title: Peripheral blood eosinophils: a surrogate marker for airway eosinophilia in stable COPD.
abstract: BACKGROUND
  Sputum eosinophilia occurs in approximately one-third of stable chronic obstructive pulmonary disease (COPD) patients and can predict exacerbation risk and response to corticosteroid treatments. Sputum induction, however, requires expertise, may not always be successful, and does not provide point-of-care results. Easily applicable diagnostic markers that can predict sputum eosinophilia in stable COPD patients have the potential to progress COPD management. This study investigated the correlation and predictive relationship between peripheral blood and sputum eosinophils. It also examined the repeatability of blood eosinophil counts.
  
  
  METHODS
  Stable COPD patients (n=141) were classified as eosinophilic or noneosinophilic based on their sputum cell counts (≥3%), and a cross-sectional analysis was conducted comparing their demographics, clinical characteristics, and blood cell counts. Receiver operating characteristic curve analysis was used to assess the predictive ability of blood eosinophils for sputum eosinophilia. Intraclass correlation coefficient was used to examine the repeatability of blood eosinophil counts.
  
  
  RESULTS
  Blood eosinophil counts were significantly higher in patients with sputum eosinophilia (n=45) compared to those without (0.3×10(9)/L vs 0.15×10(9)/L; P<0.0001). Blood eosinophils correlated with both the percentage (ρ=0.535; P<0.0001) and number of sputum eosinophils (ρ=0.473; P<0.0001). Absolute blood eosinophil count was predictive of sputum eosinophilia (area under the curve =0.76, 95% confidence interval [CI] =0.67-0.84; P<0.0001). At a threshold of ≥0.3×10(9)/L (specificity =76%, sensitivity =60%, and positive likelihood ratio =2.5), peripheral blood eosinophil counts enabled identification of the presence or absence of sputum eosinophilia in 71% of the cases. A threshold of ≥0.4×10(9)/L had similar classifying ability but better specificity (91.7%) and higher positive likelihood ratio (3.7). In contrast, ≥0.2×10(9)/L offered a better sensitivity (91.1%) for ruling out sputum eosinophilia. There was a good agreement between two measurements of blood eosinophil count over a median of 28 days (intraclass correlation coefficient =0.8; 95% CI =0.66-0.88; P<0.0001).
  
  
  CONCLUSIONS
  Peripheral blood eosinophil counts can help identify the presence or absence of sputum eosinophilia in stable COPD patients with a reasonable degree of accuracy.
authors.affiliation: Priority Research Centre for Healthy Lungs, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Hunter Medical Research Institute, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia.
authors.forename: Netsanet A
authors.initials: NA
authors.lastname: Negewo
authors.affiliation: Priority Research Centre for Healthy Lungs, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Hunter Medical Research Institute, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Department of Respiratory and Sleep Medicine, John Hunter Hospital, Newcastle, NSW, Australia; School of Nursing and Midwifery, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia.
authors.forename: Vanessa M
authors.initials: VM
authors.lastname: McDonald
authors.affiliation: Priority Research Centre for Healthy Lungs, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Hunter Medical Research Institute, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia.
authors.forename: Katherine J
authors.initials: KJ
authors.lastname: Baines
authors.affiliation: Priority Research Centre for Healthy Lungs, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Hunter Medical Research Institute, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Department of Respiratory and Sleep Medicine, John Hunter Hospital, Newcastle, NSW, Australia.
authors.forename: Peter Ab
authors.initials: PA
authors.lastname: Wark
authors.affiliation: Priority Research Centre for Healthy Lungs, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Hunter Medical Research Institute, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia.
authors.forename: Jodie L
authors.initials: JL
authors.lastname: Simpson
authors.affiliation: Institute for Infection and Immunity, St George's, University of London, London, UK.
authors.forename: Paul W
authors.initials: PW
authors.lastname: Jones
authors.affiliation: Priority Research Centre for Healthy Lungs, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Hunter Medical Research Institute, Faculty of Health and Medicine, The University of Newcastle, Callaghan, NSW, Australia; Department of Respiratory and Sleep Medicine, John Hunter Hospital, Newcastle, NSW, Australia.
authors.forename: Peter G
authors.initials: PG
authors.lastname: Gibson
mesh_terms: D000368:Aged; D019540:Area Under Curve; D003430:Cross-Sectional Studies; D004804:Eosinophils; D005260:Female; D006801:Humans; D007958:Leukocyte Count; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011379:Prognosis; D029424:Pulmonary Disease, Chronic Obstructive / Q000097:blood* / Q000175:diagnosis; D011657:Pulmonary Eosinophilia / Q000097:blood* / Q000175:diagnosis; D012372:ROC Curve; D015203:Reproducibility of Results; D013183:Sputum / Q000166:cytology; D013997:Time Factors
keywords: chronic obstructive pulmonary disease; diagnostic accuracy; sputum eosinophilia; stability of eosinophil counts