Longitudinal analysis of leukocyte differentials in peripheral blood of patients with acute respiratory viral infections.

BACKGROUND: Leukocyte counts and differentials are commonly acquired in patients with suspected respiratory viral infections and may contribute diagnostic information. However, most published work is limited to a single timepoint at initial presentation to a medical provider, which may correspond to widely varying points in the course of disease. OBJECTIVES: To examine the temporal development and time-dependent utility of routine leukocyte differentials in the diagnosis of respiratory viral infections. STUDY DESIGN: We analyzed data from recent experimental human challenges with influenza A/H3N2, human rhinovirus (HRV), and respiratory syncytial virus (RSV). Routine clinical lab cell counts and differentials were measured daily from the time period immediately prior to inoculation through the eventual resolution of symptomatic disease. RESULTS: Approximately 50% of challenged individuals developed symptoms and viral shedding consistent with clinical disease. Subpopulations of WBC showed marked differences between symptomatic and asymptomatic individuals over time, but these changes were much more profound and consistent in influenza infection. Influenza-infected subjects develop both relative lymphopenia and relative monocytosis, both of which closely mirror symptom development in time. A lymphocyte:monocyte ratio of <2 correctly classifies 100% of influenza (but not RSV or HRV) infected subjects at the time of maximal symptoms. CONCLUSIONS: Leukocyte differentials may suggest a viral etiology in patients with upper respiratory infection, but are not sufficient to allow differentiation between common viruses. Timing of data acquisition relative to the disease course is a key component in determining the utility of these tests.