hCG Test

Qualitative (reported as positive or negative) hCG testing is routinely used to confirm pregnancy. Quantitative (reported as a number) hCG testing (also called beta hCG) measures the actual amount of hCG present in the blood. It may be used to help diagnose an ectopic pregnancy, to help diganose and monitor a pregnancy that may be failing, and to monitor a woman after a miscarriage. In addition, a quantitative hCG test may be used to diagnose trophoblastic disease or germ cell tumours of the testes or ovary. It may also be used at regular intervals to monitor the effectiveness for these conditions and to detect tumour recurrence.

A qualitative urine or blood hCG test can be done as early as 10 days after a missed menstrual period if a woman wishes to confirm whether or not she is pregnant (some methods can detect hCG even earlier, at one week after conception). In certain patients, several quantitative blood hCG tests over several days may be requested to rule out an ectopic pregnancy or to monitor a woman after a miscarriage. A doctor will also request a quantitative hCG test if they suspect trophoblastic disease or the presence of germ cell tumours.

In non-pregnant women, hCG levels are normally undetectable. During early pregnancy, the hCG level in the blood doubles every two to three days. Ectopic pregnancies usually have a longer doubling time. Those with failing pregnancies will also frequently have a longer doubling time or may even show falling hCG concentrations. hCG concentrations will drop rapidly following a miscarriage. If hCG does not fall to undetectable levels, it may indicate remaining hCG-producing tissue that will need to be removed.
hCG is also used to monitor treatment in patients with trophoblastic disease and to detect recurrent disease after treatment is complete. During therapy, a falling hCG level generally indicates that a tumour is responding to treatment, while rising levels may indicate that a tumour is not responding to therapy. Increased hCG levels after treatment may indicate a recurrence of disease.

Tests performed too early in the pregnancy, before there is a significant hCG level, may give false-negative results, while blood or protein in the urine may cause false-positive results. Urine hCG tests may give a false negative result in very dilute urine. Patients should not drink large amounts of fluid before collecting a urine sample for a pregnancy test.

Certain drugs such as diuretics and promethazine (an antihistamine) may also cause false-negative urine results. Other drugs such as anti-convulsants, anti-Parkinson drugs, hypnotics, and tranquilisers may cause false-positive results.

There are reports of false positive serum hCG results due to several different compounds (not drugs) that may interfere with the test. These include certain types of antibodies that may be present in some individuals and fragments of the hCG molecule. Generally, if results are questionable, they may be confirmed by testing with a different method.