Repeatability of measures of inflammatory cell number in bronchial biopsies in atopic asthma

Airway pathology is increasingly considered to be a major outcome in asthma research. The aim of this study was to examine the intra-observer, within-section and between-biopsy repeatability, together with the implications for statistical power of a computerized quantitative analysis of inflammatory cell numbers in the lamina propria in bronchial biopsy specimens from atopic asthmatic subjects. Thirty six atopic adults (aged 19-40 yrs) with mild to moderate asthma (baseline forced expiratory volume in one second (FEV1) > or =50% of predicted value, methacholine (PC20) range 0.02-18.2 mg x mL[-1]) at various levels of treatment (25 subjects on inhaled steroids) entered the study. Biopsies were taken from the (sub)segmental carinae of the right lower and middle lobe and from the main carina. Specimens were snap-frozen and immunohistochemical staining was performed on cryostat sections with monoclonal antibodies against: (secreted) eosinophil cationic protein (EG1, EG2), mast cell tryptase (AA1), CD45, CD22, CD4, CD8, CD25, and CD45RO. Using a computerized system, the number of positively stained cells in the lamina propria was counted. When considering all cell types together, satisfactory intraclass correlation coefficients (ICC) values were obtained for intra-observer, within-section and between-biopsy repeatability, being 0.90, 0.80 and 0.81, respectively. The analysis of repeatability for individual cell types revealed ICC values ranging 0.47-0.82 for intra-observer, 0.44-0.76 for within-section and 0.37-0.67 for between-biopsy repeatability. The results imply that a sample-size between eight and 25 subjects is needed to detect at least one doubling difference in cell number per 0.1 mm2 for a particular inflammatory cell type in a study, using a within-group design with alpha=0.05 and power of 0.80. A sample-size of 13-48 subjects per group is required to detect the same difference between the groups in a parallel design.