As a general rule, blood tests for pregnancy are more reliable than urine or home pregnancy tests. But they can be wrong. That means, they can show false negative or false positive results in some women. To understand why this can happen, let us understand what blood pregnancy tests are, how they work and what do their results mean.
Blood Pregnancy Tests
A pregnancy blood test or a pregnancy serum test is a test that measures the exact amount of the pregnancy hormone, human chorionic gonadotropin (hCG), in the bloodstream.
Human chorionic gonadotropin (hCG) is a hormone that is produced by the placenta of a pregnant woman. It is detectable in the blood and urine within 10 days of fertilization. There are two types of blood pregnancy tests, namely, quantitative blood test which measures the exact amount of hCG in the blood and qualitative hCG blood test which gives a simple yes or no answer to whether you are pregnant or not.
Blood tests can detect a pregnancy earlier than a home pregnancy test (about 7-12 days from possible conception) and can also measure the concentration of hCG hormone in your blood (this is useful information for your healthcare provider in tracking certain problems in pregnancy). If you suspect any symptoms of pregnancy like delayed menstrual period, breast tenderness, pelvic pain, irregular spotting or vomiting. You must get a pregnancy blood test done to confirm or rule out pregnancy.
What do the pregnancy blood test results mean?
A healthy pregnancy is expected to show a normal pattern of HCG levels over time. In a pregnant woman the level of human chorionic gonadotropin (hCG) increases throughout the first trimester, then gradually decreases over time. In the first trimester, a woman may have this test done repeatedly to see if the level rises normally. If it doesn’t, the pregnancy is often considered in trouble. After childbirth, miscarriage, or abortion, the level should quickly decrease to zero.
Wrong results with blood pregnancy tests
The test may fail to detect human chorionic gonadotropin (hCG) even when a woman is pregnant if the test is done too early. It takes at least 7 days after fertilization for the test to become positive. In most cases, by the time a woman has missed her period, the test will be positive if she is pregnant.
Greater than normal levels of human chorionic gonadotropin (hCG) may indicate:
- a tumor of the placenta with death of the fetus
- multiple pregnancy, such as twins or triplets
- ovarian cancer and other types of cancer in some cases
- a normal pregnancy
- blood or protein in the urine, which can interfere with the result
- use of medications to prevent seizures, drugs to treat Parkinson’s syndrome, or phenothiazine drugs, such as chlorpromazine, by the mother
Lower-than-normal human chorionic gonadotropin (hCG) levels may indicate:
- ectopic pregnancy
- a miscarriage, or abortion.
How reliable are blood pregnancy tests?
Home pregnancy tests are around 97% accurate when done correctly. However, a blood test is more accurate, but not necessarily more sensitive. The results depend a lot on the lab, methodology and technique of the blood test performed.
A quantitative blood test, usually called a beta human chorionic gonadotropin (hCG) test, measures the exact units of human chorionic gonadotropin (hCG) in the blood. That means it will detect even the most minimal level. There is another type of blood test sometimes called a qualitative human chorionic gonadotropin (hCG) test. This is a test that simply gives a yes or no answer to whether you are pregnant.
Just like home pregnancy tests, labs vary in what is considered a positive pregnancy test. Common cutoffs for positive blood tests for pregnancy are 5, 10, and 25 units. A level under 5 is considered negative. A blood test that is only triggered at 25 units of human chorionic gonadotropin (hCG) is not any more sensitive than several of the home pregnancy tests.