Aim-To determine whether assays of carcinoembryonic antigen (CEA) and cholesterol in ascites add diagnostic value to cytology. Methods-The additional diagnostic eYcacy of the biochemical assays was studied in the ascitic fluid from 130 patients, of whom 57 had peritoneal carcinomatosis. All diagnoses were verified by subsequent necropsy and/or histology. Results-CEA concentrations over 5 ng/ ml indicated carcinomas, occasionally without peritoneal involvement of the tumour. However, increased values were significantly more common in cancer with peritoneal involvement (p < 0.01), giving a sensitivity of 51% and specificity of 97% for carcinomatosis. A cholesterol value exceeding 1.21 mmol/litre was found in 93% of cancers with peritoneal involvement, but it was not entirely specific (96%) for carcinomatosis. Simultaneous increases in CEA and cholesterol concentrations were specific for carcinomatosis and this combination increased the sensitivity for diagnosing carcinomatosis from 77% with cytology alone to 88%. The correct diagnosis could thus be made in five of 12 cases with inconclusive cytology. Conclusions-The measurements of both CEA and cholesterol concentrations in ascites give additional specific information about peritoneal carcinomatosis and can therefore be a useful adjunct to cytology-in particular, in inconclusive cases. (J Clin Pathol 2001;54:831-835) Keywords: ascitic fluid; carcinoembryonic antigen; cholesterol; cytology Conventional cytological examination of ascitic fluid often yields inconclusive results: apart from benign and malignant diagnoses, many cases are "suspicious for malignancy". [1][2][3] In liver cirrhosis, inflammation, chemotherapy, and irradiation it is often diYcult to distinguish exfoliated reactive mesothelial cells from highly diVerentiated cancer cells. Moreover, the cytology is often false negative because only a few neoplastic cells are present in the fluid, 4 or processing of specimens is suboptimal with lysis of tumour cells. 5 Therefore, a specific cytological diagnosis may have a sensitivity of only 40-80%. [6][7][8] Samples that are inconclusive because of some cellular atypia can be further evaluated with ancillary morphological methods, such as immunocytochemistry 1-3 9 and DNA cytometry, 2 9 10 although these techniques are of little help when only a few preserved tumour cells are present.Biochemical parameters, such as the concentrations of carcinoembryonic antigen (CEA) 6-8 10-12 and cholesterol 4 12-15 in the serous eVusion, have been studied as markers of malignancy. Such assays can be done rapidly in properly preserved eVusions after cytological examination, but their diagnostic value is still debated. It has therefore been stated that the cholesterol content of ascitic fluid is neither specific nor sensitive for a tumour.5 16 Some authors have suggested that a high concentration of CEA in the ascitic fluid is indicative of peritoneal metastases from a CEA producing tumour, 7 12 although it has been found in carcinomas without serosal involvement....