hCG pregnancy test

How is it used?

Qualitative hCG testing detects the presence of hCG and is routinely used to screen for a pregnancy. This test may be performed by a laboratory, at a doctor's office, or at home using a home pregnancy test kit. Methods will vary slightly but for most, a test strip is dipped into a collected cup of urine or exposed to a woman's urine stream. A colored line (or other color change) appears within the time allotted per instructions, usually about 5 minutes. For accurate test results, it is important to carefully follow the test directions. (See the article on Home Testing: Avoiding Errors for more on this.) If the test is negative, it is often repeated several days later. Since hCG rises rapidly, an initial negative test can turn positive within this time period.

Quantitative hCG testing, often called beta hCG (β-hCG), measures the amount of hCG present in the blood. It may be used to confirm a pregnancy. It may also be used, along with a progesterone test, to help diagnose an ectopic pregnancy, to help diagnose and monitor a pregnancy that may be failing, and/or to monitor a woman after a miscarriage.

hCG blood measurements may also be used, along with a few other tests, as part of screening for fetal abnormalities. For more information on this use, see First Trimester Down Syndrome Screen or Second Trimester Maternal Serum Screening.

Occasionally, an hCG test is used to screen for pregnancy if a woman is to undergo a medical treatment, be placed on certain drugs, or have other testing, such as x-rays, that might harm the developing baby. This is usually done to help confirm that the woman is not pregnant. It has become standard practice at most institutions to screen all female patients for pregnancy using a urine or blood hCG test before a medical intervention, such as an operation, that could potentially harm a fetus.

When is it ordered?

For confirming pregnancy, the timing of testing depends on how accurate a woman is about the day she expects her menstrual period as well as the method used for testing. In general, blood tests are more sensitive than urine tests and can be done two days before a woman would expect her period to start. A urine or blood hCG test can be done reliably by 10 days after a missed menstrual period. Even using a urine test, a woman may be able to determine whether she is pregnant the day she misses her period, but the result could be falsely negative. Testing may be repeated at a later date if the first test is negative but pregnancy is still suspected.

Quantitative blood hCG tests may be ordered over several days when a health practitioner wants to identify or rule out an ectopic pregnancy or to monitor a woman after a miscarriage. In these cases, a woman may experience the normal signs and symptoms of pregnancy at first but then may develop others that indicate that the pregnancy is not progressing as expected.

Some signs and symptoms of ectopic pregnancy include:

• Abnormal vaginal bleeding–because a woman is pregnant, she may not have a regular period but then may have light bleeding or spotting with an ectopic pregnancy
• Low back pain
• Pain or cramping in the lower abdomen or on one side of the pelvis

If untreated, signs and symptoms may get worse and may include:

• Dizziness, weakness
• Feeling faint or fainting
• Low blood pressure
• Pain in the shoulder area
• Sudden, sharp pain in the pelvic area
• Fever, flu-like symptoms
• Vomiting

The area around an ectopic pregnancy may rupture and start to bleed, and, if undiagnosed, can lead to cardiac arrest and death.

An hCG test may be ordered prior to a medical procedure or treatment that might be harmful during pregnancy.

What does the test result mean?

A negative hCG result means that it is unlikely that a woman is pregnant. However, tests performed too early in a pregnancy, before there is a significant hCG level, may give false-negative results. The test may be repeated a few days later if there is a strong possibility of pregnancy.

A positive hCG means that a woman is likely pregnant.

The blood level of hcG in a woman with an ectopic pregnancy usually rises at a slower rate than normal. Typically, hCG levels double about every two days for the first four weeks of a normal pregnancy, then slow to every 31/2 days by six weeks. Those with failing pregnancies will also frequently have a longer doubling time early on or may even show falling hCG concentrations during the doubling period. 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 (dilation and curettage - D&C).

Is there anything else I should know?

Blood or protein in the urine may cause false-positive pregnancy results. Urine hCG tests may give a false-negative result if the urine is too diluted or if testing is done too soon in the pregnancy.

Certain drugs such as diuretics and promethazine (an antihistamine) may cause false-negative urine results. Other drugs such as anti-convulsants, anti-parkinson drugs, hypnotics, and tranquilizers may cause false-positive results. The presence of protein in the urine (proteinuria), blood in the urine (hematuria), or excess pituitary gonadotropin may also cause a false positive.

There are reports of false-positive blood hCG results due to the presence of certain types of antibodies that some individuals produce or fragments of the hCG molecule. Generally, if results are questionable, they may be confirmed by testing with a different method.

What is being tested?

Human chorionic gonadotropin (hCG) is a hormone produced by the placenta of a pregnant woman. Early in pregnancy, the level of hCG increases in the blood and is eliminated in the urine. A pregnancy test detects hCG in the blood or urine and confirms or rules out pregnancy.

During the early weeks of pregnancy, hCG is important in maintaining function of the corpus luteum. Production of hCG increases steadily during the first trimester (8-10 weeks) of a normal pregnancy, peaking around the 10th week after the last menstrual cycle. Levels then fall slowly during the remainder of the pregnancy. hCG is no longer detectable within a few weeks after delivery.

When a pregnancy occurs outside of the uterus (ectopic), the level of hCG in the blood increases at a slower rate. When an ectopic pregnancy is suspected, measuring the level of hCG in the blood (quantitative test) over time may be useful in helping to make a diagnosis of ectopic pregnancy.

Similarly, the hCG blood level may be abnormal when the developing baby (fetus) has a chromosome defect such as Down syndrome. An hCG test is used routinely in conjunction with a few other tests as part of screening for fetal abnormalities. (See First Trimester Down Syndrome Screen or Second Trimester Maternal Serum Screening for more on this.)

How is the sample collected for testing?

Preferably, a urine sample is collected first thing in the morning or a blood sample is drawn from a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. However, do not drink large amounts of fluid before collecting a urine sample for a pregnancy test. This is because overly dilute urine may result in a false negative.