Molar pregnancy with false negative urine hCG: the hook effect

Urine pregnancy tests are widely used in emergency departments as the first screening test for patients of reproductive age presenting with gynaecological problems in order to rule out pregnancy. Conditions such as complete molar pregnancy, which produces large amounts of beta human chorionic gonadotropin (beta-hCG), may cause a false negative result due to an oversaturation of the assay system, known as the "hook effect". We report a case where the exclusion of pregnancy by urine testing led to the initial misdiagnosis of a molar pregnancy as a degenerative fibroid. Physicians need to be reminded of the possibility of false negative results with this commonly used test. Negative or inconclusive results in patients with a high suspicion of pregnancy should be further evaluated by serum quantification of beta-hCG and appropriate sample dilution.