hCG shares a two-subunit structure with TSH, FSH and LH. In each of these, the alpha subunit is identical and the special characteristics of the molecule are conferred by the beta subunit. Use the mouse to rotate the molecule at the right to inspect the alpha and beta subunits (colored red and blue). Immunological testing for hCG must use antibodies directed at the beta subunit to avoid cross-reactivity with the other hormones, thus these tests are often called "beta-hCG" assays. In urine, some of these tests also measure free beta subunits and beta subunit fragments while other tests are more specific for the intact molecule. Both types of test work; the correct reference range must be used for interpretation because the tests that measure fragments show higher values. hCG levels normally double every two days over the first 6 weeks of pregnancy, peaking at about 8-10 weeks of gestation. After that, the levels drop rapidly, then remain fairly steady for the rest of gestation (see the diagram below). Pregancy tests typically detect increased hCG levels 1-2 weeks after conception. Types of pregnancy tests Qualitative: Yes/No result, urine testing (includes home pregnancy tests) Quantitative: Numerical result, serum or urine; this test is also useful for evaluation of ectopic or failing pregnancy Problems with pregnancy tests If a test is not stored or performed correctly, it may yield erroneous results (can be a problem with home tests). Urine tests may give different results depending on the degree of concentration or dilution of the urine. Some tumors can produce hCG or free beta subunits. Some patients have antibodies that react against mouse antibodies (which are used in the tests) and may produce a false-positive result. These interfering antibodies also cause problems with other types of antibody-based tests and are called "heterophil" antibodies. Renal failure leads to accumulation of interfering substances that may produce incorrect results in some tests. Some patients have a low level of circulating hCG-like material. In most patients, the level of this material is not high enough to produce a positive result, but it can cause confusion in rare cases. In cases where there is a question, qualitative tests may be followed by quatitative tests several days apart to demonstrate the expected rising hCG levels. Ultrasound to demonstrate a gestational sac is also used to confirm pregnancy. Back to Outline Ectopic Pregnancy and Spontaneous Abortion Back to Pathology Overview Last modified: 1/20/97; Author: J. Harrison
hCG levels normally double every two days over the first 6 weeks of pregnancy, peaking at about 8-10 weeks of gestation. After that, the levels drop rapidly, then remain fairly steady for the rest of gestation (see the diagram below). Pregancy tests typically detect increased hCG levels 1-2 weeks after conception.
Types of pregnancy tests Qualitative: Yes/No result, urine testing (includes home pregnancy tests) Quantitative: Numerical result, serum or urine; this test is also useful for evaluation of ectopic or failing pregnancy Problems with pregnancy tests If a test is not stored or performed correctly, it may yield erroneous results (can be a problem with home tests). Urine tests may give different results depending on the degree of concentration or dilution of the urine. Some tumors can produce hCG or free beta subunits. Some patients have antibodies that react against mouse antibodies (which are used in the tests) and may produce a false-positive result. These interfering antibodies also cause problems with other types of antibody-based tests and are called "heterophil" antibodies. Renal failure leads to accumulation of interfering substances that may produce incorrect results in some tests. Some patients have a low level of circulating hCG-like material. In most patients, the level of this material is not high enough to produce a positive result, but it can cause confusion in rare cases. In cases where there is a question, qualitative tests may be followed by quatitative tests several days apart to demonstrate the expected rising hCG levels. Ultrasound to demonstrate a gestational sac is also used to confirm pregnancy. Back to Outline Ectopic Pregnancy and Spontaneous Abortion Back to Pathology Overview Last modified: 1/20/97; Author: J. Harrison
Last modified: 1/20/97; Author: J. Harrison