system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Home pregnancy tests vs. hospital or Doctor administered pregnancy tests.

What's the difference and what are the similarities between a take home test (pee stick) and the test that is administered by a medical professional? Additionally, how does this test work? Specifically, I always hear urine is sterile, but how can this be so if I can tell whether you're pregnant or not through your urine. Lastly, how accurate are the two different methods particularly when compared to each other?
gpt
Firstly, urine is not really sterile. This is a common misconception. There are bacteria in urine, even in the bladder. Additionally, going through the urethra/along the skin will add even more bacteria.

However this has nothing really to do with how pregnancy is detected via the urine. When a woman becomes a pregnant, she starts producing certain hormones (hCG). These hormones will turn up in her blood (which is how you can detect pregnancy via a blood test) and they will show up in her urine (which is how you can detect pregnancy via a stick test). 

Blood tests are more sensitive. There is also two different kinds of blood test, one that measures the exact level of hCG in your blood (useful for when you are trying to see if a pregnancy is progressing properly early on) and one that detect whether hCG is present in your urine at all. The downside is that blood tests are more expensive and take longer to get the result.

Urine tests can be taken pretty much anywhere, will give you the result quickly, and are pretty damn accurate (97% IIRC) regardless whether you get a cheap dollar store test or a fancy expensive one. However, because urine tests well only detect pregnancy once a certain threshold of hCG in your blood has been reached, you cannot test with a urine test as early as a blood test. A blood test can generally tell if you are pregnant from 7-12 days after conception, while with a urine test, the best results don't occur until 14 days after conception.