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Test ID: THCG    
Human Chorionic Gonadotropin (hCG), Quantitative, Pregnancy, Serum

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Useful For Suggests clinical disorders or settings where the test may be helpful

Diagnosing pregnancy

 

Investigation of suspected ectopic pregnancy

 

Monitoring in vitro fertilization patients

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Human chorionic gonadotropin (hCG) is a glycoprotein with 2 noncovalently bound subunits. The alpha subunit is similar to those of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). The beta subunit of hCG differs from other pituitary glycoprotein hormones, which results in its unique biochemical and immunological properties.

 

hCG is synthesized by the cells of the placenta and is involved in maintaining the corpus luteum during pregnancy. It is detected as early as 1 week after conception. The diagnostic cutoff for pregnancy is >25 IU/L. In pregnancy, the levels of hCG increase exponentially for about 8 to 10 weeks after the last menstrual cycle. Later in pregnancy, about 12 weeks after conception, the concentration of hCG begins to fall.

 

Other sources of elevated hCG values include ectopic pregnancy, threatened abortion, microabortion, recent termination of pregnancy, gestational trophoblastic disease, choriocarcinoma, and some germ cell tumors.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Negative: <5 IU/L

Indeterminate: 5-25 IU/L

Positive: >25 IU/L

 

Suggest repeat testing of indeterminate results in 72 hours.

Interpretation Provides information to assist in interpretation of the test results

Values between 5 and 25 IU/L are indeterminate for pregnancy. Consider confirming with repeat test in 72 hours. Values in pregnancy should double every 3 days for the first 6 weeks.

 

Both normal and ectopic pregnancies generally yield positive results of pregnancy tests. The comparison of quantitative human chorionic gonadotropin (hCG) measurements with the results of transvaginal ultrasonography (TVUS) may aid in the diagnosis of ectopic pregnancy. When an embryo is first large enough for the gestation sac to be visible on TVUS, the patient generally will have hCG concentrations between 1,000 and 2,000 IU/L. (These are literature values. Definitive values for this method have not been established at this time.) If the hCG value is this high and no sac is visible in the uterus, ectopic pregnancy is suggested. Elevated values will also be seen with choriocarcinoma and hydatiform mole.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

False-elevations (called phantom human chorionic gonadotropin: hCG) may occur with patients who have human antianimal or heterophilic antibodies.  

 

Some specimens may not dilute linearly due to abnormal forms of hCG.

 

Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or an imaging procedure, may have circulating antianimal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

Snyder JA, Haymond S, Parvin CA, et al: Diagnostic considerations in the measurement of human chorionic gonadotropin in aging women. Clin Chem 2005;51:1830-1835