Reliability of Home Pregnancy Tests: Are They Accurate or Is a False Positive or False Negative Possible?

Home pregnancy tests are a quick and convenient way to learn whether or not you're pregnant--but how reliable are they? The short answer is, for positive results, they're quite accurate  overall. For negative results, their reliability and accuracy is not as strong.

Available in pharmacies and drugstores over the counter, home pregnancy tests work by detecting the presence of hCG, also known as "the pregnancy hormone," in your urine. HCG is only around if you're pregnant. If you get a positive result on a home pregnancy test, that means it found the hormone--in which case, well, it found the hormone and you're very likely pregnant.

If you get a negative result, that means the test didn't find the pregnancy hormone. It may not have found it because you're not pregnant...but it also may not have found it because not enough hCG has built up in your body yet. A false positive result is therefore less likely when you use a home pregnancy kit than a false negative result is.

Home pregnancy tests when used properly give extremely accurate results, according to the National Women's Health and Information Center. Some tips to insure accuracy are:

• Follow the directions on the pregnancy kit. Follow both sampling instructions and timing instructions to the letter.
• Take the test in the morning, when the concentration of hCG tends to be greater.
• Don't drink anything right before taking the test, so as not to dilute the concentration of the pregnancy hormone in your urine, if present.
  
• Discard test kits that have expired dates.
• Study the test results key to make sure you're interpreting the colors and patterns on your test strip accurately.
  
• Test yourself with the home pregnancy kit at least a week after your missed period.
  

Why Should You Wait to Take a Home Pregnancy Test Till a Week After Your Missed Menstrual Cycle?

All pregnant women produce hCG (human chorionic gonadotropin) when they're pregnant. The hormone first appears in very low concentrations about 6 days after conception. Every day after this, the concentration increases, until there's so much of it that a home pregnancy test can reliably detect it.

If you do take the test earlier and get a positive result, it's probably a true positive--the test was simply sensitive enough to detect trace amounts of hCG in your urine.

Don't trust a negative result gotten from a test you took less than one week after your missed period, however. 10% of women experience delayed conception, anyway, so it's a good idea to follow a negative result with a repeated test a few days later.

If you do need to know whether you are pregnant before a home test kit can accurately tell you, then the Mayo Clinic advises that you go to your doctor and ask for a blood test to check for pregnancy.

Some home pregnancy test kits claim to be accurate and reliable when testing on the day you miss your period. They may be sensitive enough to do so, but it depends largely on what levels of pregnancy hormone you have in your urine--it varies from woman to woman.

The designs of the kits vary. They all do the same thing--try to detect the pregnancy hormone--but some use test strips, others powders, and some are more sensitive than others during early pregnancy. The First Response, Early Result Pregnancy Test, in a comparison with 17 other home pregnancy tests, was found in a 2004 study to accurately detect low hCG levels quite early on. But if you're testing for pregnancy at home after about 7 days after your missed period, most test kits are accurate if they show a positive result (pregnant).

How Common Are False Positives / False Negatives on Home Pregnancy Tests?

As long as the expiration date isn't expired and the test isn't defective, says the Mayo Clinic, you're unlikely to get a false positive result. There's a small chance that traces of some medications, protein or blood in your urine could yield a false positive, but this is unlikely. Check with your doctor if you are unsure.

False negatives do commonly occur, generally either because of taking the pregnancy test before your level of hCG are high enough to be detected or not following the timing instructions on the test. If you've missed your period and there's a chance you might be pregnant, it's best not to assume a negative result is conclusive.