Home pregnancy tests are designed to detect the presence of hCG, the hormone manufactured by the blatocyst (the name for the hollow clump of cells resulting from the meeting of sperm and egg) following implantation. Enough hCG is present in the urine to allow a pregnancy to be confirmed as soon as two weeks after conception, although it takes some pregnant women a little longer to test positive.
Although home pregnancy tests are proven to be about 97% accurate, false positives and false negatives can occur. The accuracy of home pregnancy tests varies because:
- The days of a woman’s menstrual cycle and ovulation can vary each month.
- The exact day of implantation of the fertilized egg can vary.
- Each home pregnancy test kit has a different sensitivity to detect hCG.
There are three basic types of home pregnancy tests.
- Dipstick or test strip. The most common types of home pregnancy tests that you either hold in the urine stream or dip into a sample of urine.
- Collection cup test. This test uses a urine collection cup or well with a testing device. To use this type of test, you place several drops of urine into a well in the testing device or you immerse the well into urine that has been collected in a cup.
- Chemical mixing test. A less common type of test which involves mixing a sample of urine in several tubes or cups with a powder or liquid to produce a chemical reaction. The presence of hCG causes a chemical reaction that produces a color change.
For any home diagnostic test, you should follow some general guidelines:
- Check the expiration date on the package. Do not use a test kit after its expiration date.
- Store the test kits as directed. Many kits need to be stored in a refrigerator or cool place.
- Read the instructions that come with your test carefully and thoroughly before doing the test. Look for any special preparations you need to take before doing the test, such as avoiding certain foods or limiting your physical activity.
- Follow the directions exactly. Do all the steps, in order, without skipping any of them.
- If a step in the test needs to be timed, use a clock. Do not guess at the timing.
- It’s best to use your first morning urine (it has a higher concentration of hCG than the urine you pass later in the day). An alternative is to use urine that has been in the bladder for at least 4 hours. Usually you have to perform the test within 15 minutes after collecting the sample.
- If you use a test that requires that you collect a sample rather than test your urine while you urinate, be sure to use a clean, soap-free container.
- Many home kits require that you be able to see color changes on a test strip. If you are color blind or have trouble distinguishing colors, have someone else read the test results for you.
- Record the results of the test so you can discuss them with your health professional.
When to check
Home pregnancy kits can be used on the first day of a missed menstrual period. However, the test results are more accurate if you wait a few days longer.
Negative. What you should do is repeat the test in one week if your menstrual period has not started. If it is still negative, consult your doctor why you aren’t having your period yet.
Also consider a false-negative result. This means the test shows a negative result, while actually you are pregnant and it should be positive. It may be caused when:
- It is a very early pregnancy and the hCG level is too low, not detected by the home pregnancy test kit.
- Using urine produced after drinking a lot of liquid (dilute urine)
Positive. What you should do is consult your doctor to confirm your test and arrange follow-up care.
Also consider a false-positive result. This means the test shows a positive result, while actually you are not pregnant and it should be negative. Some might be due:
- Detergent or soap residue in the urine collection cup.
- Reading the test at the wrong time.
- Exposing the test equipment to heat, sunlight, or vibration
- Having excess protein or blood in the urine, which could be from a urinary tract infection.
- Having hCG hormone in the urine from some other cause (such as a very rare type of ovarian or placental cancer).
- You are taking certain drugs, especially fertility medications that contain human chorionic gonadotropin.
Your doctor would perform further evaluation if it is suspected to be a false-positive result.