Extraneous tissue: a potential source for diagnostic error in surgical pathology.

"Floaters" represent a potential cause of diagnostic error and may arise during tissue processing and slide preparation. Our objective was to determine the frequency of floaters in slides and features useful in their recognition. We reviewed quality assurance (QA) records for floaters in slides and prospectively examined 1,000 additional slides for floaters and documented floater size, location, and presence in 1 or more sections. In the 521,661 slides reviewed for QA, 65 floaters were detected: 28 were present in the tissue block, and 37 were found only in a single level. In the 1,000 prospectively examined slides, 12 floaters were detected; 9 were only in a single section. Floaters represent a potential source of diagnostic error and occur in 0.01% to 1.2% of slides. Pick up of floaters from the water bath appears most common (∼60%). Floaters in only 1 level and mismatch with the specimen tissue type are clues to the extraneous nature of the floater.