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Hook-like Effect Causes False-negative Point-of-care Urine Pregnancy Testing in Emergency Patients Full Text
The Journal of Emergency Medicine, 08/15/2011
Griffey RT et al.– Manufacturers should consider that uhCG tests are routinely used at many stages of pregnancy. Characterizing urine human chorionic gonadotropin (uhCG) variants recognized by their tests and eliminating lot–to–lot variability may help improve uhCG test performance. Clinicians need to be aware of and familiarize themselves with the limitations of the specific type of uhCG point–of–care (POC) tests used in their practice, recognizing that under certain circumstances, false–negative tests can occur.
Methods- In investigating a series of late first–trimester false–negative pregnancy tests in the ED, a novel and distinct causative phenomenon was recently elucidated in the institution.
- The authors discuss uhCG POC tests, review the false–negative rate, and describe mechanisms for false negatives and potential remedies.
- The false–negative POC uhCG rate is very low, but in the setting of a large volume of tests, the numbers are worth consideration.
- In positive uhCG POC tests, free and fixed antibodies bind hCG to form a “sandwich”; hCG is present in several variant forms that change in their concentrations at different stages of pregnancy.
- When in excess, intact hCG can saturate the antibodies, preventing sandwich formation (hook effect phenomenon).
- Some assays may include an antibody that does not recognize certain variants present in later stages of pregnancy.
- When this variant is in excess, it can bind one antibody avidly and the other not at all, resulting in a false–negative test (hook–like phenomenon).
- In both situations, dilution is key to an accurate test.
