Comparative analysis of routine peripheral blood parameters in influenza and COVID-19 patients: a retrospective study.

Influenza and COVID-19 are major causes of respiratory illness, and differentiating between them remains a clinical challenge when molecular testing is unavailable. This retrospective study evaluated whether routine peripheral blood parameters could aid in this differentiation. Data from 848 participants (225 influenza, 358 COVID-19, and 265 healthy controls) were analyzed. Among all tested parameters, only white blood cell (WBC) count and age differed significantly between influenza and COVID-19 patients (median WBC: 8.45 versus 6.67×109/L, p<0.001; median age: 67 versus 71 years, p<0.001). Notably, procalcitonin (PCT) levels did not differ significantly between the two groups (p>0.05). The composite index WBC/Age amplified intergroup differences more effectively than other composite indices, including WBC×PCT, which produced a weaker effect. Receiver operating characteristic analysis revealed poor discrimination for all parameters and models: WBC count (AUC=0.67), age (AUC=0.59), WBC×PCT (AUC=0.60), and a logistic regression model combining WBC and age (AUC=0.67). Age-stratified analysis confirmed that WBC count remained significantly higher in influenza patients in both younger and older subgroups, but discriminatory performance was similarly limited (AUC=0.64 and 0.68, respectively). These findings indicate that, despite statistical differences in WBC count and age, routine blood parameters-whether alone or in combination-lack sufficient discriminatory power to reliably differentiate influenza from COVID-19 in this study. Molecular testing remains essential for accurate diagnosis.