What are chances a "brushing" of cells in bile liver duct path report during ercp stent placement are incorrect in cancer dx? says neg but pt has it?

Slight chance: Pathologists are experts for diagnosing cancer from a single cell , in order to do that must get an intact undamaged enough cells , must preserve them , properly store &; label them and send to cyto-pathology lab , even in presence of ca brush may miss it, all of the above can happen then will false negative results , 85 - 90 % time will not happen , will have correct diagnosis