LIVING THE LAB LIFE
A BLOG FOR ASCLS REGION V
Human chorionic gonadotropin (hCG) is a peptide glycoprotein hormone composed an α- and β-chain subunits. The trophoblastic cells of the placenta are the primary producer of hCG. There is also evidence that a very small amount of hCG is produced by the pituitary gland in menopausal women. The α-chain component of hCG is identical to the α-chain in luteinizing hormone, thyroid stimulating hormone, and follicle stimulating hormone; the β-chain is unique. HCG is responsible for maintaining the corpus luteum and stimulating the synthesis of progesterone. It is also has proangiogenic functions critical for maintaining blood supply to the fetus.
By far, the most common test for hCG is the urine pregnancy test. Urine pregnancy tests are available over the counter for women to test for new pregnancy. Hospital also routinely use this point-of-care testing to screen non-sterile women of child-bearing age. Urine pregnancy tests are qualitative immunoassays that detect hCG by drawing the urine specimen through a porous membrane to an immobilized section imbedded with anti-hCG. Once an hCG-anti-hCG complex forms, its presence is visualized by a colorimetric detection reaction step (the exact methodology for detection varies depending on the manufacturer).
As laboratorians, it is essential that we know the limitations of this testing so we can counsel providers on how to interpret unexpected results. False negative results can occur with dilute urine samples; first-morning urine samples are ideal. Specimens demonstrating the classic hook effect (when analyte concentration is so high that is saturates out the capture antibodies, causing erroneously low analyte detection) can also yield false negative results. In rare cases, false negatives have been reported where patients are excreting variant forms of hCG that test strips do not detect. The presence of an interfering substance causes false positives, such as various drugs.
In any cases of unexpected urine hCG results, following up with a quantitative serum (or plasma) hCG assay is recommended. Increased levels of hCG will be detectable before a positive result will show in urine. Serum hCG testing is not without potential downfalls. Most immunoassays for hCG are sandwich immunoassays, therefore the presence of heterophilic antibodies can cause false positive results. It is important to note that there is wide inter-method variability with quantitative hCG assays, therefore any patient having serial hCG levels done during her pregnancy needs to have them all done with the same method.
In normal intrauterine pregnancy, it takes about six to eight days after conception for hCG levels to increase to 10-50 IU/L, the lower end of detectability for most testing. After that, HCG levels will double every 48 hours for the next ten weeks of gestation. Providers can therefore use hCG levels to approximate date of conception (and therefore due date) of their pregnant patients. HCG levels then drop and plateau during the second and third trimester to about 10,000 IU/L. After delivery, hCG will be detectable in the patient’s serum for about two weeks. It takes about four weeks after elective or spontaneous abortion for hCG levels to drop below detectability levels due to higher levels of hCG in the earlier months of pregnancy. “Normal” ranges vary widely during pregnancy, as hCG levels can be substantially higher in multiple gestation pregnancies. Cases of eclampsia, erythroblastosis fetalis, and polyhydramnios will also present with higher-than-normal hCG levels.
HCG levels are also important when dealing with potential cases of ectopic pregnancy. Ectopic pregnancy is when the blastocyte implants outside of the uterine cavity, usually in the fallopian tubes. The fetus will then grow and rupture the tube, causing hemorrhage. Ectopic pregnancy is the most common cause of maternal death in the first trimester. Patients may present with light vaginal bleeding and abdominal pain. Risk factors for ectopic pregnancy include tubal damage from pelvic inflammatory disease, past or current sexual transmitted infections, advanced maternal age, use of assisted reproductive therapies, and smoking.
A single hCG level cannot differentiate between intrauterine and ectopic pregnancies, but serial hCG levels that do not follow the typical rate of increase in hCG suggest an abnormal or nonviable pregnancy. Ultrasound is the most commonly used method to diagnose ectopic pregnancy (although the intrauterine gestational sac is not visible on ultrasound until hCG levels increase above 6,000 IU/L). A serum progesterone level can be measured to help identify an abnormal pregnancy. Progesterone levels of greater than 25 ng/mL are typical of normal pregnancies; a level of less than 5 ng/mL is a strong indicator of an abnormal pregnancy, while levels between 5 ng/mL and 25 ng/mL do not correlate well to any clinical presentation.
Molar Pregnancy and Gestational Trophoblastic Disease
A molar pregnancy is when a (partial or complete) hydatidiform mole forms that mimics pregnancy. A mole forms in about 1 in 1,500 pregnancies. HCG levels will be abnormally high and will double at an increased rate. Molar pregnancies are treated by removing the hydatidiform mole; the patient’s hCG level then must be monitored until it is no longer detectable. If the hCG level does not drop to below analyzer limits, the patient’s condition may have progressed to gestational trophoblastic disease.
5% or partial hydatidiform moles and 20% of complete hydatidiform moles will progress to gestational trophoblastic disease (GTD), a malignant disease. Other type of GTDs include choriocarcinoma and non-invasive trophoblastic proliferative disease. Patients with GTD will have increased levels of hCG that will demonstrate an abnormal doubling rate in serial hCG levels. Patients with GTD can be treated with chemotherapy; in severe cases, full hysterectomy may be necessary.
HCG also has applications as a marker for other tumors as well. In these patients, measuring not just hCG but the β-chain of hCG adds to the diagnostic utility of the results. 40% of patient with pure seminoma have elevated β-hCG levels, while only 15% have elevated hCG levels. In the case of germ cell disorders, hCG levels correlate with overall patient prognosis. A value of greater than 50,000 IU/L indicates a poor prognosis.
Clarke, W. (Ed.). (2011). Contemporary Practice in Clinical Chemistry (2nd ed.). Washington DC: AACC Press.
Mais, D. D. (2014). Practical Clinical Pathology. Hong Kong: ASCP Press.