Abstract

Introduction: The primary complications of influenza are secondary infections, particularly pneumonia, which contribute to increased morbidity and mortality. Currently, no reliable methods exist to differentiate secondary infections occurring alongside influenza. This study aimed to evaluate the role of procalcitonin (PCT) and lymphocyte levels in predicting mortality and diagnosing secondary infections in patients hospitalized with influenza pneumonia.

Materials and Methods: Patients with confirmed influenza and radiological evidence of lung infiltration on chest X-ray or computed tomography were included. Medical records were reviewed retrospectively. Patients were classified into two groups: those with influenza alone and those with influenza and a secondary infection. The highest PCT level and the lowest lymphocyte count recorded during hospitalization were analyzed for their association with secondary infection risk and mortality.

Results: Among 66 patients, 30 (45%) were treated in the intensive care unit (ICU), while 36 (55%) received care in the general ward. Secondary infections were identified in 29 patients (43.9%). Although ICU admission rates did not differ between groups, mortality was 38.4% in patients with secondary infections and 3% in those with influenza alone. During the 5-day influenza treatment period, C-reactive protein and PCT levels showed no significant differences between groups. The highest median PCT levels in discharged and deceased patients were 1.63 and 9.8 μg/L, respectively (p=0.005). The mean lowest lymphocyte counts in discharged and deceased patients were 300 cells/mL and 100 cells/mL, respectively (p=0.008). Among patients with a lowest lymphocyte count below 200 cells/mL, the secondary infection rate was 73% compared to 35.3% (p=0.031) in those with a count above 200 cells/mL. Additionally, mortality was 46% vs. 9.8% (p=0.001), and hospital stay was longer at 20 (13-40) days vs. 15 (9-19) days (p=0.047), respectively.

Conclusion: Patients hospitalized with influenza frequently develop secondary infections, which are linked to higher mortality. A lymphocyte count below 200 cells/mL is associated with an risk of secondary infection, prolonged hospitalization, and higher mortality. Although elevated PCT levels were also linked to an increased risk of secondary infections and mortality, this association was not statistically significant.