Chronic obstructive pulmonary disease (COPD) is a major global health burden, representing the third leading cause of death worldwide, responsible for approximately 3.23 million deaths in 2019. In India, COPD affects an estimated 55 million individuals, with a prevalence of 7.2% in adults aged above 30 years, contributing to nearly 10.4% of all COPD-related global deaths. The disease is characterized by persistent respiratory symptoms and airflow limitation, resulting from airway and alveolar abnormalities caused by significant exposures to noxious particles or gases — with tobacco smoking and biomass fuel combustion being the dominant risk factors in the Indian context.

Acute exacerbations of COPD (AECOPD) — defined as acute worsening of respiratory symptoms beyond normal day-to-day variation that requires a change in medication — are the central events that drive disease progression, accelerate lung function decline, impair health-related quality of life, and account for the majority of COPD-related healthcare costs and hospitalizations. Each severe exacerbation requiring hospitalization is associated with a 10–22% risk of in-hospital mortality and a substantially elevated risk of readmission. Early identification of patients at high exacerbation risk is therefore a clinical priority.

Blood eosinophil count has attracted considerable attention as a candidate biomarker in COPD management over the past decade. Eosinophilic inflammation, long recognized as central to asthma pathophysiology, is now understood to be present in a clinically significant subset of COPD patients — estimated at 20–40% — and constitutes a distinct inflammatory endotype with implications for both disease behavior and therapeutic response. The landmark GOLD (Global Initiative for Chronic Obstructive Lung Disease) strategy document from 2019 onwards incorporated blood eosinophil count as a biomarker to guide inhaled corticosteroid (ICS) therapy in COPD, suggesting thresholds of 100 and 300 cells/microL for decision-making.

Mechanistically, eosinophils may contribute to COPD exacerbation susceptibility through several pathways: enhanced airway inflammation and hyperresponsiveness; augmented responses to viral and bacterial triggers; release of eosinophil-derived mediators (major basic protein, eosinophil cationic protein) that damage the airway epithelium; and cross-talk with mast cells and type 2 innate lymphoid cells perpetuating type-2 airway inflammation. Notably, eosinophilic COPD patients appear to mount a greater corticosteroid-responsive component during exacerbations, explaining the differential ICS benefit.

Despite growing evidence from Western cohorts — notably the ECLIPSE study, the SUMMIT trial, and the Sputum Eosinophil Guided Therapy trial — data from Indian COPD patients are limited. Indian patients have distinct characteristics: predominance of biomass fuel-related COPD in women, lower BMI, high prevalence of comorbid tuberculosis-associated obstructive lung disease (TOAD), and different patterns of exacerbation triggers (particularly infectious). Whether the eosinophil thresholds validated in Western populations are directly applicable to this population remains uncertain.

This prospective cohort study was therefore designed to: (1) determine the relationship between baseline blood eosinophil count and frequency of AECOPD in Indian patients over 12 months; (2) identify an optimal eosinophil threshold through ROC analysis; and (3) determine independent predictors of exacerbation risk using multivariable Cox regression.

2.1 Study Design and Setting This was a single-center, prospective cohort study conducted at the Outpatient Department of Pulmonology and Respiratory Medicine, [Tertiary Care Teaching Hospital], [City], India, from January 2022 to June 2023 (18 months: 6 months enrollment, 12 months follow-up). The study was approved by the Institutional Ethics Committee (IEC Ref: XXXX/2021) and registered with CTRI (CTRI/2021/XXXXXX). All procedures were conducted in accordance with the Declaration of Helsinki. 2.2 Study Population Inclusion criteria: Age 40-80 years; spirometry-confirmed COPD (post-bronchodilator FEV1/FVC <0.70) per GOLD 2021 criteria; GOLD spirometric stage II-IV (FEV1 20-79% predicted); clinically stable state at enrollment (no exacerbation in preceding 6 weeks); ability to attend follow-up visits; written informed consent. Exclusion criteria: Active asthma or asthma-COPD overlap syndrome (ACOS); current or recent (within 6 weeks) AECOPD; concurrent conditions causing eosinophilia (allergic bronchopulmonary aspergillosis, parasitic infections, eosinophilic granulomatosis with polyangiitis, hypereosinophilic syndrome, active malignancy); systemic corticosteroid use in the preceding 6 weeks; active pulmonary tuberculosis; inability to comply with follow-up schedule. 2.3 Sample Size Based on the expected proportion of patients with eosinophil count >300 cells/microL in COPD being approximately 30-40%, and an anticipated hazard ratio of 2.0 for exacerbation in the eosinophil-high group with 80% power and 5% two-sided significance level, a minimum of 180 patients was required. Accounting for 10% loss to follow-up, 200 patients were enrolled. 2.4 Data Collection at Baseline At enrollment, the following data were collected for all participants: • Demographic and clinical details: age, sex, BMI, smoking history (pack-years), biomass fuel exposure (>10 years), occupation, comorbidities (hypertension, diabetes, ischemic heart disease, cor pulmonale) • COPD severity: GOLD spirometric stage, Modified Medical Research Council (mMRC) dyspnoea scale, COPD Assessment Test (CAT) score • Exacerbation history: number of moderate-to-severe exacerbations in the preceding 12 months • Medication review: current ICS use, long-acting bronchodilator use (LABA, LAMA), theophylline • Spirometry: pre- and post-bronchodilator FEV1, FVC, FEV1/FVC ratio (ATS/ERS 2019 standards) • Blood investigations: complete blood count with differential (absolute eosinophil count and eosinophil percentage), serum IgE, CRP • 6-minute walk test (6MWT) distance 2.5 Eosinophil Count Assessment Absolute blood eosinophil count (cells/microL) and eosinophil percentage (%) were determined from a fasting venous blood sample collected using an automated haematology analyser (Sysmex XN-1000) at a single accredited laboratory. Patients were categorized as eosinophil-high (>300 cells/microL) or eosinophil-low (<= 300 cells/microL) based on the GOLD-recommended threshold, and subsequently re-stratified using the ROC-derived optimal threshold for sensitivity analyses. 2.6 Follow-up and Outcome Ascertainment All enrolled participants were followed up at clinic visits at months 1, 3, 6, 9, and 12, with telephone contact at months 2, 4, 7, and 10 to capture exacerbation events in the interim. A structured event diary was provided to each participant. Primary outcome: Annualized rate of moderate-to-severe AECOPD per patient-year. Moderate exacerbation was defined as requiring a course of systemic corticosteroids and/or antibiotics (outpatient or clinic); severe exacerbation as requiring emergency department visit or hospitalization. Secondary outcomes: Time-to-first exacerbation; proportion of patients with 2+ exacerbations/year (frequent exacerbators); hospitalization rate; all-cause mortality; change in CAT score and FEV1 at 12 months. 2.7 Statistical Analysis Data were analyzed using SPSS v26 and MedCalc v20. Continuous variables are reported as mean +/- SD or median (IQR); categorical variables as frequency (%). Independent t-test or Mann-Whitney U test was used for between-group comparisons as appropriate. Receiver operating characteristic (ROC) curve analysis was used to determine the optimal eosinophil threshold for predicting exacerbation (outcome: >=2 exacerbations in 12 months). Kaplan-Meier survival curves with log-rank test compared time-to-first exacerbation between eosinophil strata. Univariate and multivariate Cox proportional hazards regression identified independent predictors of AECOPD. A p-value <0.05 was considered statistically significant.

3.1 Study Flow and Baseline Characteristics

Of 236 COPD patients screened, 200 met eligibility criteria and were enrolled. Twelve patients were lost to follow-up (6%) leaving 188 for the primary analysis (intention-to-treat analysis included all 200). Mean age was 58.4 +/- 9.8 years; 76% were male. Tobacco smoking was the primary risk factor in 68%; biomass fuel exposure in 24%; mixed exposure in 8%. The eosinophil-high group (>300 cells/microL) comprised 84 patients (42%) with a mean eosinophil count of 486 +/- 189 cells/microL, compared to 128 +/- 74 cells/microL in the eosinophil-low group. Baseline characteristics were well-balanced between groups (Table 1).

3.2 Exacerbation Outcomes by Eosinophil Stratum

During the 12-month follow-up, the eosinophil-high group experienced significantly more exacerbations (Table 2). The mean exacerbation rate was 2.18 +/- 0.91 per patient-year in the eosinophil-high group versus 0.94 +/- 0.63 in the eosinophil-low group (p<0.001). The proportion of frequent exacerbators (>=2 exacerbations/year) was 64.3% versus 31.0% (p<0.001). Hospitalization rate was also significantly higher (41.7% vs. 19.8%, p=0.001). Mean time-to-first exacerbation was shorter in the eosinophil-high group (3.8 +/- 1.9 vs. 7.4 +/- 2.6 months, p<0.001). Kaplan-Meier analysis confirmed significantly earlier and more frequent exacerbation events in the eosinophil-high group (log-rank p<0.001).

3.3 ROC Analysis — Optimal Eosinophil Threshold

ROC curve analysis using frequent exacerbation (>=2 events/year) as the outcome yielded an AUC of 0.78 (95% CI: 0.72-0.84, p<0.001) for blood eosinophil count. The optimal cut-off by Youden index was 270 cells/microL (sensitivity 74.2%, specificity 71.8%, positive predictive value 68.4%, negative predictive value 77.1%, positive likelihood ratio 2.64). The AUC for eosinophil percentage (>=2%) was 0.74 (95% CI: 0.67-0.80). The GOLD 300 cells/microL threshold had lower sensitivity (58.9%) with higher specificity (79.4%). The 100 cells/microL threshold had high sensitivity (91.2%) but very low specificity (31.4%), limiting its clinical utility as a rule-in test (Table 3).

3.4 Predictors of AECOPD — Cox Regression Analysis

On univariate Cox regression, blood eosinophil count >270 cells/microL, prior exacerbation history (>=2 in preceding year), GOLD stage III-IV, mMRC score >=2, lower 6MWT distance, and absence of ICS use were associated with exacerbation risk. On multivariate Cox regression adjusting for all covariates, three variables remained as independent predictors: eosinophil count >270 cells/microL (HR 2.34, 95% CI 1.61-3.40, p<0.001), prior exacerbation history (HR 2.91, 95% CI 1.98-4.27, p<0.001), and GOLD stage III-IV (HR 1.87, 95% CI 1.22-2.87, p=0.004) (Table 4).

Table 1: Baseline Characteristics by Eosinophil Stratum (N=200)

BMI = Body Mass Index; GOLD = Global Initiative for Chronic Obstructive Lung Disease; FEV1 = Forced Expiratory Volume in 1 second; mMRC = Modified Medical Research Council; CAT = COPD Assessment Test; ICS = Inhaled Corticosteroids

Table 2: Exacerbation Outcomes at 12 Months by Eosinophil Stratum

Rx = treatment; FEV1 = Forced Expiratory Volume in 1 second; CAT = COPD Assessment Test

3.5 Eosinophil Count and ICS Response

In a pre-specified sub-group analysis, among eosinophil-high patients (>270 cells/microL), those receiving ICS-containing regimens (n=38) had significantly fewer exacerbations compared to those not on ICS (n=46): 1.62 +/- 0.78 vs. 2.66 +/- 0.94 per patient-year (p=0.001). No significant ICS benefit was observed in the eosinophil-low group (0.91 +/- 0.64 with ICS vs. 0.97 +/- 0.62 without ICS, p=0.68), supporting the concept of eosinophil-guided ICS prescription in COPD.

Table 3: Diagnostic Performance of Different Eosinophil Thresholds for Predicting Frequent Exacerbation (>=2/year)

*Optimal threshold by Youden Index; PPV = Positive Predictive Value; NPV = Negative Predictive Value; AUC = Area Under ROC Curve; GOLD = Global Initiative for Chronic Obstructive Lung Disease threshold

Table 4: Cox Proportional Hazards Regression — Predictors of AECOPD

HR = Hazard Ratio; CI = Confidence Interval; GOLD = spirometric stage; mMRC = Modified Medical Research Council dyspnoea scale; ICS = Inhaled Corticosteroids; 6MWT = 6-Minute Walk Test. Multivariate model adjusted for age, sex, BMI, and smoking pack-years.

This prospective cohort study provides robust evidence that blood eosinophil count is a clinically meaningful, independent predictor of AECOPD in Indian patients. The eosinophil-high group (>300 cells/microL) experienced more than double the exacerbation rate of eosinophil-low patients (2.18 vs. 0.94 per patient-year), a difference that is both statistically highly significant and clinically important. The ROC-derived optimal threshold of 270 cells/microL, with an AUC of 0.78, demonstrates acceptable discriminatory accuracy — comparable to, and in some respects superior to, the GOLD-recommended 300 cells/microL threshold in this population.

The biological plausibility of eosinophils as exacerbation predictors in COPD is well established. Eosinophilic airway inflammation, even in stable COPD, is associated with greater bronchial hyperresponsiveness, more pronounced airway remodeling, and heightened vulnerability to viral and atypical bacterial triggers. The ECLIPSE study — the largest longitudinal COPD biomarker study to date — demonstrated that blood eosinophil count was a reproducible and consistent trait in a subset of COPD patients, not merely a transient finding, with approximately 37% of patients persistently maintaining eosinophil counts above 2% over 3 years. Our findings replicate and extend this observation in an Indian cohort.

The identification of 270 cells/microL as the optimal threshold in our population — slightly lower than the 300 cells/microL used in GOLD recommendations — may reflect population-specific differences. Indian patients have generally lower absolute eosinophil counts due to differences in baseline atopic status and helminthic exposure. Additionally, the lower BMI and higher prevalence of TOAD in our cohort may influence eosinophil biology. This finding has practical implications: using 270 cells/microL would improve sensitivity (74.2% vs. 58.9% for 300 cells/microL) while maintaining adequate specificity, enabling earlier risk stratification of a larger proportion of high-risk patients.

The prior exacerbation history emerged as the strongest predictor (HR 2.91) in our multivariable model, consistent with the well-replicated finding that past exacerbation is the best predictor of future exacerbations — the so-called frequent exacerbator phenotype. Importantly, blood eosinophil count remained a significant independent predictor (HR 2.34) after adjustment for prior exacerbation history, GOLD stage, and other covariates, confirming its additive value beyond established predictors. This has clinical relevance: in a patient with no prior exacerbation history but elevated eosinophils, the biomarker may help identify latent high-risk individuals who might otherwise be under-managed.

The sub-group analysis showing differential ICS benefit in eosinophil-high versus eosinophil-low patients aligns with evidence from the IMPACT, TRIBUTE, and ETHOS trials, which consistently demonstrated that the benefit of ICS (on exacerbation reduction) is concentrated in patients with higher eosinophil counts. Our observation that ICS use was associated with a 39% relative reduction in exacerbation rate in eosinophil-high patients, but no significant benefit in eosinophil-low patients, supports the GOLD recommendation for eosinophil-guided ICS escalation decisions in COPD management.

Limitations of this study include: single-center design potentially limiting generalizability; eosinophil count measured at a single timepoint (intra-individual variability not assessed); open-label follow-up with potential ascertainment bias in moderate exacerbation identification; and insufficient power for sub-group analyses stratified by exacerbation trigger type (viral vs. bacterial vs. environmental). Future studies should address eosinophil count stability over time and combine it with other biomarkers (fibrinogen, CRP, club cell protein-16) in composite risk prediction models.

Blood eosinophil count is a simple, inexpensive, and independently predictive biomarker for acute exacerbations in COPD. A threshold of 270 cells/microL, derived from ROC analysis in this Indian cohort, offers clinically actionable discrimination with good sensitivity and specificity. Patients with elevated eosinophils have more than twice the exacerbation risk, shorter time to first exacerbation, and show differential benefit from ICS therapy. These findings support the routine measurement of blood eosinophil count in COPD management for risk stratification and therapeutic decision-making, and suggest that population-specific threshold calibration may improve the predictive accuracy of this biomarker in Indian patients.

1. Global Initiative for Chronic Obstructive Lung Disease (GOLD). Global Strategy for Prevention, Diagnosis and Management of COPD: 2024 Report. Available at: www.goldcopd.org
2. Salvi S, Agarwal A. India has a huge burden of COPD but little awareness of the disease. Chest. 2012;142(5):1085-1086.
3. Bafadhel M, McKenna S, Terry S, et al. Acute exacerbations of chronic obstructive pulmonary disease: identification of biologic clusters and their biomarkers. Am J Respir Crit Care Med. 2011;184(6):662-671.
4. Wedzicha JA, Seemungal TA. COPD exacerbations: defining their cause and prevention. Lancet. 2007;370(9589):786-796.
5. Vedel-Krogh S, Nielsen SF, Lange P, Vestbo J, Nordestgaard BG. Blood Eosinophils and Exacerbations in Chronic Obstructive Pulmonary Disease. The Copenhagen General Population Study. Am J Respir Crit Care Med. 2016;193(9):965-974.
6. Pascoe S, Locantore N, Dransfield MT, et al. Blood eosinophil counts, exacerbations, and response to the addition of inhaled fluticasone furoate to vilanterol in patients with chronic obstructive pulmonary disease. Lancet Respir Med. 2015;3(6):435-442.
7. Siddiqui SH, Guasconi A, Vestbo J, et al. Blood eosinophils: a biomarker of response to extrafine beclomethasone/formoterol in chronic obstructive pulmonary disease. Am J Respir Crit Care Med. 2015;192(4):523-525.
8. Watz H, Tetzlaff K, Wouters EF, et al. Blood eosinophil count and exacerbations in severe chronic obstructive pulmonary disease after withdrawal of inhaled corticosteroids: a post-hoc analysis of the WISDOM trial. Lancet Respir Med. 2016;4(5):390-398.
9. Bafadhel M, Davies L, Calverley PM, et al. Blood eosinophil guided prednisolone therapy for exacerbations of COPD: a further analysis. Eur Respir J. 2014;44(3):789-791.
10. Yilmaz I, Pazarli AC. The Relationship Between Eosinophil Count and Exacerbation in People with COPD. Int J Chron Obstruct Pulmon Dis. 2019;14:2391-2397.
11. Hastie AT, Martinez FJ, Curtis JL, et al. Association of sputum and blood eosinophil concentrations with clinical measures of COPD severity: an analysis of the SPIROMICS cohort. Lancet Respir Med. 2017;5(12):956-967.
12. Papi A, Vestbo J, Fabbri L, et al. Extrafine inhaled triple therapy versus dual bronchodilator therapy in chronic obstructive pulmonary disease (TRIBUTE): a double-blind, parallel group, randomised controlled trial. Lancet. 2018;391(10125):1076-1084.
13. Lipson DA, Barnhart F, Brealey N, et al. Once-Daily Single-Inhaler Triple versus Dual Therapy in Patients with COPD (IMPACT). N Engl J Med. 2018;378(18):1671-1680.
14. Rodriguez-Roisin R. Toward a consensus definition for COPD exacerbations. Chest. 2000;117(5 Suppl 2):398S-401S.
15. Kerkhof M, Sonnappa S, Postma DS, et al. Blood eosinophil count and exacerbation risk in patients with COPD. Eur Respir J. 2017;50(1):1700761.
16. Miller MR, Hankinson J, Brusasco V, et al. ATS/ERS Task Force. Standardisation of spirometry. Eur Respir J. 2005;26(2):319-338.

17. Jones PW, Harding G, Berry P, et al. Development and first validation of the COPD Assessment Test. Eur Respir J. 2009;34(3):648-654.