2.1. Study design and participants
The present investigation was designed as a retrospective study involving a total of 476 COVID-19 patients admitted consecutively to the Union Hospital of Huazhong University of Science and Technology and the Wuhan Asia General Hospital, from December 2019 to March 2020. The clinical outcomes, discharge from hospital, or death in hospital, were recorded up to March 31, 2020. Both hospitals are located in Wuhan, Hubei Province, and were designated by the Chinese government as hospitals responsible for the treatment of COVID-19 patients. The disease was diagnosed according to the WHO guidelines.
Exclusion criteria were as follows: (1) patients that were pregnant or under 18 years old; (2) patients with severe or critical COVID-19 at admission; (3) patients receiving irregular treatment before the admission (Fig. 1). Based on the exclusion criteria, 338 COVID-19 patients were included in the final analyses.
All patients admitted to the hospital with mild or moderate COVID-19 were divided into the improvement group and the progression group. The improvement group included patients who recovered after admission and were discharged from the hospital. The progression group included patients who after the admission progressed to severe or critical condition, or died in the hospital, regardless whether the final outcome was discharged or death,
The protocol of the study was approved by the Research Ethics Committee of the Tongji Medical College. The data used in the study were anonymized, and the requirement for informed consent was waived by the Committee.
2.2. Data collection
Complete clinical data for all COVID-19 patients were collected from the medical records of the patients. They included demographic, clinical, laboratory, imaging, treatment, and outcome information. The laboratory data and CT scan of the lung were collected corresponded to the results of the first test performed upon admission. Prior to the analysis, the patient information was de-identified and anonymized. The NLR was calculated from the results of neutrophil and lymphocyte counts. SII was calculated according to the results of a routine blood test, according to the formula: SII=N×P/L, where N, P, and L represents the count of neutrophils, platelets, and lymphocytes, respectively (6).
2.3. Laboratory examination
Laboratory confirmation of the infection of SARS-CoV-2 was conducted by local CDC in accordance with the Chinese CDC protocol. Pharyngeal swab samples were collected from all patients and the samples were stored in a viral-transport medium for laboratory testing. Real-time RT-PCR was performed to exclude infection with other respiratory viruses including influenza A virus, coxsackie virus, influenza B virus,respiratory syncytial virus, enterovirus and parainfluenza virus. Automatic biochemical analyser, AU5800(Beckman Coulter, USA), was used for measuring the concentrations of ALT, AST, albumin, and creatinine. Liquid Assayed Multiqual was performed in QC procedures. Automated blood analyser, XE-2100 (Sysmex, Japan), was used for measuring the count of white blood cells, neutrophils, lymphocytes, and platelets. The lymphocyte test kit (Beckman Coulter Inc., FL, USA) was used for lymphocyte subset analysis. Plasma cytokines (IL10, IL6, IL4, IL2, IFN – γ and TNF - α) were detected using the human Th1/2 cytokine kit II (BD Ltd., Franklin lakes, NJ, USA). All laboratory tests were conducted in accordance with the product manual.
2.4. Study definitions
The classification of the severity of COVID-19 was based on the “Diagnosis and Treatment Protocol for COVID-19 (Trial Version 7)” (4). The clinical classifications of illness severity of COVID-19 were as follows: (1) mild type, with mild clinical symptoms and the absence of signs of pneumonia on imaging; (2) moderate type, with fever, respiratory tract symptoms, and signs of pneumonia were identified on imaging; (3) severe type, characterized by one of the following: a) respiratory distress, respiratory rate ≥ 30 breaths/min; b) mean oxygen saturation ≤ 93% in the resting state; c) oxygenation index ≤ 300 mmHg; and (4) critical type, characterized by one of the following: a) shock; b) respiratory failure requiring mechanical ventilation; c) organ failure requiring ICU admission. Fever was defined as an axillary temperature of at least 37.3°C(20). The duration of viral shedding was defined as the time from the onset of illness to the second negative nucleic acid test (4).
2.5. Statistical analysis
Summary statistics of the demographic data, clinical characteristics, laboratory results, and radiographic findings were expressed as median and the interquartile range (IQR) for continuous variables and as frequencies and proportions for categorical variables. If the variance in the improvement group and the progression group was the same, continuous variables were compared by the Student’s t-test; otherwise, the Welch’s t-test was used. Categorical variables were compared by the χ2 test or Fisher's exact test.
The relationship between the NLR treated as a continuous variable, and the progression of COVID-19 was examined first; subsequently, the relationship was evaluated considering NLR as a categorical variable according to the best threshold value. These relationships were examined using univariate and multivariate logistic regression analyses, and odds ratio (OR) and 95% confidence interval (CI) were calculated. After univariate logistic regression analysis, only the variables with a P-value of less than 0.1 were considered for multivariate analysis to identify predictors of progression of COVID-19. To avoid overfitting in multivariate analysis, six variables were selected on the basis of clinical constraints and previous studies. Previous investigations have documented that older age was associated with poor prognosis in COVID-19 patients (20). Recent studies indicated that patients with severe COVID-19 had higher C-reactive protein (CRP) and lactate dehydrogenase (LDH) levels than patients with non-severe COVID-19 (13). Moreover, in comparison with patients with mild COVID-19, the levels of IL-10 and IFNγ in patients with severe COVID-19 were significantly increased in the early stage of the disease, and most detected cytokines peaked in the serum 3-6 days after the onset (21, 22). Therefore, NLR, CRP, LDH, interleukin-10(IL-10), and interferon γ(IFNγ) were selected as the six variables for the multivariable logistic regression analysis.
To further validate the association between elevated NLR and disease progression, propensity score matching (PSM) was used to eliminate confounding bias (23). All COVID-19 patients were divided into two groups, the high NLR (≥3.75) group and the low NLR (<3.75) group, based on the best threshold value of NLR predicting disease progression. The propensity score was calculated using the logistic regression model in which age, gender, hypertension, coronary heart disease, diabetes, chronic obstructive pulmonary disease, fever, and cough were considered. The matching was performed using a 1:1 ratio. By this approach, patients in the high NLR group were matched with patients in the low NLR group having the closest propensity score. Patients selected by PSM were enrolled in a new cohort and subjected to further analysis of the association between NLR elevation and disease progression. The best threshold value of NLR was calculated according to the Youden index. The two-sided P-value of less than 0.05 was considered to indicate a statistically significant difference. SPSS 24.0 software was used to perform PSM and for statistical analysis.