False-negative results in point-of-care qualitative human chorionic gonadotropin (hCG) devices due to excess hCGbeta 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 hCGbeta (hCGbeta cf). We identified 3 urine specimens with apparent false-negative results using the OSOM(R) 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 false-negative hCG results. Purified hCG variants were added to urines positive for hCG and tested using the OSOM, ICON(R) 25 hCG (Beckman Coulter), and hCG Combo SP(R) 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 hCGbeta cf occurred in molar excess of intact hCG. Addition of purified hCGbeta cf to hCG-positive urines caused false-negative hCG results using the OSOM and ICON qualitative urine hCG devices.

Conclusions: Increased concentrations of hCGbeta cf can cause false-negative results on the OSOM and ICON qualitative urine hCG devices.