Influenza is a self-limiting disease caused by various highly contagious viruses that primarily affect the respiratory tract, characterized by rapid transmission and general susceptibility in the population (1, 2). Based on differences in nucleoprotein (NP) and matrix protein (M) antigenicity, influenza viruses are classified into four subtypes: A, B, C, and D. Among these, influenza A and B viruses exhibit high mutational variability, with widespread susceptibility across populations, and are more likely to cause systemic toxic symptoms such as high fever, sore throat, fatigue, cough, and myalgia. In the late stages of infection, these cases may progress to pneumonia or even severe disease (3). Globally, influenza viruses are estimated to cause approximately 3–5 million severe cases and 290,000–650,000 deaths annually (4). Previous studies have shown that early treatment of influenza-related symptoms is an effective strategy to prevent viral transmission and reduce the incidence of severe disease and mortality (5, 6). Therefore, rapid and accurate treatment in the early stages of influenza infection is of particular importance.

Currently, the detection methods for influenza virus include viral culture, viral antigen detection, nucleic acid–based tests (NATs), serological assays, next-generation sequencing (NGS), as well as other techniques such as gene chip analysis. Each method has its own advantages and limitations. Viral culture, considered the “gold standard” for laboratory diagnosis of influenza, requires the longest turnaround time. In contrast, nucleic acid testing and sequencing technologies enable more rapid diagnosis but demand advanced laboratory conditions and incur higher costs, making them less suitable for routine screening. Antigen detection, based on the principle of antigen–antibody specificity, offers relatively high sensitivity, technical maturity, rapid turnaround, and moderate cost, making it suitable for large-scale influenza screening. However, due to the high mutational variability of influenza A and B viruses, antigen detection may yield false-negative results (7, 8). Therefore, influenza diagnosis cannot rely solely on antigen detection and should be supported by additional diagnostic evidence.

Complete blood count (CBC) is one of the most commonly performed laboratory tests in outpatient settings, and its combined analysis with influenza virus antigen detection can improve diagnostic sensitivity. Lupovitch et al. reported that patients with influenza A exhibit elevated neutrophil and monocyte counts accompanied by decreased lymphocyte levels (9). Similarly, Vangeti demonstrated that monocyte levels are positively correlated with disease severity following influenza virus infection (10). The lymphocyte-to-monocyte ratio (LMR) and the neutrophil-to-lymphocyte ratio (NLR) have been widely reported as novel inflammatory markers (11, 12). It has been documented that NLR increases significantly after influenza virus infection in children (13), while Russell’s study indicated that decreased LMR is associated with influenza virus infection (14). However, these studies do not encompass all patient populations, and comparative analyses of CBC parameters between influenza A and B remain limited across different age groups. This may explain the inconsistency observed among previous study findings.

In previous studies on the diagnosis and investigation of influenza virus infection, relative values or ratios of hematological parameters have often been shown to outperform absolute counts, as they are less likely to be overlooked when alterations in blood components occur (15, 16). In the present study, we analyzed hematological parameters including neutrophil, lymphocyte, and monocyte ratios, as well as LMR and NLR, in patients with different types of influenza. Furthermore, patients were stratified by age groups to evaluate age-related changes in these parameters and to explore their associations with different influenza virus subtypes.

Patients

This study enrolled a total of 1,667 individuals aged 4–79 years who attended Chengdu Second People’s Hospital between October 2024 and January 2025. According to the Age-Based Grouping Criteria in Medicine (17), participants were categorized into the following groups: pediatric group (0–19 years), comprising children (4–13 years) and adolescents (14–19 years); young adult group (20–39 years); middle-aged group (40–59 years); and older adult group (60–79 years). Within 48 hours of confirmed influenza diagnosis, all patients underwent influenza virus antigen testing of respiratory secretions and complete blood count (CBC) analysis. Diagnostic criteria for influenza A and B were determined based on A Brief Review of Influenza Virus Infection (18), and inclusion required patients presenting with symptoms such as high fever and chills, headache, general fatigue, severe myalgia and arthralgia, conjunctival hyperemia, sore throat, dry cough, and rhinitis. Exclusion criteria were: fever lasting longer than 24 hours; presence of malignancy; pregnancy or lactation; concurrent infections (e.g., pneumonia or urinary tract infections); other viral infections (e.g., respiratory syncytial virus, adenovirus, Mycoplasma pneumoniae, or Chlamydia pneumoniae); and patients with immune system disorders, hematological diseases, or severe hepatic or renal dysfunction. The study was approved by the Ethics Committee of Chengdu Second People’s Hospital (approval number: [KY] PJ2025409).

Detection of routine blood parameters

Venous or capillary blood samples were collected and transferred into tubes containing EDTA-K2 anticoagulant, followed by immediate routine hematological examination. Analyses were performed using a standard hematology analyzer (XN-2800, SYSMEX, Hyogo, Kobe, Japan). The following parameters were recorded: white blood cell (WBC) count; absolute neutrophil count (N) and neutrophil ratio (N ratio); absolute lymphocyte count (L) and lymphocyte ratio (L ratio); absolute monocyte count (M) and monocyte ratio (M ratio). Ratios were defined as the proportion of each leukocyte subtype relative to the total WBC count, expressed as decimals rather than percentages. In addition, derived hematological indices were calculated, including the lymphocyte-to-monocyte ratio (LMR), defined as the ratio of lymphocytes to monocytes, and the neutrophil-to-lymphocyte ratio (NLR), defined as the ratio of neutrophils to lymphocytes.

Application of Colloidal Gold Antigen Detection for Influenza Virus

Influenza A and B virus antigens were detected using a commercially available rapid antigen detection kit (Influenza A/B Virus Antigen Test Kit, Abbott Diagnostics, Chicago, IL, USA), following the manufacturer’s instructions. Briefly, six drops of sample dilution buffer were added to nasopharyngeal swab specimens and mixed thoroughly. The results were interpreted within 15 minutes. The uppermost line served as the quality control line, indicating the validity of the sample and reagents. The appearance of red detection lines below corresponded to positive antigen results for influenza A and B, respectively, confirming influenza virus infection.

1.Patient characteristics

Patients who met the inclusion and exclusion criteria for influenza-like symptoms and tested positive for influenza A or B antigens were assigned to groups accordingly. A total of 590 patients who were positive only for influenza A antigen were included in the influenza A group (A+), and 506 patients who were positive only for influenza B antigen were included in the influenza B group (B+). In addition, 571 healthy individuals who tested negative for both influenza A and B antigens during routine physical examination were selected as the control group (Con). All participants were stratified into five age groups: 4–13 years, 14–19 years, 20–39 years, 40–59 years, and 60–79 years. Within each age group, no statistically significant differences were observed in the distribution of age and sex among the A+, B+, and Con groups (P > 0.05) (Table 1).

2.Differences in routine blood parameters among the clinical groups in the three age groups

In the 4–13-year age group, compared with the Con group, both the A + group and B + group exhibited significantly increased white blood cell (WBC) counts and monocyte ratio (M ratio) (P < 0.05), along with a significant decrease in lymphocyte-to-monocyte ratio (LMR) (P < 0.05). Additionally, in the A + group, the lymphocyte ratio (L ratio) was significantly reduced (P < 0.05), whereas neutrophil-to-lymphocyte ratio (NLR) and neutrophil ratio (N ratio) were significantly elevated (P < 0.05) (Table 2, Fig. 1A).

In the 14–19-year age group, compared with the Con group, both the A + group and B + group showed a significant increase in M ratio (P < 0.05) and a significant decrease in LMR (P < 0.05). In the A + group, N ratio and NLR were significantly elevated (P < 0.05), while L ratio was significantly reduced (P < 0.05) (Table 3, Fig. 1B).

In the 20–39-year age group, compared with the Con group, both the A + group and B + group demonstrated a significant increase in M ratio (P < 0.05) and a significant reduction in LMR (P < 0.05). Specifically, in the A + group, NLR was significantly elevated (P < 0.05) and L ratio was significantly decreased (P < 0.05), while in the B + group, N ratio and NLR were significantly decreased (P < 0.05) (Table 4, Fig. 1C).

In the 40–59-year age group, compared with the Con group, both the A + group and B + group showed a significant increase in M ratio (P < 0.05) and a significant reduction in LMR (P < 0.05). In the A + group, L ratio was significantly decreased (P < 0.05), while in the B + group, N ratio and NLR were significantly decreased (P < 0.05), and L ratio was significantly increased (P < 0.05) (Table 5, Fig. 1D).

In the 60–79-year age group, compared with the Con group, the A + group showed significantly decreased L ratio and LMR (P < 0.05), whereas in the B + group, N ratio and NLR were significantly decreased (P < 0.05) and M ratio was significantly increased (P < 0.05) (Table 6, Fig. 1E).

M ratio, N ratio, and L ratio = the fractions of monocytes, neutrophils, and lymphocytes relative to the total white blood cell count; MON, NEU, and LYM = absolute counts of monocytes, neutrophils, and lymphocytes; LMR = lymphocyte-to-monocyte ratio; NLR = neutrophil-to-lymphocyte ratio.

M ratio, N ratio, and L ratio = the fractions of monocytes, neutrophils, and lymphocytes relative to the total white blood cell count; MON, NEU, and LYM = absolute counts of monocytes, neutrophils, and lymphocytes; LMR = lymphocyte-to-monocyte ratio; NLR = neutrophil-to-lymphocyte ratio.

M ratio, N ratio, and L ratio = the fractions of monocytes, neutrophils, and lymphocytes relative to the total white blood cell count; MON, NEU, and LYM = absolute counts of monocytes, neutrophils, and lymphocytes; LMR = lymphocyte-to-monocyte ratio; NLR = neutrophil-to-lymphocyte ratio.

M ratio, N ratio, and L ratio = the fractions of monocytes, neutrophils, and lymphocytes relative to the total white blood cell count; MON, NEU, and LYM = absolute counts of monocytes, neutrophils, and lymphocytes; LMR = lymphocyte-to-monocyte ratio; NLR = neutrophil-to-lymphocyte ratio.

M ratio, N ratio, and L ratio = the fractions of monocytes, neutrophils, and lymphocytes relative to the total white blood cell count; MON, NEU, and LYM = absolute counts of monocytes, neutrophils, and lymphocytes; LMR = lymphocyte-to-monocyte ratio; NLR = neutrophil-to-lymphocyte ratio.

Influenza A

In the 4–13-year age group, using the Con group as a reference, the areas under the curve (AUCs; 95% confidence intervals), ranked from highest to lowest, were: LMR, 0.6973 (0.6280–0.7667); MON, 0.6288 (0.5552–0.7023); LYM, 0.6250 (0.5507–0.6992); NLR, 0.6129 (0.5382–0.6877); and NEU, 0.5764 (0.5000–0.6527). The optimal cutoff values were 1.83, 0.50, 0.70, 2.63, and 3.20, respectively (Fig. 2A).

In the 14–19-year age group, the AUCs (95% CI), ranked from highest to lowest, were: LMR, 0.6860 (0.6193–0.7528); LYM, 0.6277 (0.5571–0.6983); NLR, 0.6211 (0.5501–0.6921); MON, 0.6020 (0.5301–0.6740); and NEU, 0.5934 (0.5209–0.6660). The optimal cutoff values were 1.55, 0.187, 3.55, 1.00, and 4.20, respectively (Fig. 2B).

In the 20–39-year age group, the AUCs (95% CI), ranked from highest to lowest, were: LMR, 0.7302 (0.6666–0.7938); MON, 0.6375 (0.5676–0.7075); LYM, 0.6085 (0.5376–0.6793); NLR, 0.5978 (0.5266–0.6690); and NEU, 0.5590 (0.4867–0.6312). The optimal cutoff values were 1.80, 0.60, 0.90, 4.71, and 5.40, respectively (Fig. 2C).

In the 40–59-year age group, the AUCs (95% CI), ranked from highest to lowest, were: LMR, 0.6405 (0.5716–0.7093); MON, 0.6061 (0.5353–0.6769); LYM, 0.5662 (0.4938–0.6386); NLR, 0.5568 (0.4842–0.6293); and NEU, 0.5140 (0.4407–0.5872). The optimal cutoff values were 1.10, 0.40, 0.90, 4.22, and 5.50, respectively (Fig. 2D).

In the 60–79-year age group, the AUCs (95% CI), ranked from highest to lowest, were: LMR, 0.6083 (0.5321–0.6845); LYM, 0.5565 (0.4777–0.6353); NLR, 0.5541 (0.4752–0.6330); NEU, 0.5513 (0.4721–0.6305); and MON, 0.5375 (0.4593–0.6158). The optimal cutoff values were 1.60, 1.00, 2.66, 5.80, and 0.30, respectively (Fig. 2E).

Influenza B

In the 4–13-year age group, using the Con group as a reference, the areas under the curve (AUCs; 95% confidence intervals), ranked from highest to lowest, were: MON, 0.7396 (0.6741–0.8050); LMR, 0.6334 (0.5592–0.7077); NEU, 0.5802 (0.5038–0.6565); NLR, 0.5379 (0.4605–0.6153); and LYM, 0.5179 (0.4401–0.5957). The optimal cutoff values were 0.80, 2.16, 3.00, 5.44, and 1.00, respectively (Fig. 3A).

In the 14–19-year age group, the AUCs (95% CI), ranked from highest to lowest, were: MON, 0.6734 (0.5851–0.7617); NEU, 0.6216 (0.5294–0.7137); NLR, 0.6105 (0.5165–0.7044); LYM, 0.6089 (0.5146–0.7032); and LMR, 0.5350 (0.4351–0.6349). The optimal cutoff values were 0.70, 9.00, 7.91, 0.70, and 1.66, respectively (Fig. 3B).

In the 20–39-year age group, the AUCs (95% CI), ranked from highest to lowest, were: MON, 0.7747 (0.7152–0.8342); LMR, 0.6504 (0.5816–0.7191); NEU, 0.6239 (0.5529–0.6949); NLR, 0.5626 (0.4902–0.6349); and LYM, 0.5435 (0.4709–0.6162). The optimal cutoff values were 0.80, 1.37, 3.60, 6.80, and 0.70, respectively (Fig. 3C).

In the 40–59-year age group, the AUCs (95% CI), ranked from highest to lowest, were: MON, 0.7495 (0.6880–0.8109); NEU, 0.6604 (0.5928–0.7279); NLR, 0.6181 (0.5483–0.6879); LYM, 0.6052 (0.5347–0.6756); and LMR, 0.5711 (0.4996–0.6426). The optimal cutoff values were 1.00, 8.10, 2.85, 0.90, and 3.00, respectively (Fig. 3D).

In the 60–79-year age group, the AUCs (95% CI), ranked from highest to lowest, were: MON, 0.6734 (0.5851–0.7617); NEU, 0.6216 (0.5294–0.7137); NLR, 0.6105 (0.5165–0.7044); LYM, 0.6089 (0.5146–0.7032); and LMR, 0.5350 (0.4351–0.6349). The optimal cutoff values were 0.70, 5.20, 8.22, 1.00, and 1.66, respectively (Fig. 3E).

Influenza A virus (IAV) and influenza B virus (IBV) are major respiratory pathogens in humans and are responsible for seasonal influenza epidemics. IAV possesses a broader host range and a higher mutation rate, which enables it to cause global pandemics and poses a substantial threat to public health (19). Clinical and immunological studies have suggested that IAV and IBV elicit age-dependent differences in host responses: IAV tends to induce a stronger inflammatory response in young and middle-aged adults, whereas this difference diminishes in the elderly (20, 21). In contrast, IBV has been shown to trigger sustained monocytosis and alterations in lymphocyte populations across a wider age spectrum, including both children and older adults (22). These findings highlight age as a critical factor in modulating virus-specific immune responses.

Through a cross-sectional analysis of complete blood count (CBC) parameters in patients aged 4–79 years with influenza A (IAV) and influenza B (IBV) compared with age-matched healthy controls, we found that both types of influenza virus infection induced characteristic peripheral blood cell alterations, with notable commonalities as well as distinct differences. Compared with controls, the neutrophil ratio (N ratio) was significantly increased in the IAV group aged 4–19 years, whereas it was significantly decreased in the IBV group aged 20–79 years. The monocyte ratio (M ratio) was significantly higher in both IAV and IBV groups across most age groups, with the exception of IAV patients aged 60–79 years, in whom no significant difference was observed.The lymphocyte ratio (L ratio) in the IAV group was consistently and significantly reduced across all age groups (4–79 years), reflecting a pronounced influenza-associated lymphopenic trend (16). In the IBV group, the L ratio tended to decrease in the 4–19-year age group, but from age 20 onwards showed an upward trend, becoming significantly higher than controls in the 40–59-year age group (with a borderline increase at 60–79 years, P = 0.0521). As a consequence, the derived hematological indices also showed distinct alterations. The lymphocyte-to-monocyte ratio (LMR) was generally and significantly reduced in both IAV and IBV groups across all ages, except for IBV patients aged 60–79 years where the difference disappeared. The neutrophil-to-lymphocyte ratio (NLR), however, displayed divergent patterns: in the IAV group, NLR was significantly elevated in patients aged 4–39 years, while in the IBV group, it was significantly decreased from age 20 through 79 years. Taken together, these results indicate that both IAV and IBV infections lead to substantial alterations in leukocyte subsets, yet the hematological impact is virus- and age-dependent. IAV infection was generally characterized by decreased L ratio and increased N ratio, whereas IBV infection showed relatively stable L ratio in childhood and adolescence (4–19 years), followed by increased L ratio in adulthood (40–59 years). Both viruses were associated with elevated M ratio, but the direction of N ratio changes differed between IAV and IBV. These findings suggest that IAV and IBV may induce distinct age-related immune cell response patterns.

In this study, NLR exhibited completely divergent alterations between influenza A and B virus infections. This bidirectional pattern not only suggests virus type–specific differences in host immune responses but also reflects age-dependent features of inflammatory regulation. The increase in NLR observed in influenza A virus (IAV) infection is consistent with previous findings (23). Substantial evidence indicates that IAV, particularly the H1N1 and H3N2 subtypes, induces a pronounced systemic inflammatory response characterized by enhanced neutrophil recruitment and concomitant lymphopenia(24–26). Elevated NLR represents a peripheral manifestation of these immunological changes. In both pediatric and adult populations, increased NLR has been correlated with disease severity and even with higher mortality risk (27). Mechanistically, this may be linked to viral virulence factors such as non-structural protein 1 (NS1), which augments innate immune activation and cytokine release (28). Thus, our finding of increased NLR in IAV patients further reinforces its potential as an indicator of inflammatory activity and disease severity.In contrast, the significant reduction of NLR in influenza B virus (IBV) patients differs from previous reports. Some studies have suggested that NLR in IBV patients does not differ markedly from that in healthy controls, or shows only mild elevation (13), however, these observations were limited to pediatric cohorts, while changes in NLR among middle-aged and elderly IBV patients have not been reported. Our findings suggest that this discrepancy may be explained by age-related immune remodeling. Elderly individuals often exhibit features of immunosenescence and “inflammaging,” characterized by impaired neutrophil chemotaxis and dysregulated lymphocyte function(29, 30). Against this background, IBV may fail to elicit a robust neutrophil-driven inflammatory response, while relatively preserved or even compensatory lymphocyte levels could lead to a reduced NLR. Another plausible explanation is the lower overall virulence of IBV, as epidemiological data indicate that IBV infections are less frequently associated with systemic complications and severe disease compared with IAV (22). Accordingly, the reduced NLR observed in our study may reflect a combined effect of viral pathogenicity and host immune aging, providing new evidence for the generally milder clinical course of IBV infection. It should be noted, however, that the diagnostic accuracy of NLR alone is limited. Recent studies have demonstrated that combining NLR with other acute-phase reactants, such as C-reactive protein (CRP) and serum amyloid A (SAA), can markedly improve diagnostic and prognostic evaluation of influenza (31). Future investigations should therefore integrate NLR with additional biomarkers and clinical parameters, while stratifying analyses by virus type and patient age, to further elucidate its predictive value.

With regard to leukocyte parameters, both absolute counts and relative values or ratios of lymphocytes, neutrophils, and monocytes exhibited significant alterations during influenza A (IAV) and influenza B (IBV) infection (Tables 2–6). However, as the total white blood cell (WBC) count also fluctuated, discrepancies were observed between absolute values and relative ratios. Previous studies have suggested that proportional indices may enhance sensitivity in distinguishing infection types and evaluating disease status (32). For leukocyte subsets with relatively low abundance, absolute values are more strongly affected by changes in total WBC counts (9). Moreover, in childhood—particularly during infancy—monocyte absolute counts are higher than in adults, while relative values remain comparatively stable; in contrast, elderly individuals often show increased monocyte counts due to immunosenescence (33, 34). To account for these differences and facilitate comparisons across age groups, we adopted relative values and ratios in our statistical analyses.

Interestingly, we observed that both IAV and IBV infections were associated with elevated monocyte ratio (M ratio). In particular, IBV patients demonstrated a consistently significant increase in M ratio across the 4–79 age range, whereas in IAV patients aged 60–79 years, this increase lost statistical significance. This suggests a sustained and robust association between IBV and M ratio. ROC curve analysis further revealed that M ratio served as the most reliable indicator for IBV, while in IAV, lymphocyte-to-monocyte ratio (LMR) showed stronger correlations across age groups. The absence of a significant increase in M ratio among elderly IAV patients may be related to age-related immune remodeling, as studies have reported profound alterations in monocyte subset composition and function in older individuals, including shifts in classical and non-classical monocyte populations (35, 36). These changes may attenuate the responsiveness of M ratio to IAV infection in the elderly. In addition, IAV infection is typically characterized by marked lymphopenia and systemic inflammation, particularly in younger patients, which may explain why LMR more accurately reflects disease status in IAV (31, 37). In contrast, IBV patients exhibited significantly elevated M ratio across all age groups, suggesting a distinct immunological profile compared with IAV. Although IBV is generally regarded as less virulent (22), our findings indicate that persistent M ratio elevation, independent of age, may be a hallmark hematological feature of IBV infection. Previous pediatric studies also reported monocyte elevation during influenza infection, with differing hematological patterns between IAV and IBV (38, 39). Our results not only support these observations but also extend the evidence by showing sustained M ratio elevation in IAV among younger and middle-aged populations, while for the first time demonstrating consistent M ratio elevation in IBV across most age groups. This highlights M ratio as a stable hematological marker of IBV infection. Given that monocytes are a major source of proinflammatory cytokines such as IL-6 and TNF-α (40), it is possible that IBV drives inflammation through monocyte-mediated pathways distinct from IAV. Unlike IAV, which is characterized by prominent lymphocyte reduction and neutrophil-driven inflammation, IBV appears to be marked by a more pronounced and persistent elevation in M ratio, thereby establishing its distinct hematological signature.

In summary, this study leveraged a large sample size and broad age range to systematically compare changes in complete blood count (CBC) parameters in patients with influenza A (IAV) and influenza B (IBV). By focusing on relative values and conducting detailed analyses of leukocyte subsets with relatively low abundance, this study enhances our understanding of the distinct pathogenic characteristics of these two influenza viruses. Unlike previous studies that primarily focused on blood count changes in IAV or failed to distinguish between virus types, our results reveal important differences in the inflammatory cell responses between IAV and IBV. For instance, IAV infection induces more pronounced increases in neutrophil ratio (N ratio) and decreases in lymphocyte ratio (L ratio) in younger and middle-aged individuals, whereas IBV infection is characterized by increased M ratio with relatively preserved L ratio across all age groups (4–79 years). This distinct hematological pattern may reflect differences in the pathogenic mechanisms of the two viruses. Another key finding of our study is the validation of routine hematological markers for distinguishing between IAV and IBV infections. Previous studies have shown that indices such as NLR and LMR can differentiate between healthy individuals and influenza patients in adults and assist in disease monitoring and prognosis evaluation (41, 42). Our findings support these observations, demonstrating that IAV patients typically show a reduction in LMR, while NLR is notably elevated in IAV patients. This aligns with the characteristic hematological changes associated with viral infections, as opposed to bacterial infections (43). Moreover, our study suggests that when routine blood tests reveal a significant reduction in L ratio combined with an increase in M ratio (leading to a substantial decrease in LMR), clinicians should consider the possibility of influenza, particularly IAV infection. On the other hand, when M ratio is abnormally elevated, IBV infection should be strongly suspected. Overall, this study provides the first large-scale cross-sectional comparison of CBC changes in IAV and IBV, confirming the unique significance of M ratio elevation in IBV infection and emphasizing the value of integrating NLR, LMR, and other indices to improve the recognition and assessment of influenza. These findings offer valuable insights for clinical diagnosis and monitoring of influenza, as well as serve as a foundation for future studies exploring the immunological differences between the two influenza viruses.

Both influenza A (IAV) and influenza B (IBV) patients showed a reduction in lymphocyte-to-monocyte ratio (LMR) and an increase in monocyte ratio (M ratio) in the 4–59 age group. Among these, IAV was consistently associated with a decrease in lymphocyte ratio (L ratio) across most age groups, with the strongest correlation observed between LMR and IAV. In contrast, for IBV, neutrophil-to-lymphocyte ratio (NLR) was significantly reduced across all age groups (4–79 years). Additionally, M ratio not only increased in the 4–59 age group but remained significantly elevated in the 60–79 age group, with a strong correlation between M ratio and IBV infection.