The “String Sign” for Diagnosis of Mucinous Pancreatic Cysts

Abstract

Purpose: To assess the performance characteristics of the “string sign” in pancreatic cyst fluid for the diagnosis of mucinous cysts, and the utility of a novel sequential interpretation model for cyst fluid tests. Methods: Medical records of all patients undergoing EUS FNA (fine needle aspiration) of pancreatic cystic lesions at one institution from 2003 to 2012 were reviewed. String sign was performed prospectively, and results were obtained from EUS reports. The string sign was performed at the time of EUS FNA by placing a drop of fluid between two gloved fingers, or watching the fluid drip from the FNA needle. A positive sign was defined as formation of a string at least 1 cm in length and lasting for at least 1 second. Diagnosis for each cystic lesion was independently determined using standardized criteria including histology results. A sequential cyst fluid test interpretation model was created, in which the string sign was interpreted first, followed by additional tests interpreted sequentially. Results: String sign results were available for 318 cystic lesions, of which 232 had an independent basis for diagnosis, including 118 mucinous lesions. For 98 histologically proven cases the sensitivity, specificity, PPV, NPV, +LR, and -LR of the string sign for diagnosis of mucinous cysts was 58%, 95%, 94%, 60%, 11.3, and 0.45 respectively. Results were similar for all 232 lesions. String sign performance was comparable to cyst fluid CEA at a diagnostic threshold of 800 ng/ml. The string sign had a superior sensitivity to mucin staining (p=0.029) and cytology (p=0.002). In 10 cases the string sign was positive and cyst fluid CEA was < 200 ng/ml; 6 of these were IPMN. The remaining 4, which were not mucinous, all had cyst fluid CEA < 5 ng/ml. The sequential testing model is shown in the table. When the string sign was positive, specificity was high without further cyst fluid test interpretation. When the string sign was negative, additional stepwise test interpretation improved the sensitivity for diagnosis of a mucinous lesion from 58% to 96%.Table: Table. Performance of a sequential test interpretation modelConclusion: The string sign is highly specific for mucinous pancreatic cysts, and improves overall diagnostic accuracy of pancreatic cyst fluid analysis. A sequential test interpretation model incorporating the string sign yields high diagnostic specificity and sensitivity for mucinous pancreatic lesions.