Human chorionic gonadotrophin (hCG) Test
Most pregnancy tests can be carried out from the first day of a missed period. Some sensitive pregnancy tests can be done even before you miss a period, from as early as 8 days after conception. However if you test too soon you may get a false negative test result. If you have a negative test but think you might be pregnant, you can repeat the test after a few days. To be confident that a negative result is correct, wait at least 21 days after you last had unprotected sex.
A doctor or nurse may request a pregnancy test if they think that your symptoms suggest ectopic pregnancy, a miscarriage, trophoblastic disease or germ cell tumours.
Human chorionic gonadotrophin (hCG) is also tested as part of the prenatal screening program for Down’s syndrome.
Either a urine sample or a blood sample taken from a vein in the arm.
Human chorionic gonadotrophin (hCG) is a hormone produced in the placenta of a pregnant woman, , which starts to be produced around 6 days after fertilisation. A pregnancy test is a specific blood or urine test that can detect hCG and confirm or exclude pregnancy.
During the early weeks of pregnancy, hCG is important in maintaining function of the corpus luteum (which is formed from the ruptured ovarian follicle following ovulation). Production of hCG increases steadily during the first trimester of a normal pregnancy, peaking around the 10th week after the last menstrual cycle. Concentrations then fall slowly during the remainder of the pregnancy.
hCG is no longer detectable within a few weeks after delivery. hCG is also produced by some germ cell tumours and increased levels are seen in trophoblastic disease.
How is the sample collected for testing?
hCG is most commonly measured in urine, for confirmation or exclusion of pregnancy. The preferred specimen is a random urine sample, which can be collected any time of day. Hormone levels are highest first thing in the morning so in early pregnancy it is usually best to test urine collected first thing in the morning to reduce the chance of false negative test results. hCG can also be measured in blood taken from a vein in the arm.
How is it used?
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.
When is it requested?
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.
What does the test result mean?
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.
Is there anything else I should know?
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.
How does the test that I do at home myself compare with the results of a test done in a lab?
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.
What is an ectopic pregnancy?
An ectopic pregnancy is the condition when the fertilised ovum (blastocyst) implants somewhere other than in the uterus. This is a serious condition needing immediate treatment. Patients with ectopic pregnancies often have abdominal pain and uterine bleeding. Usually, abnormal levels of hCG are produced in ectopic pregnancies with slower-than-normal rates of increase.
What is gestational trophoblastic disease?
Gestational trophoblastic disease refers to a group of disorders that can affect a woman’s uterus. The disorders include benign tumours, molar pregnancies and malignant choriocarcinoma. The tumours start in the trophoblast cells (cells that form the placenta in a normal pregnancy). Blood hCG tests can be used to help to diagnose and monitor gestational trophoblastic disease.
When is a blood hCG test requested instead of a urine hCG?
The blood hCG test actually measures the amount of hCG present in the bloodstream; the urine hCG is used to detect the presence of hCG. Since hCG is not normally detected in the urine of a non-pregnant woman, the urine hCG test is usually enough to confirm a pregnancy. Sometimes, it is important to know how much hCG is present: to evaluate a suspected ectopic pregnancy, to monitor a woman following a miscarriage, to monitor the treatment of a tumour or to detect recurrence. In these circumstances, your doctor will request a blood hCG test.
How many days after a miscarriage would it take for a urine pregnancy test to show a negative result?
Urine hCG disappears at about the same rate as blood hCG, which can take anywhere from 9 to 35 days, with a median of 19 days. However, the timeframe for when an hCG result will be negative is dependent on what the hCG level was at the time of the miscarriage. Frequently miscarriages are monitored with quantitative blood hCG testing. If the levels of hCG do not fall to undetectable levels, some hCG-producing tissue may remain and have to be removed.