What are the chances a path report from a "brushing" of a liver bile duct during ercp is incorrect in a cancer dx? 87yo. severe jaundice- no pain?

Slight chance: Pathologists are experts for diagnosing cancer from a single cell , in order to do that must get an intact undamaged cell , must preserve it , properly store & label it 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 , good luck to your dad.

False Positive: Bile duct brush, regardless of pain not likley. Painless jaundice commonly is cancer. Issue of location (pancreas, intra- or extra- hepatic ducts, may mitigate treatment options. Speak with a bile duct surgeon even if you do not want an operation.