Both urine and blood pregnancy tests work by detecting human chorionic gonadotropin, or hCGm, levels. Blood tests can detect hCG earlier in pregnancy than urine tests. Blood tests can typically do this around the time of implantation, seven to nine days after fertilization or even prior to it, while urine tests take a few days longer to detect hCG. Although both tests are generally reliable and accurate, there are many reasons why either a false positive or negative result might be obtained.
Types of Blood Pregnancy Tests
According to virtualmedicalcentre.com, there are two types of blood pregnancy tests. The quantitative blood serum test, also known as the beta hCG test, measures the exact amount of hCG in the blood. It is a very accurate test and can be used to either confirm a pregnancy or to assess whether pregnancy is progressing normally. The qualitative blood serum test, on the other hand, can only confirm whether or not the pregnancy hormone, hCG, is present, and thus, whether or not a woman is pregnant or not. Its level of accuracy is about the same as that of a home urine test.
How the Test is Taken
MedlinePlus, a publication of the National Institutes of Health, states that the pregnancy blood test is done just like any other blood test. This is done by drawing a single tube of blood from a vein from the inside of the elbow or the back of the hand. The blood is then sent to a laboratory. The results for the test can be received within hours, but, if the laboratory is busy, can take up to three days.
Different Levels to Determine Pregnancy
BabyHopes.com states that it will take about 5 to 25 milli-International Units per milliliter (mIU/ml) of hCG in the blood for most blood pregnancy tests to detect pregnancy. According to a study published in "Fertility and Sterility” in 1982 by Dr. E.A. Lenton, approximately 5 mIU/ml of hCG can be found at the time of fertilization and approximately 25 mIU/ml of hCG is present when the embryo connects itself with the walls of the uterus, or when the implantation occurs. Although a pregnancy test may be able detect a low level of hCH, some laboratories may not report this. There is a considerable variation between laboratories in what they consider as a positive pregnancy test. Common cutoffs for positive blood tests for pregnancy are 5, 10, or 25 mIU/ml of hCG in the blood. It is always a good idea to ask what the level was, instead of simply asking whether the test was negative of positive.
Medications That Might Give False Positive Results
According to the website All Blood Tests, false positive results are typical after taking medication that contains hCG. Many fertility medications include this hormone, but also medications to such conditions as Parkinson's disease and seizure prevention. Phenothiazine drugs that are used to treats such conditions as schizophrenia, or hypnotics ,such as Ambien, may also increase hCG levels. Besides these drugs, a false positive result from a blood or urine pregnancy test can be received if the pregnancy has ended in miscarriage but the hCG levels are still high. Dr. T.W. Saddler states in his book "Langman's Medical Embryology," published in 2006, that approximately 30 percent of pregnancies end up in miscarriage before the zygote has even implanted.
Medications That Might Give False Negative Results
Drugs that can give false negative results are quite rare. Among them are diuretics. According to RxList, some anti-histamine medication such as Promethazine can either increase or decrease hCG levels and can give a false negative or positive results. The most common reason for a false negative results is, however, that the test was taken too early.
References
- MedlinePlus: Pregnancy test
- All Blood Tests: Pregnancy Testing False Positive Pregnancy Test
- Virtual Medical Centre: Pregnancy test
- Babyhopes: Can Blood Tests For Pregnancy Be Wrong?
- “Fertility and Sterility”; Plasma concentrations of human Chorionic Gonadptrophin from the time of implantation until the second week of pregnancy; Lenton E.A., Neal L.M. & Sulaiman R.; 1982
