Falsely Decreased Human Chorionic Gonadotropin (hCG) Results Due to Increased Concentrations of the Free  Subunit and the  Core Fragment in Quantitative hCG Assays David G. Grenache, 1† Dina N. Greene, 1† Anand S. Dighe, 2 Corinne R. Fantz, 3 Daniel Hoefner, 4 Christopher McCudden, 5 Lori Sokoll, 6 Carmen L. Wiley, 7 and Ann M. Gronowski 8* BACKGROUND: Earlier studies have shown that increased concentrations of certain human chorionic gonadotro- pin (hCG) variants can cause false-negative results in some qualitative hCG devices. The objective of this study was to determine if increased concentrations of hCG and hCG core fragment (hCGcf) cause falsely decreased results on 9 commercially available quanti- tative hCG assays. METHODS: Several concentrations of purified hCG and hCGcf were added to 2 sets of 6 serum samples with and without a fixed concentration of intact hCG. We examined 9 widely used immunoassays to measure immunoreactive hCG. Falsely decreased results were defined as those in which the measured hCG concen- tration was 50% of expected. RESULTS: High concentrations of hCG (240 000 pmol/L) produced falsely decreased hCG measurements in 2 assays known to detect this variant. Similarly, high concentrations of hCGcf (63 000 pmol/L) produced falsely decreased hCG measurements in 3 assays that do not detect purified hCGcf. Two assays were identified that detected both hCG and hCGcf, and neither pro- duced falsely decreased results in the presence of high concentrations of these variants. CONCLUSIONS: Extremely high concentrations of hCG variants can cause falsely decreased results in certain quantitative hCG assays. Of the 9 assays examined, none exhibited falsely decreased results in the presence of hCG concentrations typically associated with hCG- producing malignancies. Two assays exhibited decreased (50%) hCG results in the presence of hCGcf concen- trations found during normal pregnancy. © 2010 American Association for Clinical Chemistry Human chorionic gonadotropin (hCG) 9 is a molecu- larly heterogeneous molecule. The biologically active hormone (intact hCG) is composed of noncovalently linked  and  subunits. However, partially degraded variants of hCG and other variants with modified pro- tein and/or carbohydrate structures are also detectable in serum and urine. These variants include hyperglyco- sylated hCG, nicked hCG, hCG missing the -subunit C-terminal peptide, hCG free -subunit (hCG), hy- perglycosylated free -subunit, nicked hCG, and the hCG core fragment (hCGcf) (1–4). hCGcf is the variant predominantly detected in urine after 5 weeks of pregnancy. The relative abundance of each of these hCG vari- ants in serum or urine depends on the physiological and/or pathological state of the individual. For in- stance, hyperglycosylated hCG is predominantly in- creased in urine and serum during early pregnancy but, as pregnancy progresses, hCGcf becomes the pre- dominant variant in urine (3). By gestational day 35, mean concentrations of urine hCGcf have been re- ported to be 65 000 pmol/L and can reach concentra- tions of 1 000 000 pmol/L (4–6). Many trophoblastic and nontrophoblastic tumors produce hCG (7). Very high concentrations of this variant suggest aggressive disease and are strongly as- sociated with a poor prognosis (7). Lempia ¨ninen et al. 1 University of Utah Health Sciences Center, Department of Pathology, Salt Lake City, UT; 2 Harvard Medical School, Department of Pathology, Cambridge, MA; 3 Emory University, Department of Pathology and Laboratory Medicine, Atlanta, GA; 4 Marshfield Clinic, Division of Laboratory Medicine, Marshfield, WI; 5 Uni- versity of North Carolina School of Medicine, Department of Pathology and Laboratory Medicine, Chapel Hill, NC; 6 Johns Hopkins Medical Institutions, Department of Pathology, Baltimore, MD; 7 Providence Sacred Heart Medical Center, Department of Laboratory Medicine, Spokane, WA; 8 Washington Uni- versity School of Medicine, Department of Pathology and Immunology, St. Louis, MO. † David G. Grenache and Dina N. Greene contributed equally to the work, and both should be considered as first authors. * Address correspondence to this author at: Department of Pathology and Immunology, Washington University School of Medicine, Box 8118, 660 S. Euclid, St. Louis, MO 63110. Fax 314-362-1461; email gronowski@wustl.edu. Received January 13, 2010; accepted August 25, 2010. Previously published online at DOI: 10.1373/clinchem.2010.143479 9 Nonstandard abbreviations: hCG, human chorionic gonadotropin; hCG, hCG free -subunit; hCGcf, hCG core fragment; IRR, international reference reagent; FDA, Food and Drug Administration. Clinical Chemistry 56:12 1839–1844 (2010) Endocrinology and Metabolism 1839