Background : Thyroid carcinomas are the most common of all endocrine system malignancies.The pathological diagnosis of papillary thyroid carcinoma(PTC) is usually easily achieved if the histological structure and nuclear features are present, but in the absence of these features the diagnosis can be difficult .We evaluated the diagnostic value of CD56 and claudin-1 expression in PTC , follicular lesions and follicular neoplasms.Methods : Seventy five cases diagnosed as PTC and 75 cases diagnosed as follicular lesions(follicular adenoma,follicular carcinoma, hyperplastic nodule) were stained immunohistochemically with both CD56 and claudin-1 antibodies. A positive membranous immunostaining with or without cytoplasmic reactivity in more than 10% of the neoplastic cells with CD56 antibody qualified the case as positive .A positive membranous immunostaining in more than 5% of the neoplastic cells with claudin-1 antibody was considered positive. Results: CD56 expression was positive in 69 (92%) cases of follicular lesions. On the other hand CD56 expression was negative in 62(82.7%) cases of PTC (p value<0.001). CD56 as a negative marker for diagnosing PTC had the 82.7% sensitivity, 92% specificity, PPV OF 91.2% and NPV of 84.1%. Claudin-1 expression was positive in 72 cases (96%) of PTC and 56 (74.7%) cases of thyroid follicular lesions (p value<0.001). Claudin-1 as a positive marker for diagnosing PTC had a 96% sensitivity,74.7% specificity,79.1% of PPV and 94.9% of NPV. The panel of (claudin-1+/CD56-) had the specificity and PPV of 100%. In other words a thyroid nodule with positive claudin-1 and negative CD56 reactivity is definitely PTC.The panel of(claudin1-/CD56+) had the specificity and PPV of 100%.A thyroid nodule with positive CD56 and negative claudin-1 reactivity is definitely a Non- PTC lesion. Conclusion: claudin-1 expression is a sensitive immunohistochemical marker for differentiating PTC from other thyroid follicular lesions.CD56 is a less sensitive but more specific marker for identification of PTC. Combination of these two antibodies as a panel has the specificity and PPV of 100%.
