A blood pregnancy test can determine if an unintended pregnancy has occurred due to contraception failure by examining whether the pregnancy hormone, human chorionic gonadotropin (hCG), is present. These tests will analyze the blood to try to detect the presence of the hcG hormone.
Doctors use two types of blood pregnancy tests. Blood tests can detect hCG earlier in a pregnancy than urine or home pregnancy tests can. These tests can confirm a pregnancy about 8 to 10 days after ovulation. Blood pregnancy tests will yield a positive result if they detect 5 mIU (milli-International Units per milliliter) of HCG in the blood. This level is usually seen shortly after conception.
Quantitative Blood Serum Test:
This test, also known as the beta hCG test, can measure the exact amount of hCG in the blood. Because it can detect even trace amounts of hCG, this is a very accurate test. HCG levels generally double approximately every 2 days for the first four weeks of pregnancy. A quantitative blood test can be helpful in confirming a pregnancy and can help assess whether or not a pregnancy is progressing normally.
Qualitative Blood Serum Test:
This test can only confirm whether or not the pregnancy hormone, hCG, is present and, therefore, whether a woman is pregnant or not. The qualitative hCG blood test tends to be about as accurate as a home urine test.
How the Blood Pregnancy Test is Performed:
This test is completed just like a typical blood test. The puncture site (most likely the forearm or the back of the hand) is cleaned with an antiseptic. A tourniquet will be placed around the upper arm to apply pressure. A needle will then be inserted, and the blood will be collected in an airtight vial or a syringe. Unless the medical office has an in-house lab, the blood sample will be sent out to a lab to be analyzed.
Blood Test Results:
Blood tests performed at a doctor’s office tend to have a 98 to 99 percent accuracy rate. These tests can be conducted seven days after you ovulate (which is about a week before your period is due) and still provide accurate results.
As with urine pregnancy tests, it is possible to end up with false results (both negative and positive) from a blood pregnancy test.
- False negative results (test is negative, but you are actually pregnant) usually occur if the test was performed too early. This is because there may not be enough hCG in the blood to detect a pregnancy.
- False positive results (test is positive, but you are not pregnant) may appear if you are taking a medication that contains hCG.
Medications That Can Affect Blood Pregnancy Test Results:
Medications that can decrease hCG measurements include:
- Promethazine (used as an antihistamine in combination cough and cold products to treat allergy symptoms and to treat nausea or vomiting from illness or motion sickness)
- Promethazine (can increase or decrease hCG levels)
- Hypnotics (like Ambien)
- Medications to prevent seizures
- Drugs to treat Parkinson's disease
- Prescription medicates that contain hCG, (Pregnyl, A.P.L., Profasi, Chorex, Novarel, or Ovidrel)
- Phenothiazine drugs, such as Chlorpromazine (Thorazine)
Disadvantages of Blood Pregnancy Tests:
It can take longer to receive results from a blood test as compared to a urine test. The time it takes to receive the results from a pregnancy blood test varies from lab to lab and can range from an hour to a few days.
These tests must also be performed at a medical office, so they take more time out of one's schedule and are more expensive than home pregnancy tests (with the price varying based on doctor’s and lab fees).
Possible Risks and Considerations Regarding Blood Pregnancy Tests:
Just as with any blood test, there is always the risk of feeling lightheaded, fainting, excessive bleeding, infection at the puncture site where the skin is broken, and/or hematoma (blood accumulating under the skin).
Also, since veins and arteries differ in size from one person to another and from one side of the body to the other, obtaining a blood sample could be more difficult for some people than for others. To acquire the blood sample needed for this test, it may require multiple pricks to locate a vein.