Interference of exogenous HCG on pregnancy tests.

New urinary tests of pregnancy have recently been developed that are exceptionally sensitive to HCG. Our study was undertaken to explore the possibility that exogenous HCG, administered to induce ovulation, may interfere with these low-threshold tests of pregnancy. Six healthy volunteers participated in our study. Each woman received three sequential 5,000-U doses of HCG by im route on days 3, 5, and 7 after ovulation. Blood and urine for HCG assay were taken daily from each subject. Urine specimens were subjected to a traditional high-threshold pregnancy test (sensitivity limit = 2000 U/L) and to a low-threshold pregnancy test (sensitivity limit = 50 U/L); HCG in plasma and urine was determined by ELISA assay. Plasma HCG levels increased gradually under HCG dosing and attained a peak on day 8 (average = 73.7 U/L; range 24-124 U/L). As regards urinary HCG levels, the average value on days 8, 9, and 10 was above the 50-U/L detection limit of the low-threshold test, which was in fact positive in the majority of patients. Also on days 11 and 12, however, urinary HCG levels were above this limit in a certain proportion of subjects (day 11, 2/6; day 12, 1/6); accordingly the low-threshold pregnancy test was positive with a frequency of 2/6 on day 11 and of 1/6 on day 12. By contrast, the traditional high-threshold pregnancy test was negative in urine samples of all subjects. Our study indicates that the increased sensitivity of modern low-threshold pregnancy tests may cause false positive results in those cases where exogenous HCG has been administered during the luteinic phase.