Effect of urinary proteins on in vitro immunological tests for pregnancy

A study of the possible interference of urinary proteins with all the commercial in vitro pregnancy products on the market in the United States today was carried out by our group. Six of these tests were of the tube type and seven were slide tests. Radioimmunoassays and receptor assays for beta human chorionic gonadotropin (beta-HCG) were not tested. The products examined showed interferences by urinary proteins presented at various levels. Interference by human serum albumin (HSA) (in five tube tests) occurred only at a physiologically improbable, high concentration of HSA (5 g/dL). However, in certain cases interferences by serum proteins (false positive type) occurred at concentrations lower than 0.06 g/dL which are physiologically probable. The four tests formulated to work with both serum and urine did not show any urinary protein interference with HCG measurements. There was no clear difference in urinary protein interferences between the tube and the slide tests; the tube tests were slightly more sensitive to the presence of HSA. Those tests employing latex beads, as opposed to erythrocytes, showed greater sensitivity to protein interferences.