Human chorionic gonadotrophin (hCG) Test

Qualitative (reported as positive or negative) hCG testing (urine pregnancy test) 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 diagnose and monitor a pregnancy that may be failing, and to monitor a woman after a miscarriage.

hCG analysis is also included as part of the prenatal Downs screening program. Higher values of hCG are typically observed in trisomy 21 (Down’s syndrome) than in unaffected pregnancies.

In addition, a quantitative blood 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 of treatment  for these conditions and to detect tumour recurrence.

A qualitative urine hCG test can be done from the first day of 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 a germ cell tumour.

A blood test which includes hCG as part of the prenatal screening program for Downs syndrome is conducted during the 2nd trimester of pregnancy.

In non-pregnant women, hCG is normally undetectable in blood and urine. During early pregnancy, the amount of hCG 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 analysis is combined with other tests as part of the prenatal screening program for Down’s syndrome. The results of the combined tests are used to assess a woman’s risk of having a baby affected by Down’s syndrome.

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 amount of hCG present in the blood or urine, may give false-negative results, while blood or protein in the urine may cause false-positive result. 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.

Home pregnancy testing is very similar to qualitative urine hCG testing in the laboratory, but there are factors surrounding its use that are important to note.

• Home pregnancy tests come with very specific instructions that must be followed explicitly. If you are using a home pregnancy test kit, always follow the instructions provided extremely carefully.
• Home pregnancy tests are sometimes performed too soon after the missed menstrual cycle (period) to result in a positive test - it takes 10 days after conception before the presence of hCG can be detected by the urine test. That is why a home test is sometimes negative when the test done by your doctor is positive - the doctor's test is often done several days later when hCG levels have become detectable.
• All urine qualitative hCG tests should, if possible, be done on a first morning urine sample. Urine becomes much more dilute after ingestion of liquids (tea, coffee, juice, water, etc.) and urine hCG concentrations may become too low to register as positive.

Generally, when used correctly, the home pregnancy test should produce the same result as the urine hCG test done by your doctor. Quantitative blood testing for hCG is more sensitive than urine hCG testing, so sometimes a blood test will indicate pregnancy when the urine test is negative. Quantitative hCG testing requires a blood sample and testing must be performed in a laboratory.