Handling false positives in the genomic era.

Both procedures used iodinated contrast media, iohexol (Omnipaque 350; GE Healthcare) in this case. The cause of the “floating gel” was an increase in serum density from residual contrast dye still present in the catheter used for the procedures (1 ). Laboratory tests should not be performed, even if the serum is extracted from beneath the gel barrier. The effect of residual dye is unknown, but laboratory results may be falsely low because of dilution with contrast media or be erroneous owing to interference with the assay itself (2, 3 ). This phenomenon can be avoided by properly flushing lines prior to sample collection (4 ) and/or waiting at least 1 half-life (approximately 20 min) before collecting a peripheral blood sample.