False-negative results in point-of-care qualitative human chorionic gonadotropin (hCG) devices due to excess hCGβ core fragment

BACKGROUND: During pregnancy, human chorionic gonadotropin (hCG) immunoreactivity in urine consists of intact hCG as well as a number of hCG variants including the core fragment of hCGβ (hCGβcf). We identified 3 urine specimens with apparent false-negative results using the OSOM® hCG Combo Test (Genzyme Diagnostics) qualitative hCG device and sought to determine whether an excess of 1 of the fragments or variants might be the cause of the interference. METHODS: We measured concentrations of hCG variants in the urine from 3 patients with apparent falsenegative hCG results. Purified hCG variants were added to urines positive for hCG and tested using the OSOM, ICON® 25 hCG (Beckman Coulter), and hCG Combo SP® Brand (Cardinal Health) devices. RESULTS: Dilution of these 3 urine samples resulted in positive results on the OSOM device. Quantification of hCG variants in each of the 3 patient urine specimens demonstrated that hCGβcf occurred in molar excess of intact hCG. Addition of purified hCGβcf to hCGpositive urines caused false-negative hCG results using the OSOM and ICON qualitative urine hCG devices. CONCLUSIONS: Increased concentrations of hCGβcf can cause false-negative results on the OSOM and ICON qualitative urine hCG devices.