavir/ritonavir for 12 days. Vancomycin was commenced 4 days after admission. SCr level before vancomycin use was 0.56 mg/dL, SCr a day after the commencement of vancomycin use was 1.11 mg/dL, and SCr 2 days after the commencement of vancomycin was 1.20 mg/dL (peak); indicating AKI (stage 2). After that, although vancomycin was maintained for one week, SCr improved to baseline Cr level. The last recorded SCr level was 0.54 mg/dL. The three patients' laboratory data and clinical events are shown in Fig. 1.

In all 66 patients, routine urinalysis with a dipstick test was performed at the time of or during hospitalization. Trace and + albumin on a dipstick test was found in nine (13.6%) patients (trace in one patient, 1+ in two patients, and 2+ in six patients).

All patients were randomly tested for urine PCR and ACR more than once. In 20 (30.3%) out of 66 patients, urine PCR was higher than the normal range, 11 (16.7%) had severely increased proteinuria, and 9 (13.6%) had moderately increased proteinuria. Of the 11 patients with severely increased proteinuria, 10 had increased albuminuria, and one patient had no albuminuria. None of the nine patients with moderately increased proteinuria had albuminuria. Ten of 66 patients showed hematuria with red blood cell (RBC) counts > 3/HPF on urine microscopy.13 Eight of them were patients with severely increased proteinuria, and two were young women whose hematuria was not clinically meaningful due to the overlapping menstrual period.

The recovery of 20 patients with elevated urine PCR was compared. For patients with moderately increased proteinuria, on follow-up, six out of nine had not improved, and three had recovered to a normal range. For patients with severely increased proteinuria, on follow-up, 1 out of 11 had not improved, 4 had improved (a reduction of over 50% compared with their highest urine PCR levels), and 6 had recovered to a normal range.

Among COVID-19 patients, the clinical classification was done according to the National Health Commission of China guidelines (version 7) into mild, moderate, and severe cases.14 There were 21 mild cases, 37 moderate cases, and 8 severe cases. A total of three patients with AKI were included in the severe cases (P = 0.001). There was a high frequency of both the urine PCR and ACR in the severe cases, and proteinuria and albuminuria were also more severe in the severe COVID-19 group (P < 0.001) (Table 4).

No patients died of COVID-19 during the study period.