One of the most frequent questions we get here on Sexual Health Connection is from women wanting to know if they are pregnant. I addressed this issue in my last post entitled "Am I Pregnant? How to Know" A variation of this theme are women who have taken a home pregnancy test or even several of them and they turn out negative. The question we then get is: "Can I still be pregnant even though my home pregnancy test is negative?" So for this post I researched into the reasons why these tests can sometimes yield an inaccurate result and how you can up the odds for having more accurate test results.
How do Home Pregnancy Tests Work?
The National Women's Health Information Center tells us that all types of pregnancy tests attempt to detect a hormone called human chorionic gonadotropin or hCG. This "pregnancy hormone" is made when a fertilized egg implants in the uterus. It varies from woman to woman how long this implantation can take. For most women this happens about six days after the egg is fertilized by the sperm but for some women implantation can occur as late as the first day of the missed period. Each day after implantation, the amount of hCG in your system will increase rapidly.
There are two different types of pregnancy tests. One type of pregnancy test is a blood test given by your doctor. A blood test combined with a pelvic exam can be considered the most reliable way to see if you are pregnant. The second type of pregnancy test uses a urine sample and this can be performed at the doctor's office or at home using a home pregnancy test. If you use a home pregnancy test, most commonly you will be instructed to hold a stick in your urine stream where an absorbent strip will collect the urine for testing. Directions are given with each test kit to determine if you are pregnant such as seeing a plus or minus sign and now there are some tests which even show the word "pregnant" in the test results window of the test stick. So it is now easier than ever to test for pregnancy in the privacy and convenience of your own home. But are home pregnancy tests always accurate? It seems the answer depends mostly upon how you use the test.
What types of errors occur with using a Home Pregnancy Test?
There are two ways that a home pregnancy test can be inaccurate. One type of error is to have a false positive reading. This means that although your pregnancy test indicates that you are positive for pregnancy, you are not really pregnant. The other error which can occur is that you get a false negative result. This means that although your test says that you are not pregnant, you really are pregnant.
Why do these errors occur?
Most times when there is an error with home pregnancy tests it is caused by not following the directions given with the test. Bastian et al (1998) found that most false negative results are due to users not using the kits properly. If used correctly most home pregnancy tests have a high degree of reliability although some brands may be more accurate than others.
Does the brand of Home Pregnancy Test make a difference in reliability and accuracy?
Indeed the brand of test you use can impact upon the accuracy of results. A Wellsphere article entitled, "Sensitivity Chart Home Pregnancy Tests" provides us with an easy to read chart comparing the Sensitivity (or the hCG threshold at which a positive result is indicated). One brand of home pregnancy test which shows a high level of sensitivity for detecting hCG and has been cited in the literature consistently as being the most reliable and accurate test is the First Response Early Result Pregnancy Test. In 2003 Consumer Reports senior editor Nancy Metcalf talked about the best pregnancy tests on CBS's The Morning Show. Consumer Reports had tested 18 brands of home pregnancy tests and the First Response Early Result Pregnancy Test was found to be the most reliable and sensitive test kit, detecting hCG at concentrations as low as 6.5 mIU/ml. Metcalf reported that this test was super sensitive enough to detect any pregnancy soon after implantation.
Possible reasons for a False Negative (You really are pregnant but your HPT shows a negative test result):
- It is estimated that in 10% of pregnant women, the embryo does not implant until after the first day of the missed period and as late as a week after a missed period, 3% of pregnancies still hadn't implanted. So if you take the test too early your hCG levels might not be detected due to implantation occurring later than usual. (source: "The Best Pregnancy Tests" by Tatiana Morales)
- The urine is too dilute.
- The test is done too early or too late in the pregnancy. The level of hCG begins to decrease a couple of months after conception.
- The pregnancy is developing abnormally and a miscarriage is about to occur.
- Sometimes an ectopic pregnancy (a dangerous situation where the egg implants outside of the uterus, usually in the fallopian tubes) will not appear as positive on a home pregnancy test.
- Miscalculation of when your period was actually due leading to taking the test too early to detect hCG levels.
Possible Reasons for a False Positive Test result (Your pregnancy test is positive for pregnancy but you really are not pregnant):
- Some medications and drugs which reportedly can cause you to have a false positive home pregnancy test result. These drugs include: Methadone, methyldopa (used to treat hypertension), diuretics and promethazine (used to treat allergy symptoms and nausea). Some tranquilizers and antidepressants are reported to be able to cause a false positive result, as well as large doses of aspirin.
- You are taking a fertility drug which contains hCG.
- Some medical conditions such as rare cancers, trophoblastic disease, ovarian cysts, and thyroid disorders can sometimes create false-positive results.
- The egg never implants properly or else spontaneously aborts soon after implantation. It is estimated that up to a third of all conceptions implant and then fail. If you take the test too early you may be in for disappointment when your period comes despite a positive test result.
- You let the test sit too long before reading the result. If you read the test results after the instructed time period, a faint positive may appear which will be inaccurate if read at that time.
Ways to get the most accurate test results from a home pregnancy test:
- The literature is consistent with advice to take a home pregnancy test a week after your missed period. This will give most eggs a chance to implant and also your hCG levels will be more detectable.
- Use your first morning urine as it will be more concentrated and hCG will be more easily detected.
- Use a reliable and sensitive brand of home pregnancy test.
- Make sure to follow the directions given with the home pregnancy kit. Always read the results in the time frame indicated or they may be inaccurate.
- Make sure that you are not using an expired test. Check the expiration date on the box.
- Buy at least two tests. If you are doubtful or unsure of the first test results, take another test in a week.
- If you have taken two or more pregnancy tests and you still have concerns please do schedule an appointment with your doctor or gynecologist. They will be able to give you a blood test for pregnancy as well as conduct a pelvic exam to see what is going on. For peace of mind see your doctor.
I hope this post was helpful to anyone who is wondering whether or not they may be pregnant. It can be an especially anxious time. Home pregnancy tests are mostly reliable but some things can affect the accuracy of your results. The best advice I can give is to be patient. It can be difficult to wait but if you want to be more certain of your test results, wait at least a week after your period to take a test. And then check with your doctor if there is any confusion.
And now it is your turn. Have any of you taken a pregnancy test only to find that the results were not accurate? Let us know your story. We love to hear from you!
Bastian, L.A. et al (1998) Diagnostic efficiency of home pregnancy test kits. Archives of Family Medicine; 7: 5, 465-469
Carlson, K.J., Eisenstat, S.A., and Ziporyn, T. Harvard University Press (2004) Pregnancy Testing. The New Harvard Guide to Women's Health. 489-491
"Pregnancy Tests" (2006) National Women's Health Information Center, U.S. Department of Health and Human Services
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