Screening for Lynch Syndrome by Immunohistochemistry of Mismatch Repair Proteins: Significance of Indeterminate Result and Correlation With Mutational Studies

Context.— Immunohistochemical expression of mismatch repair (MMR) protein is a well-accepted method for routine screening for Lynch syndrome with relatively high sensitivity and specificity. Occasionally, however, immunohistochemistry (IHC) can yield an equivocal result with poor reproducibility and the potential for misdiagnosis. Objective.— To determine the frequency and significance of indeterminate MMR IHC expression in patients routinely screened for Lynch syndrome and correlation with germline mutation studies. Design.— Semiquantitative scoring of MMR IHC was performed by image analysis in 479 cases, of which 380 were colorectal and 99 endometrial cancer. Scores of 10% or more, less than 10%, and 0% were used as cutoffs for retained, indeterminate, and loss of expression respectively. Negative and indeterminate IHC results were confirmed by mutational studies. Results.— Four hundred eighteen of 479 cases (87.2%) were reported as retained expression, 45 (9.3%) as loss of expression, and 16 (3.3%) as ...