2.4 Statistical analysis
In total, 1107 Flu-p patients were identified and enrolled in this study. These patients were then randomly assigned to derivation and validation cohorts (80% and 20% of patients, respectively), which were respectively used to develop and validate our prognostic model.
In addition, these 1107 patients were separated into two groups based upon whether or not they underwent IMV within 14 days following admission. Baseline characteristics were then compared between these two patient groups, and all variables which yielded a P < 0.1 in these initial univariate analyses were incorporated into a multivariate stepwise logistic regression model to identify risk factors associated with 14-day IMV rates. To ensure model simplicity, each risk factor was assigned an integer score value associated with its corresponding regression coefficient (β) value. Model cutoff scores were then defined using receiver operating characteristic (ROC) curves based upon Youden’s index. Kaplan-Meier analyses were conducted to compare rates of IV between patients above and below this cutoff score (high-risk and low-risk, respectively). The area under the ROC curve (AUROC) was used to gauge the prognostic performance of this model based upon overall sensitivity and specificity values.
A Kolmogorov–Smirnov test was used to assess result normality, with normally and non-normally distributed variables being presented as means ± standard deviation and medians, respectively. Continuous variables were evaluated with Mann-Whitney U tests or Student’s t-tests, whereas categorical variables were assessed with Fisher’s exact test or chi-squared tests. A two-tailed P < 0.05 was indicative of significance. SPSS 22.0 or MedCalc 19.0 were used for all statistical testing.
Results3.1 Patient screening
A total of 3405 hospitalized patients who were found to be positive for influenza viral RNA during the study period were screened for eligibility, of whom 1107 with laboratory-confirmed Flu-p were enrolled in this study. Flu-p was associated with influenza A virus and influenza B virus infections in 683 and 424 of these patients, respectively ( Figure 1).