How Soon (The short Answer):
The best case scenario, is 6 days after possible conception.
How Soon (The long answer):
The hCG Hormone
All pregnancy tests, test for the existence of the hormone hCG (human chorionic gonadotrophin). This hormone is the key to knowing how soon pregnancy can be detected. It will start to be present in your blood and urine after the fertilized egg implants itself into the wall of the uterus. The length of time between this, and the fertilization of the egg, is different from woman to woman, but normally is 6 to 12 days. The levels of hCG in the blood and urine start doubling roughly each day around this time.
A blood test can test levels of hCG as low as 1mlU/mL. There are urine tests that can detect levels as low as 5mlU/mL. It’s important to check the pack before buying so you understand what the limits are.
Be aware that when taking a blood test, you may not get an immediate result. This differs from practice to practice.
So The Answer:
So the short answer above of 6 days, is the “best case scenario” of the egg attaching itself to the uterus, and, a good urine test (one capable of detecting levels of hCG as low as 5mlU/mL).
It’s also important to know that a positive test means you are almost certainly pregnant. A false test may be inaccurate depending on the factors explained above. If you have had unprotected sex in the last 6 days, and have gotten a false reading from the urine test, you now understand that the egg may not have implanted itself to your ovaries yet, or, the levels of hCG are too low, and a test tomorrow, or the next day, are a good idea!