Influenza remains an important disease despite the COVID-19 pandemic. Given the fact that COVID-19 is likely to be a long-term threat, simultaneous transmission of both diseases during the flu season continues to be a great concern. Nosocomial influenza jeopardizes the normal functioning of the hospital system, endangers the health of the inpatients with underlying comorbidities, and increases the risk of infection to the medical staff. Although nuclear acid testing using PCR is a reliable diagnostic tool, it requires adequate resources and has a long turnaround time.
CBC is a fast and convenient test that offers valuable information about the differential concentration of various blood cells as well as the ratios of certain cell components. It is a routine test conducted for newly admitted patients or those with a sudden onset of fever in daily practice in our center. This study revealed a clear two-way curve when LMR, NLR, and PLR were calculated and compared between the time points of admission, influenza symptom onset, and recovery. In ROC curve analysis, all three ratios showed large AUC values, along with relatively high sensitivity and specificity; however, LMR is the preferred indicator for influenza diagnosis for the following reasons: first, it involves two remarkably changed parameters (lymphocytes and monocytes); second, the specificity is higher than NLR and LPR; third, although not shown in the present study, previous studies have reported that NLR is significantly elevated among COVID-19 patients and can predict their outcome.15–17 The reduction in LMR is universal regardless of the patients’ sex, age, or underlying diseases. There was no significant difference in LMR levels between different influenza types, years, or wards (surgical vs. medical). Notably, our study shows that parameters such as LMR can save more than one day in the diagnosis of influenza with a much cheaper price, which is a remarkable advantage in the context of nosocomial influenza detection and prevention.
Studies using lymphocyte count and leukocyte ratios for influenza diagnosis date back to the H1N1 pandemic in 2009.18 A lymphocyte to monocyte ratio below 2 was proposed as a screening tool for influenza. Zheng et al13 described a series of 15 cases of PCR-proven influenza-infected hospitalized adult patients, of whom 80.0% had an LMR of < 2 and 93.3% had an LMR < 2.5 at the point of influenza diagnosis. McClain et al. measured LMR daily in patients experimentally infected with influenza H3N2 (n = 17), respiratory syncytial virus (RSV, n = 20), and human rhinovirus (HRV, n = 20).19 LMR < 2 predicted 100% of all symptomatic influenza-infected patients on day 3 with peak symptom severity, but was less predictive for symptomatic RSV (60%) or HRV (18%). Another study compared LMR in patients in the emergency department diagnosed with H1N1 influenza (n = 18) or culture-proven Streptococcus pneumoniae community-acquired pneumonia (CAP; n = 18) and found that LMR < 2 was associated with influenza (67% vs. 38%, p = 0.05). Yuan et al. al20evaluated the application of neutrophil-lymphocyte ratio in the diagnosis of influenza among preschool children, and found that among 378 children with an influenza-like illness, 99 (26.19%) cases were positive for influenza A. Furthermore, there was a significant difference in the NLR between patients with influenza A infection and children without infection (p < 0.05). When NLR was 0.42, the sensitivity for the prognosis was 86.1%, specificity was 93.2%, and AUC was 0.594.
A decrease in LMR can result from lymphopenia and/or monocytosis.21 Previously, relative lymphopenia was found to be an early and reliable laboratory finding in adult influenza A.22,23 Cunha et al. claimed that, similar to human seasonal influenza A, relative lymphopenia appears to be a laboratory marker of H1N1.22 However, the exact mechanism remains to be elucidated. Dynamic changes in the lymphocyte count in adult patients with severe pandemic H1N1 influenza A were also observed.24 Lymphocyte proportions and absolute counts returned to normal or remained slightly higher than normal within 2–3 weeks after disease onset. Lymphopenia may be associated with apoptosis induced by viral infection.25 Lymphopenia may also result from the redistribution and migration of lymphocytes to the respiratory system to combat the virus.24 There is evidence that influenza virus infection could induce apoptosis of lymphocytes and temporally destroy this line of defense of the immune system.26 Analysis of lymphocyte subpopulations after exposure to influenza A virus showed that a portion of the CD3+, CD4+, CD8+, and CD19+ lymphocytes became apoptotic.27 Lymphocyte apoptosis likely represents part of an overall beneficial immune response but could also be a possible mechanism of disease pathogenesis. Recruitment of monocytes is essential for the effective control and clearance of viral infections, including influenza.28 Monocytes egress from the bone marrow into the circulation mediated by CC-chemokine receptor 2 (CCR2). It has been shown that the increase in circulating monocytes that is associated with infection or sterile inflammation is mediated by CCL2 (CC ligand 2, also known as MCP1) and CCL7 (MCP3), triggering CCR2 signaling in LY6Chi monocytes in the bone marrow.29
LMR has also been demonstrated as a new systemic inflammatory indicator in many diseases, including various active infections, solid organ malignancies, hematological malignancies, and rheumatic diseases.9,10,12,30 As a result, the diagnostic value of this parameter may be considered doubtful or obscure. Such parameters are unlikely to be useful as definitive diagnostic criteria for certain diseases. However, nosocomial influenza is an ideal disease entity for an effective diagnostic parameter for the following reasons: First, PCR tests are expensive and have a long turnaround time. Second, influenza has a high transmission capacity, and timely diagnosis is required for implementing effective infection control measures. Third, influenza can be treated and prevented by oseltamivir, but treatment should be implemented within 48 h of onset, which again calls for a fast diagnosis. Our study has proven that parameters including LMR and NLR can be useful, timely, and cost-efficient diagnostic tools during hospital outbreaks of influenza, especially when leading cases are already confirmed by PCR or antigen testing.
Our study has several limitations. First, it was a retrospective study conducted in a single center, and the sample size was relatively small. Second, it was difficult to identify another clearly defined nosocomial outbreak as a control group. Instead, we used COVID-19 patients diagnosed in our center based on the similar presentation of the two diseases; COVID-19 patients were derived from the community rather than a hospital. Third, although ≥ 48 h after admission is the criterion for hospital infection, it is possible that they were acquired in the community. Fourth, appropriate and timely discovery of a nosocomial influenza outbreak can be challenging, especially among those with atypical influenza-like symptoms; consequently, application of our study can be limited. Therefore, further controlled studies comprising a greater number of patients from different centers are needed to validate the clinical value of lymphocytes, LMR, NLR, and PLR, and extend their generalizability and applicability for influenza diagnosis.