Reproducibility of the WHO/IASLC grading system for pre-invasive squamous lesions of the bronchus: a study of inter-observer and intra-observer variation

Although many workers have graded pre‐invasive squamous lesions arising in the bronchus, there has been no consensus classification system until the latest edition of the WHO/IASLC histological classification of pulmonary and pleural tumours. Because the value of any such system is dependent on its reproducibility, we have circulated a series of such lesions to a panel of histopathologists to assess interobserver and intra‐observer variation when the WHO/IASLC classification was applied. Colour transparencies of 28 pre‐invasive squamous lesions were assessed by six histopathologists (two with a special interest in pulmonary pathology, two generalists and two trainees) on three separate occasions over a period of 3 months, using the criteria of the WHO/IASLC (mild, moderate and severe dysplasia, and in‐situ carcinoma). An additional category of metaplasia was added for those cases that showed no dysplasia. Weighted kappa coefficents of agreement (Kw) were used to evaluate paired observations with a standard quadratic weighting being employed, such that kappa coefficients corresponded to intra‐class correlation coefficients. Wilcoxon's sign‐ranked test was used to measure the statistical significance of group trends, when comparing kappa values for the three grading systems. Various 3‐point systems were also assessed, through combination of the above groups. Intra‐observer agreement was substantially better than interobserver variation (mean: 0.71 vs. 0.55). Between the various pathologist groups, inter‐observer variation was relatively minor, although intra‐observer variation was higher within the trainee pathologist group. Using weighted kappa values, there was no significant difference in either inter‐observer or intra‐observer agreement between the five point grading system and a 3‐point system of metaplasia/mild, moderate and severe/in‐situ grades. However, there was a significant increase in variation when a 3‐point system of metaplasia/mild, moderate/severe and in‐situ carcinoma was used. This study shows levels of interobserver and intra‐observer variation similar to those found in other grading systems in histopathology, with no significant decrease in variability found by abridging the system. The WHO/IASLC system is therefore recommended for future use in both clinical and research fields.