Are home pregnancy tests accurate?
Home pregnancy tests work by measuring HCG (human chorionic gonadotropic, the pregnancy hormone) in the urine. Unlike blood tests for HCG, which measure the actual amount of hormone, home pregnancy tests just register the presence or absence of HCG.
A positive home pregnancy test is quite accurate because the test will not register the presence of HCG unless it is actually there. If your test is positive, you are pregnant.
A negative test is far less accurate for several reasons. Each brand of test has a lower limit below which it cannot measure HCG. For most tests, the lower limit is 25. This level is reached at about 14 days after conception, which is usually the date on which you expect your period. Therefore, most pregnancy tests cannot pick up a pregnancy until the day you miss your period. If you test before then, and you get a negative result, it is meaningless, since you may have tested before the HCG level was high enough to detect.
Many women have irregular periods and they don’t know when they could have conceived. Therefore, it is impossible to identify the day when a pregnancy test will turn positive if they are indeed pregnant. Again, a negative test may be wrong because it was done too soon. In a normal pregnancy, the HCG level doubles approximately every 48 hours. If you have a negative test, but suspect that you are pregnant, you can test again in 2-7 days. If you are pregnant, the test should turn positive in that time.
Finally, not all tests meet the sensitivity levels that are claimed by the brand. So while the average pregnancy test of a brand that claims it can detect HCG as low as 25 may be able to do so, individual tests may not reach the same level of quality and may not register positive until a higher level of HCG is reached. An individual test stick may be negative, even though another test stick will be positive because the HCG level really is 25 or above.
All these caveats apply only in very early pregnancy. Once the HCG is level is above 100 (usually within 3 weeks after conception) any pregnancy test will turn positive.
Finally, it is important to remember that a home test only measures the presence or absence of HCG. It cannot tell you whether a pregnancy is healthy or whether you have had a miscarriage. You can have a miscarriage and your pregnancy test will remain positive for days or even weeks. That’s because it will take that long for the HCG level to drop back under 25.
The bottom line: A positive result means that you are pregnant. A negative result is only valid as long as you have not tested too soon.