Origin of a false positive urine pregnancy test in a patient with membranoproliferative glomerulonephritis type I

Background: Discrepant qualitative urine and quantitative serum hCG results were observed in a patient with a history of membranoproliferative glomerulonephritis. Further studies were performed to investigate this discrepancy. Methods: Urine and serum specimens were analyzed using alternate methods and heterophilic antibody blocking and dilution studies were performed. The potentially interfering substance was identified and subsequent specimens from the patient's hospital stay were analyzed for hCG and the effect of the interferent was assessed. Results: Initially, the patient's urine specimen was positive for the presence of hCG based on the qualitative QuickVue + hCG point-of-care device. However, quantitative serum testing was negative for pregnancy on the Tosoh AIA-1800 analyzer (< 2 mIU/ml). The serum specimen was also positive on the QuickVue + device, but upon incubating urine or diluted serum with heterophilic antibody binding reagent, hCG results shifted from positive to negative. Although the patient had a non-elevated human anti-mouse antibody (HAMA) level, the patient exhibited an elevated serum rheumatoid factor concentration (270. IU/ml; reference interval: ≤ 20 IU/ml). Application of a calibrator solution containing rheumatoid factor at levels above 175. IU/ml resulted in a positive reading on the QuickVue + point-of-care card. Conclusion: The discrepancy between the qualitative urine and quantitative hCG results was due to an interfering substance causing a false positive on the QuickVue + urine point-of-care device. Subsequent studies identified rheumatoid factor as the potential interferent in this case.