Q: How accurate are home pregnancy tests compared to a blood test?
Answer from Peter Roberts, M.D., medical director of obstetrics and gynecology at Providence Portland Medical Center: Home pregnancy tests have improved and, if done at the proper time and according to directions, are now essentially as reliable as a blood test. A blood test, however, will generally give an accurate result earlier than a urine test.
How soon is that? If you're pregnant, a blood test can usually detect it seven to 10 days after conception, which is defined as the moment when the sperm attaches to the egg. Home tests may take up to four days longer to detect a pregnancy.
Some home tests advertise themselves as giving early results, but check the fine print on the box. You may see that their reliability increases greatly on or after the date you expected your period, which might be 12 to 14 days after conception.
hCG: the miracle messenger Both blood and urine tests detect pregnancy by looking for a hormone called hCG, or human chorionic gonadotropin, which is produced only when a woman is pregnant. As soon as the fertilized egg attaches to the wall of the uterus, the developing placenta starts pumping hCG into the blood. The hormone is a vital part of the machinery that enables the body to switch gears to develop a fetus: It signals the ovaries to keep making estrogen and progesterone, which are needed to stop menstruation and maintain the pregnancy.
The amount of hCG in your blood and urine doubles about every two days during the early weeks of pregnancy, and it doesn’t take long before the levels are high enough to be detected by a pregnancy test.
Blood tests, which are more sensitive to hCG, can detect as little as 5 to 10 mIU/mL (milli International Units per milli Liter). Home tests, depending on the brand, require a higher concentration – anywhere from 20 units to 100 units of hCG – to show a positive reading.
Home test how to's Research has shown that the accuracy of home tests very much depends on following instructions. Here are some pointers.
Start by reading the package. Make sure the product has not expired. Store it at the suggested temperature range. Check if the manufacturer mentions medications that interfere with the test results. If you are taking one of those, call your doctor for advice.
Read the leaflet. Follow the instructions to the letter.
For best results, administer the test first thing in the morning, when the hCG will be more concentrated in the urine. If that isn't possible, use urine that has been in your bladder for at least four hours.
Don't chug extra liquids before your urine test – that can dilute the concentration of hCG.
Everyone's body is a bit different, and the production of hCG early in pregnancy can vary a bit. If your test result is negative, but you continue feeling symptoms of pregnancy, repeat the test several days later. (Some manufacturers sell two-packs of the home tests.) If still in doubt, ask your doctor for a blood test or exam.
Plan and prepare before you test positive I always like to remind women that it's best to prepare for pregnancy before your test reads positive. Start taking prenatal vitamins a few months before you start trying to conceive, or as soon as you think you could be pregnant. Among other things, the vitamins contain folic acid, a nutrient that greatly helps prevent certain birth defects.
Finally, talk with your physician about other aspects of your health that could have an impact on your pregnancy and your developing baby, such as your weight, your exercise habits, the medicines you use and the amount of alcohol you consume.
