Clinical Information

Due to exposure to animal antigens, some patients have developed antibodies that interfere with immunoassay testing. These heterophilic antibodies can bind to animal antibodies used in immunoassays. It has been found that a significant percentage of certain sandwich immunoassay results are false-positive results caused by heterophilic antibody interference. The most frequently reported assay interference effect of heterophilic antibodies is a false-positive assay result. False-negative assay results have also been reported in the literature. Manufacturers add blocking agents to their reagents, but occasional patient samples containing heterophile antibodies are incompletely blocked. Subsequent reporting of erroneous results can have adverse effects on patient management, especially with tumor marker assays.

Among immunometric assays, human chorionic gonadotropin (hCG) assays have been found uniquely susceptible to heterophile antibody interference, resulting in occasional false-positive results. The current assay has proven robust in this respect, but rare interferences still occur. Typically, the observed false-positive elevations are modest, ranging from just above the reference range to levels of 50 to 60 IU/L. If such results are seen and are discordant with the clinical picture or other biochemical or imaging tests, then the laboratory should be alerted. After additional blocking treatment, repeat analysis of the specimen in question may resolve the issue. Dilution of the specimen prior to assay performance often yields unexpected nonlinear results in the presence of interfering substances, such as heterophile antibodies. Heterophile blocking tube treatment is used for troubleshooting samples that yield results that are either nonlinear or do not match the clinical picture of the patient and are suspected of containing heterophile antibodies. Finally, assessment of an analyte, such as hCG with an alternative assay will often lead to apparent discrepant results in the presence of a heterophile antibody, as heterophile antibodies often interact differently with alternative assay antibodies.

Human chorionic gonadotropin (hCG) is a glycoprotein hormone (molecular weight [MW] approximately 36,000 Da) consisting of 2 noncovalently bound subunits. The alpha subunit (92-amino acids; "naked" protein MW 10,205 Da) is essentially identical to that of luteinizing hormone (LH), follicle-stimulating hormone, and thyrotropin. The alpha subunit is essential for receptor transactivation. The different beta subunits of the above hormones are transcribed from separate genes, show less homology, and convey the receptor-specificity of the dimeric hormones. The chorionic gonadotropin, beta gene (coding for a 145-amino acid, "naked" protein MW 15,531 Da; glycosylated subunit MW approximately 22,500 Da) is highly homologous to the beta subunit of LH and acts through the same receptor. However, while LH is a classical tropic pituitary hormone, hCG does not usually circulate in significant concentrations. In pregnant primates (including humans), it is synthesized in the placenta and maintains the corpus luteum and, hence, progesterone production, during the first trimester. Thereafter, the placenta produces steroid hormones, diminishing the role of hCG. HCG concentrations fall, leveling off around week 20, significantly above prepregnancy levels. After delivery, miscarriage, or pregnancy termination, hCG levels fall, with a half-life of 24 to 36 hours, until prepregnancy levels are reached.

Outside of pregnancy, hCG may be secreted by abnormal germ cell, placental, or embryonal tissues, in some seminomatous and nonseminomatous testicular tumors; ovarian germ cell tumors; gestational trophoblastic disease (hydatidiform mole and choriocarcinoma); and benign or malignant nontesticular teratomas. Rarely, other tumors, including hepatic, neuroendocrine, breast, ovarian, pancreatic, cervical, and gastric cancers may secrete hCG, usually in relatively modest quantities.

During pathological hCG production, the highly coordinated secretion of alpha and beta subunits of hCG may be disturbed. In addition to secreting intact hCG, tumors may produce disproportionate quantities of free alpha-subunits or, more commonly, free beta-subunits. Assays that detect both intact hCG and free beta-hCG, including the electrochemiluminescent immunoassay assay, tend to be more sensitive in detecting hCG-producing tumors.

With successful treatment of hCG-producing tumors, hCG levels should fall with a half-life of 24 to 36 hours and, eventually, return to the reference range.

The alternate testing method is an enzymatic immunoassay. Values obtained with different assay methods or kits may be different and cannot be used interchangeably.