From the Guidelines
False positive urine human chorionic gonadotropin (hCG) pregnancy tests are most commonly caused by pituitary hCG, hypogonadism, and heterophilic antibodies.
Causes of False Positive Results
- Pituitary hCG/hypogonadism: Unilateral orchiectomy and chemotherapy can cause low testosterone levels, leading to increased production of luteinizing hormone (LH) and hCG by the pituitary gland, which can cross-react with some assays for hCG 1.
- Heterophilic antibodies: These antibodies have been reported to result in false-positive hCG results in women 1.
- Other cancers: Other cancers can produce moderately elevated levels of hCG, so elevations of hCG are not diagnostic of a germ cell tumor in patients with poorly differentiated cancers 1.
- Tumor lysis: Serum tumor marker levels may rise during the first week of chemotherapy because of tumor lysis 1.
Diagnosis and Management
When a false positive is suspected, assessment of the urine hCG can be helpful as cross-reactive molecules in the blood that cause false positives rarely get into the urine 1. A positive urine hCG excludes a false-positive serum result. Clinicians should be aware of the potential problems with false-positive results and measure the hCG on a different assay when results do not fit the clinical picture 1.
From the Research
Causes of False Positive Urine hCG Pregnancy Tests
- False-positive serum human chorionic gonadotropin results are estimated to occur in 1 in 10^3 to 1 in 10^4 tests 2
- Most of these false-positive results are due to interference by non-human chorionic gonadotropin substances (especially human luteinizing hormone and anti-animal immunoglobulin antibodies) and the detection of pituitary human chorionic gonadotropin 2
- Circulating factors in serum, such as heterophilic antibodies or nonactive forms of hCG, can interact with the hCG antibody and cause unusual or unexpected test results 3, 4
- False-positive urine beta-HCG tests can occur in patients with certain medical conditions, such as a tubo-ovarian abscess 5
- The mechanism for the false-positive reaction in some cases is unknown, but it may be related to the interaction between the test kit and certain substances present in the patient's urine or serum 5
Characteristics of False-Positive hCG Results
- Serum levels that are generally <1000 mIU/mL 2
- Absence of human chorionic gonadotropin in the urine 2
- Nonparallelism of the human chorionic gonadotropin levels in serial dilutions of the serum with the human chorionic gonadotropin standard 2
- Results that are not consistent with the clinical or operative findings 2
- Absence of a substantial change in the serum levels over time or after therapy 2
- Finding of a negative result when an alternative type of human chorionic gonadotropin assay is used 2