When I began practicing many years ago, the urine pregnancy tests we used were inaccurate until 6-8 weeks of pregnancy. Now they become positive 1 week after conception, even before the first missed period (conception occurs about 2 weeks after the last normal period, if periods occur regularly every 28-30 days). I assume this is true for all brands purchased at pharmacies or supermarkets, etc., but unfortunately, I don’t know for sure.
Take into account that all types of urine tests may be inaccurate if the urine is very dilute (false-negative). In medical settings, we determine that by a very simple test for specific gravity; anything under 1.005 is too dilute to be sure (normal urine is around 1.010 – 1.020; pure water is 1.000). If urine looks darkish, it’s probably fine, but if it looks very pale, almost like water, don’t trust the test without checking the specific gravity.
Also, some people don’t follow directions well, including hurried health care professionals. I’ve seen people call a test “negative” without waiting the full period of time called for in the instructions. There are blood pregnancy tests which are much more accurate; in an ER they can be done promptly, but in the office or clinic setting results won’t be back for a few days. So if that’s where we work, they rarely help us.
On the other hand, there’s a very rare phenomenon called the “prozone effect” or “hook effect.” In this case, there is so much of the HCG hormone of pregnancy being produced that it interferes with the test itself. To rule this out, the urine should be purposely diluted. We would only suspect this if pregnancy seems very likely, but the urine test remains negative. This phenomenon might be suspected if a woman has severe nausea & vomiting, and has missed periods, but the pregnancy test is negative.