False-positive hCG

Serum hCG testing relies on detecting two antibodies on the hCG molecule. The antibodies are polyclonal or monoclonal antibodies derived from various animals: mouse, rabbit, goat or sheep. Humans with heterophilic (or cross-species) antibodies bind the antibodies in the assay, leading to a false-positive result. This was a common problem with one of the commercially available assays until it was re-engineered in 2003. Heterophilic antibodies cannot cross the glomerular filtration barrier, so the performance of a urinary hCG can eliminate this source for a positive test result. The urine sample should be run using the same system generally reserved for serum, as opposed to over-the-counter urine-pregnancy tests, to avoid decreased sensitivity in the latter.