Comparison of two antibodies for immunohistochemical evaluation of epidermal growth factor receptor expression in colorectal carcinomas, adenomas, and normal mucosa

BACKGROUND. Immunohistochemical staining for epidermal growth factor receptor (EGFR) has been used as a criterion for the selection of patients with colon cancer for anti-EGFR therapy. Two antibodies, the PharmDx kit and the 31G7 clone, are used commonly for immunohistochemistry by various laboratories. No comparative studies on the performance of these 2 antibodies are available. METHODS. EGFR status was evaluated in 744 tissue microarray core samples from primary and metastatic colorectal carcinomas, colorectal adenomas, and normal colorectal mucosa with both the PharmDx kit and the clone 31G7 monoclonal antibodies. The stains were compared for staining intensity by using an automated image-analysis system. The intensity of positive staining (brown color) was measured on a scale from 0 to 255. The staining intensity also was scored manually as 0, 1 +, 2 +, and 3 +. RESULTS. Statistically, the median staining intensities scored by the automated system between the 2 antibodies did not differ significantly, although, within each category of samples (normal, adenoma, carcinoma, and metastases), the PharmDx antibody staining was slightly more intense than the clone 31G7 antibody staining. There was a linear correlation between automated image-analysis and manual scoring categories. The median automated image-analysis intensity scores for the 4 manual scoring categories with the PharmDx kit were as follows: 0 staining, 67.5; 1 + staining, 75.5; 2 + staining, 89.6; and 3 + staining, 106.0. The median automated image-analysis intensity scores for the 4 manual scoring categories with the clone 31G7 antibody were as follows: 0 staining, 71.3; 1 + staining, 73.6; 2 + staining, 84.6; and 3 + staining, 99.1. The classification of tumors as EGFR-negative (0 staining) or positive (1 +, 2 +, or 3 + staining) was concordant in 151 of 160 carcinomas (94.4%) with 2 antibodies using manual scoring. Five samples (3%) that scored 1 + with the PharmDx kit antibody scored 0 with the clone 31G7 antibody; whereas 4 samples (2.5%) that scored 1 + with the clone 31G7 antibody scored 0 with the PharmDx kit antibody. CONCLUSIONS. The EGFR expression results obtained by immunohistochemistry using both the EGFR PharmaDx kit and the 31G7 clone were comparable. Either antibody may be used for immunohistochemical detection of EGFR in colorectal carcinomas. In addition, manual scoring had an excellent correlation with automated scoring.