Garrido-Estepa et al. BMC Cancer 2010, 10:485 http://www.biomedcentral.com/1471-2407/10/485 Results Intra-observer agreement Wolfe’s classification Table 1 shows agreement between the first and second measurements using the Wolfe scale, with 308 of the 375 (82.13%) images being consistently classified. Only 1.86% of the observations showed disagreement in two categories. The kappa value was 0.73 (p < 0.0001) and the weighted kappa value was 0.84 (p < 0.0001). Using aggregated data in two categories (low- and high-risk), the kappa statistic was 0.85 (p < 0.0001). Kappa values for analog and digital measures were 0.78 (p < 0.0001, 95% CI: 0.706-0.846) and 0.67 (p < 0.0001, 95% CI: 0.575-0.764), and weighted kappa values were 00.87 (p < 0.0001, 95% CI: 0.808-0.916) and 0.78 (p < 0.0001, 95% CI: 0.686-0.856), respectively Tabár’s classification Comparisons between first and second measures with Tabár classification are shown in Table 2. The percen- tage of total agreement was 82.93% (311 of 375 mea- sures were exactly the same). Only 4.27% of the measures differed in two categories. The kappa value was 0.72 (p < 0.0001) and that of weighted kappa 0.71 (p < 0.0001). When the scale was divided into low and high risk, the kappa value was 0.80 (p < 0.0001) Kappa values for analog and digital measures were 0.77 (p < 0.0001; 95% CI: 0.685-0.838) and 0.62 (p < 0.0001; 95% CI: 0.497-0.721), and weighted kappa values were 0.75 (p < 0.0001; 95% CI: 0.641-0.845) and 0.64 (p < 0.0001; 95% CI: 0.494-0.764), respectively. BI-RADS classification Using the BI-RADS scale, 315 of 375 images were classi- fied in the same category (84% agreement). No observa- tions differed in more than one category, as can be seen in Table 3. The kappa value for BI-RADS classification was 0.76 (p < 0.0001) and weighted kappa was 0.90 (p < 0.0001). High/low-risk classification resulted in a kappa value of 0.82 (p < 0.0001). Kappa values for analog and digital measures were 0.76 (p < 0.0001; 95% CI: 0.683-0.829) and 0.76 (p < 0.0001; 95% CI: 0.676-0.842), and weighted kappa values were 0.90 (p < 0.0001; 95% CI: 0.864-0.932) and 0.90 (p < 0.0001; 95% CI: 0.860-0.938), respectively. Boyd’s classification Table 4 summarizes the agreement between the first and second measure using the Boyd scale. Total agree- ment was 74.93% (281 of 375 measures were identically classified). There were no observations with disagree- ment in more than one category. The kappa value was 0.68 (p < 0.0001) and weighted kappa was 0.92 (p < 0.0001). Using the two-category scale, the kappa statistic was 0.82 (p < 0.0001). Page 5 of 8 Kappa values for analog and digital measures were 0.70 (p < 0.0001; 95% CI: 0.626-0.774) and 0.63 (p < 0.0001; 95% CI: 0.547-0.724), and weighted kappa values were 0.92 (p < 0.0001; 95% CI: 0.890-0.940) and 0.91 (p < 0.0001; 95% CI: 0.884-0.936), respectively Comparability among scales High/low-risk categorizations enabled the four scales to be compared. The bivariate study of interscale agree- ment is summarized in Table 5. Wolfe’s classification Wolfe classified 53.39% of the mammograms as low-risk and 46.61% as high-risk. Total agreement between Wolfe’s and the Boyd, BI-RADS and Tabár scales was 75.25%, 75.22% and 90.37% with kappa values of 0.49, 0.49 and 0.80, respectively. Tabár’s classification Using Tabár’s classification, 37.32% mammograms were classified as high-risk. Whereas Tabár displayed almost perfect agreement with Wolfe (90.37% agreement and a kappa value of 0.80 (p < 0.00001)), agreement with BI-RADS and Boyd was lower (84.46% and 84.49%, respectively), with kappa values of 0.64 for both (p < 0.0001) BI-RADS classification BI-RADS classified 21.89% mammograms as high-risk. BI-RADS agreement with the other classifications was as follows: Wolfe, 75.22%, and a kappa value of 0.48 (p < 0.0001); Tabár, 84.46%, and a kappa value of 0.64; BI- RADS and Boyd, almost perfect (99.97% total agreement and a kappa value of almost 1.00, namely, 0.99992 with p < 0.0001). Boyd’s classification Boyd classified 78.14% of mammograms as low-risk and 21.86% as high-risk. It showed almost perfect agreement with BI-RADS (99.97% total agreement and a kappa value of almost 1, namely, 0.9992, p < 0.0001), good agreement with Tabár (84.49% agreement and a kappa value of 0.64, p < 0.0001), and moderate agreement with Wolfe (75.25% agreement and a kappa value of 0.49, p < 0.0001). Graphic distribution study of qualitative scales of mammographic density measurement (Wolfe, Tabár and BI-RADS) with respect to a semi-quantitative scale (Boyd) Figure 1 shows the distribution of the different qualita- tive (i.e., pattern-based) scales of mammographic density measurement with respect to Boyd’s scale. Low-risk categories are represented in blue and high-risk cate- gories in violet-purple. Boyd categories A, B, C and D correspond to low risk, and E and F to high risk. None of the mammograms of low-risk categories of the Wolfe scale, N1 and P1, were included in the high- risk categories of the Boyd scale (E and F) and only one