If a urine pregnancy test showed a positive result and later, a blood pregnancy test gave a negative result, there are several possible explanations. First, and most likely, the woman was pregnant and the pregnancy ended in early miscarriage. It is also possible that the first test was a false positive test. According to Rxlist.com, some medications contain human chorionic gonadotropin, or hCG hormone and might give you a false positive result. It is also possible that the second test was a false negative. This is the least unlikely explanation, but it is possible to get a negative result when a woman is pregnant.
How Do Pregnancy Tests Work?
Pregnancy tests can use either urine or blood. Virtualmedicalcenter.com states that they both work by detecting the presence of the hormone human chorionic gonadotropin, or hCG. If done correctly, both of these methods are very reliable. According to BabyHopes.com, urine pregnancy tests are 97 percent accurate, and a quantitative pregnancy blood test can be 99.5 to 100 percent accurate, according to the virtualmedicalcentre.com. Both of these tests have their problems, however, and both tests could produce false positive and false negative results.
Blood and Urine Tests
One of the key differences between blood and urine pregnancy tests is the earliest time they can be used to test for pregnancy. Blood tests can be used as early as the time of fertilization, but are generally considered more accurate seven to nine days after ovulation, when the embryo implants, or attaches itself to the walls of the uterus. BabyHopes.com states that it takes about 5 to 25 milli-International Units per milliliter (mIU/ml) of hCG in the blood for most blood pregnancy tests to detect pregnancy. Urine pregnancy tests can take a few days longer to detect pregnancy. They are typically not recommended until a woman misses her period, although some of them can detect a pregnancy a few days before it. They usually require approximately 25 mIU/ml of hCG in the blood to detect pregnancy, according to BabyHopes.com.
False Positive
Taking medication that contain hCG can result in false positive readings in pregnancy tests, says allbloodtest.com. According to RxList, many fertility medicines contain this hormone but there are other medicines such as those used for conditions like Parkinson’s, seizures, schizophrenia and insomnia that might cause a false positive result. Ovarian cancer especially, and some other types of cancer, can cause false positive results, according to BabyHopes.com
False Negative
The most common reason to get a false negative reading from a pregnancy test is that the test has been taken too early. Yet, in this case, the first pregnancy test was positive so the second pregnancy test could not have been taken too early. A handful of drugs can cause a false negative reading. RxList reports that certain antihistamine-based drugs, such as Promethazine, can both increase and decrease hCG levels in those who use them. Diuretics can give false negative results in some cases.
There is great variance between laboratories in the hCG levels that they require to report that a woman is pregnant. Therefore, ask what the hCG level was instead of asking whether a woman is pregnant or not. Common cutoffs for positive blood tests for pregnancy are 5, 10, or 25 mIU/ml of hCG in the blood.
Miscarriage
Unfortunately, if a positive result was received from a urine pregnancy test, but the blood test came back as negative, chances are that a woman had an early miscarriage around the time the first pregnancy test was taken. In his book "Langman's Medical Embryology," Dr. T. W. Saddler explains that roughly 30 percent of pregnancies end up in miscarriages before the embryo has had the chance to attach itself to the walls of the uterus. These finding are supported by the fact that typically blood testing is more accurate than urine tests. It takes some time for the hCG levels to drop in the blood, so a pregnancy test might give a positive result even after a woman has already miscarried. To be on the safe side, after receiving contradictory results from different pregnancy tests, repeat the test after one week to verify the results.
References
- RxList: Phenergan
- MedlinePlus: Pregnancy Test
- Virtual Medical Centre: Pregnancy Test
- Baby Hopes: Is a Blood Test for Pregnancy More Accurate Than a Urine Test?
- "Langman's Medical Embryology”; T. W. Saddler; 2006.
