During influenza epidemics, influenza-like illnesses (ILIs) viruses cocirculate with influenza strains. If positive, rapid influenza diagnostic tests (RIDTs) identify influenza A/B, but false-negative RIDTs require retesting by viral polymerase chain reaction (PCR). Patient volume limits testing during influenza epidemics, and non-specific laboratory findings have been used for presumptive diagnosis pending definitive viral testing. In adults, the most useful laboratory abnormalities in influenza include relative lymphopenia, monocytosis, and thrombocytopenia. Lymphocyte: monocyte (L:M) ratios may be even more useful. L: M ratios <2 have been used as a surrogate marker for influenza, but there are no longitudinal data on L: M ratios in hospitalized adults with viral ILIs. During the 2015 influenza A (H3N2) epidemic at our hospital, we reviewed our experience with L: M ratios in 37 hospitalized adults with non-influenza viral ILIs. In hospitalized adults with non-influenza A ILIs, the L: M ratios were >2 with human metapneumovirus (hMPV), rhinoviruses/enteroviruses (R/E), and respiratory syncytial virus (RSV), but not human parainfluenza virus type 3 (HPIV3), which had L: M ratios <2. HPIV-3, like influenza, was accompanied by L: M ratios <2, mimicking influenza A (H3N2). In influenza A admitted adults, L: M ratios <2 did not continue for >3 days, whereas with HPIV-3, L: M ratios < 2 persisted for >3 days of hospitalization.
