False-Positive hCG Assay Results Leading to Unnecessary Surgery and Chemotherapy and Needless Occurrences of Diabetes and Coma

Much concern has been raised by the unraveling at the hCG Reference Service of six cases of persistent phantom human chorionic gonadotropin (hCG). These are false-positive hCG results, which are likely attributable to human anti-mouse IgG or to heterophilic antibodies (1)(2)(3). The hCG Reference Service, started in January 1998 to aid with the interpretation of irregular or discordant hCG immunoassay results, requests parallel serum and urine samples. Each is tested in four separate two-step microtiter plate ELISAs (assay 1 detects intact hCG, assay 2 detects nonnicked or bioactive hCG only, assay 3 detects the hCG free β-subunit only, and assay 4 detects the hCG β-core fragment only) at three different concentrations (undiluted, a 1:2 dilution, and a 1:5 dilution). From the data, inferences are made about the nature (nonnicked or nicked hCG, free β-subunit, and β-core fragment) and likely source (trophoblast disease, pituitary hCG, cancer, or phantom hCG) of the hCG immunoreactivity. In all six cases of phantom hCG tested by the service, increased hCG concentrations were detected in serum samples (Table 1), but no detectable hCG, free β-subunit, or β-core fragment was found in the parallel urine samples (<3 IU/L). In all cases, the presence of phantom hCG was confirmed by at least two of the following three criteria: the finding of serum concentrations that were nonlinear on dilution; the finding of hCG concentrations in a two-step assay that were 20% or less of values in a one-step (single incubation with both coating and tracer antibodies) sandwich assay or that differed by 80% in two different hCG assays; and the finding of measurable β-core fragment immunoreactivity (not usually detectable in serum) in serum samples. Summary of clinical findings and laboratory data. Patients i, ii, and iii are described in more detail in Cole (3). Range of hCG concentrations in the 3–11 months (depending on case) after initial detection. Summary of clinical findings and laboratory data. Patients i, ii, and iii are described in more detail in Cole (3). Range of hCG concentrations in the 3–11 months (depending on case) after initial detection. The six cases had similar histories. Each started with an incidental pregnancy test (Table 1). The pregnancy test was positive (69–285 IU/L), and the patient was sent to an obstetrician. In each case, ultrasound failed to reveal a fetal sac, laparoscopy did not reveal an ectopic pregnancy, and dilation and curettage revealed no recent history of pregnancy or trophoblast disease. In each case, the false-positive hCG persisted for an additional 3–11 months (5–451 IU/L) before samples were sent to the hCG Reference Service. In all six cases, the patients were referred to an oncologist or gynecologic oncologist with a suspected diagnosis of trophoblast disease or postgestational choriocarcinoma. Four of the six cases received multiple courses of methotrexate chemotherapy, and one of the six received in addition EMACO chemotherapy, with no major quantitative reduction in false-positive hCG results. After chemotherapy, two patients underwent a hysterectomy and one other patient underwent an oophorectomy, all without a major reduction in measured hCG concentrations. One patient developed type 1 diabetes as a complication of the chemotherapy and became comatose. All therapies came to a halt with the finding by the hCG Reference Service that the persistent hCG results, the sole basis for treatment, were in fact false-positive or “phantom” hCG. We have now heard that in two of our earliest phantom hCG cases (tested in Spring 1998), the false-positive hCG results eventually, after 10–14 months, subsided. The cases came from all parts of the United States (one from the West Coast, three from the Midwest, and two from the Northeast). These cases of phantom hCG found their way through word of mouth to the hCG Reference Service, a new, unadvertised facility, in a 9-month period. We wonder how many other similar cases may exist. Four of the six false-positive cases had been detected and followed with the Abbott Diagnostics AxSym hCGβ test (Table 1), and one of six had been followed with the sister assay, the Abbott Diagnostics IMx hCGβ test, which uses the same antibody/chemical set. Thus, five of the six cases were detected with this one type of assay. We do not know if this type of assay, among the >40 quantitative hCG test sold in the US (4), is particularly prone to false-positive results. Laboratory directors and managers need to be aware of this potential problem, especially if they are performing the AxSym or IMx hCGβ type test. They need to be available to help physicians quickly exclude or identify phantom hCG, which can be done by simply running quantitative urine hCG tests. In phantom hCG cases, no hCG immunoreactivity may be detected (<5 IU/L) in urine samples. Other simple methods to exclude phantom hCG are to test serum samples with competitive hCGβ RIAs (which do not detect phantom hCG), or to demonstrate nonlinearity in dilutions of the serum samples in an hCG immunoassay (phantom hCG may give grossly nonlinear results). Alternatively, help can be sought from the hCG Reference Service.