Garrido-Estepa et al. BMC Cancer 2010, 10:485 http://www.biomedcentral.com/1471-2407/10/485 Page 6 of 8 Table 5 Bivariate study for high/low risk classification among scales Percentage of total agreement; Kappa statistics (95% CI) Scale Wolfe Tabár BI-RAD Boyd % Agreement: Kappa (95% CI): % Agreement: Kappa (95% CI): % Agreement: Kappa (95% CI): % Agreement: Kappa (95% CI): Wolfe 92.55%* 0.851 (0.798-0.904)* 90.37% 0.804 (0.785-0.823) 75.22% 0.485 (0.460-0.510) 75.25% 0.485 (0.460-0.511) Tabár 90.37% 0.804 (0.785-0.823) 90.16%* 0.800 (0.739-0.861)* 84.46% 0.638 (0.612-0.663) 84.49% 0.638 (0.612-0.664) BI-RADS 75.22% 0.485 (0.460-0.510) 84.46% 0.638 (0.612-0.663) 93.35% * 0.815 (0.746-0.885)* 99.97% 0.999 (0.998-1.000) Boyd 75.25% 0.485 (0.460-0.511) 84.49% 0.638 (0.612-0.664) 99.97% 0.999 (0.998-1.000) 93.62%* 0.822 (0.754-0.891)* *Data drawn from intra-observer agreement study (sub-sample of 320 mammograms) mammogram classified as low risk, category III, using Tabár scale was included in the high risk category E of Boyd scale. Categories N1 (Wolfe) and II (Tabár) are classified mainly into categories A or B of Boyd scale (≤ 10% of density) and categories P1 and III into B, C or D (> 0-50%). On the other hand, 24.75% of the mam- mograms included in high-risk categories P2 and DY using Wolfe method, were included in low-risk cate- gories, almost all in D category, using Boyd scale. For Figure 1 Graphic distribution study of mammographic density measured by qualitative scales with respect to Boyd’s low-risk (A, B, C, D) and high-risk categories (E, F). Tabár scale the percentage was lower, 15.48% (15.34% in D). Category-D mammograms, namely, those with 25% to 50% density, displayed a more heterogeneous distribu- tion under Wolfe’s and Tabár’s systems. Discussion Different methods in use, subjective classifications, and the lack of a gold standard mean that misclassification is a potential problem when assessing mammographic density and potential breast cancer risk. Previous studies have measured inter- and intra-observer variability using experienced radiologists. Most of these studies report good or very good agreement [14-23]. Reliability studies of Wolfe’s classification show kappa values ranging from 0.69 to 0.88 for inter-observer agreement, and 0.69 to 0.87 for intra-observer agreement [14-18]. In previous studies: Tabár inter-observer agreement obtained kappa values of 0.63 and of 0.75 for intra-observed agreement [19]; BI-RADS classification displayed only moderate agreement, with an overall kappa value for intra-obser- ver agreement of 0.43-0.59 [20,21]; and Boyd’s classifica- tion registered inter-observer agreement of 0.89 [22] and a kappa value of 0.74 [18], with a weighted kappa value for intra-observer agreement of 0.68-0.89 [19,23]. In our study, good intra-observer reproducibility was observed, particularly when weighted kappa was used. Only the Wolfe and Tabár scales showed disagreement in more than one category in 7 (1.86%) and 16 (4.27%) cases, respectively: six of these cases were classified on both scales with disagreement in two categories, perhaps due to specific characteristics of these mammograms which hindered their evaluation. Only one classification -Boyd’s- registered a kappa value of under 0.70, namely, 0.68. Nevertheless, all the disagreements observed (25.06%) corresponded to differences in only one cate- gory. It should be noted that Boyd’s classification is divided into six categories, with three categories -A, B and C- classifying densities under 25 percent with nar- rower range intervals than the rest. Half of all