Home pregnancy tests on the market today vary in sensitivity. Some can detect HCG one day after the missed period. Others require one to two weeks. Some tests must be done on a urine specimen obtained in the morning, when the concentration of HCG is the highest; others can be performed on a specimen collected anytime. Some tests react within one minute, but others require one to two hours. If you follow the directions carefully, the results are 90 to 95 percent accurate.
The tests are easy to perform. You immerse a trip of paper into a collected urine sample, or you urinate on a strip of paper. Positive tests usually are indicated by the formation of a lone or a plus sign on the paper. Now, there are even digital tests that display the results in a window so that there is no need to interpret lines or plus signs.
Even though these tests are extremely sensitive and accurate, it is possible to receive a false-positive or false-negative result. If you are taking fertility drugs, they may affect the outcome of the test and produce a false-positive result -- that is, the test result is positive even though the woman is not pregnant. However, more common than a false-positive test result is a false-negative one -- that is, the test result is negative even though the woman is pregnant. This usually occurs when the test is done too early after the missed period.
In some cases, the level of HCG is too low to detect, or the embryo hasn't become implanted yet. In approximately ten percent of women, the embryo implants after the first day of the missed period. Low levels of HCG may also be caused by an ectopic pregnancy (a pregnancy that develops outside the uterus). If the first test result is negative and your period doesn't start, repeat the test in five to ten days. If it is still negative, and you believe you may be pregnant, consult your doctor.
Your doctor will probably order a urine pregnancy test similar to the home pregnancy tests. If your doctor needs to know if you are pregnant at a time too early for the urine test to be used or if he or she suspects a false-negative test result, your doctor may order a blood test that is more sensitive and specific. Because it can measure very small amounts of HCG, your doctor can use it to diagnose pregnancy before a missed period (seven to nine days after fertilization) or to diagnose a tubal pregnancy (one that develops in one of the fallopian tubes). The test takes longer (two to three hours) to complete, and it is more expensive because it requires special equipment and personnel. Therefore, it is not used routinely to diagnose pregnancy.