Serum laboratory hCG test occasionally give false positive hCG or erroneously hCG results. When the test is repeated it may once again give a false positive hCG results. The principal of the hCG test is animal antibodies. One animal antibody to one part of the hCG molecule is used to immobilized the hCG on the automated test vial. A second antibody to a distant part of the hCG molecule, binds the immobilized hCG and adds a label to the molecule. Usually this antibody is labeled with a chemilumenescent compound. The amount of this chemilumenescent tracer immobilized corresponds to the concentration of hCG. As such, hCG is detected by linking the capture animal antibody and labeled animal antibody together.
The problem is that some people have heterophilic antibodies in their circulation. These are human antibodies against human antibodies, that because they recognize a part of the antibody that is common in humans and animals, will bind animal antibodies. All antibodies bind two molecules which they are raised against. As such a heterophilic human antibody can bind the hCG immobilized antibody and tracer antibody used in an hCG test and cause a false positive hCG test. The USA hCG Reference Service has seen false positive cases with false serum hCG results as high as 1000 mIU/ml.
There is one plus to false positive hCG. Heterophilic antibodies are very large molecules and do not cross the kidney and enter the urine. If a person is positive for hCG in the serum and negative for hCG in the urine, then the serum test was false positive due to heterophilic antibodies. The problem is that in the USA the FDA does not allow sensitive serum hCG tests be used for urine. As such urine hCG is measured with insensitive point-of-care tests. The problem is that these tests being negative do not necassarily mean that a person has false positive hCG, it could be negative because of the low sensitivity (12-50 mIU/ml sensitivity). One home pregnancy urine hCG test is sold that it very sensitive (sensitivity 3.3 mIU/ml), this is First Response manual test. This test can show if a person is truly positive or negative for hCG in urine. If hCG is positive in urine then in all likelihood the hCG detected in serum is real.