Claudin-18 Immunohistochemical Staining Facilitates the Identification of Metastatic Carcinoma of Gastric or Pancreatic Origin in Effusion Specimens.

Determining the primary origin of a malignant effusion remains a common challenge for cytopathologists. Although immunohistochemical (IHC) markers are available for most primary sites, ideal IHC markers for metastatic gastric adenocarcinoma and pancreatic ductal adenocarcinoma are lacking, and related interpretation is often hindered by mesothelial cells. We recently revealed that claudin-18 IHC staining is useful for identifying the stomach and pancreas as the primary sites of metastatic adenocarcinoma. Thus, we assessed the use of claudin-18 IHC staining in 111 cell blocks obtained from various metastatic cancers and specimens negative for malignancy. Positive membranous claudin-18 staining was noted in all 10 (100%) metastatic pancreatic ductal adenocarcinomas, 9 (90%) of 10 gastric adenocarcinomas, and 1 (9%) of 11 nonmucinous lung adenocarcinomas. The cases of metastatic mucinous carcinomas of lung origin (1 case) and ovarian origin (1 case) were also positive for claudin-18. The other remaining 89 cases showed variable cytoplasmic staining on some cells (73 cases) or complete absence of staining (16 cases). After normalization to the tumor frequency, the sensitivity and specificity for identifying the stomach or pancreas as primary tumor sites in ascites were 95% (confidence interval: 0.83-0.99) and 99% (confidence interval: 0.94-1), respectively. In conclusion, membranous claudin-18 staining is a useful marker for metastatic gastric adenocarcinoma and pancreatic ductal adenocarcinoma in effusion specimens.