BackgroundWe described a patient who exhibited a gradual increase in carbohydrate antigen 19-9 (CA19-9) concentrations for 4 years at three hospitals, with no associated clinical manifestations; however, we were unable to define the cause of this increase, forcing us to consider whether it was a false-positive result. MethodsGiven the potential for interference, this study used multiple system detection, gradient dilution, Polyethylene glycol (PEG) precipitation and heterophilic antibody blocking assay to evaluate the reliability of CA19-9 concentration increase. ResultsAnalysis of the patient sample using multiple systems indicated that CA19-9 concentrations showed an obvious increase (154.0, and 889.2 IU/ml, respectively) using the Cobas E602 and Advia Centaur XP systems, and were within the reference ranges (<10 IU/ml) on other modules. PEG precipitation on the Cobas E602 and Advia Centaur XP systems reduced the CA19-9 concentration, as did heterophilic blocking tube (HBT-6, HBT-1) blockade. ConclusionCA19-9 was incorrectly identified to increase due to the presence of heterophilic antibodies. We recommend that heterophilic antibodies should be evaluated in cases with elevated CA19-9 level but no associated clinical manifestations to prevent false positives.