Pregnancy tests rely on the presence of the hormone human chorionic gonadotropin (hCG), a glycoprotein that is secreted by the placenta shortly after fertilization.
The placenta begins developing after the fertilized egg implants in a woman's uterus, which happens about six days after conception, so the earliest these tests can be used to detect pregnancy is about six days post-conception.
Wait to Take Test
Fertilization does not necessarily take place the same day as intercourse, so most women are advised to wait until they miss their period before trying a pregnancy test. Levels of hCG double about every two days in a pregnant woman, so the test increases in reliability over time
The tests work by binding the hCG hormone, from either blood or urine to an antibody and an indicator. The antibody will bind only to hCG; other hormones will not give a positive test result.
The usual indicator is a pigment molecule, present in a line across a home pregnancy urine test. Highly sensitive tests could use a fluorescent or radioactive molecule attached to the antibody, but these methods are unnecessary for an over-the-counter diagnostic test.
The tests available over-the-counter versus those obtained those at the doctor's office are the same. The primary difference is the decreased chance of user error by a trained technician.
Blood tests are equally sensitive at any time. Urine tests tend to be most sensitive using urine from early morning, which tends to be more concentrated (meaning it would have the highest levels of hCG.)
False Positives and Negatives
Most medications, including birth control pills and antibiotics, do not affect the results of pregnancy tests. Alcohol and illegal drugs do not affect the test results either.
The only drugs that can cause a false positive are those containing the pregnancy hormone hCG in them (usually used for treating infertility.) Some tissues in a non-pregnant woman can produce hCG, but the levels are normally too low to be within the detectable range of the tests.
Also, about half of all conceptions don't proceed to pregnancy, so there may be chemical "positives" for a pregnancy that won't progress.
For some urine tests, evaporation may form a line that could be interpreted as a "positive." This is why tests have a time limit for examining the results. It's untrue that urine from a man will give a positive test result.
Although the level of hCG rises over time for a pregnant woman, the quantity of hCG produced in one woman is different from the amount produced in another. This means some women may not have enough hCG in their urine or blood at six days post-conception to see a positive test result.
All tests on the market should be sensitive enough to give a highly accurate result (~97%-99%) by the time a woman misses her period.