Definition - What does Hook Effect mean?
The hook effect is a falsely-negative pregnancy test result on a home pregnancy test that occurs due to the presence of excess human chorionic gonadotropin (hCG). Because of the hook effect, the pregnancy test of an otherwise pregnant woman will falsely show a negative result.
A false-negative test rate due to a hook effect is, however, considered to be very low.
FertilitySmarts explains Hook Effect
The urine pregnancy tests rely on the detection of hCG. In a positive pregnancy test, two different antibodies (free and fixed) bind to hCG to form a “sandwich,” which gives a positive result. The hCG levels have to be in a certain range to form a sandwich and yield a positive test result. Too high or too low hCG levels can lead to a misguided result.
Very high hCG levels can occur with an undiluted urine sample (such as when a woman is dehydrated). The excess hCG then saturates the antibodies, impeding sandwich formation and giving a falsely negative result. This inability of the antibodies and hCG to form a sandwich is referred to as the hook effect. In such cases, the pregnancy detection by home pregnancy tests fails.
If a hook effect is suspected, a repeat testing after diluting the urine sample can clear the confusion.