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Well, there's so many factors that involve with pregnancy tests.

First, optimal time. According to most instructions, the best time to test is first thing in the morning when the HcG hormone is the most concentrated. So, testing an hour after taking a previous test is not the best way to get a substanial and accurate results. The key is following the instructions perfectly.

Also, are you testing before the first possible missed period? The best time to test is a week after the first missed period day.

It is more likely to get a false negative than a false positive.

With false positives, the options are:
(which I just asked this question recently and got this answer)
You can only acheive a false positive if:

1. It's an ectopic pregnancy
2. It's a chemical pregnancy (early miscarriage)
- BOTH of which are ACTUALLY pregnancies, just not the kind that you want

3. You are taking fertility drugs that introduce HcG to your system
4. You have some kind of weird freakshow medical problem like tumors that make teeth and hair and such.

Lunashock did a very good job of explaining this. If you get a positive, go see your doctor right away for a double check. You need to get on a good diet very early on for your child to develope a healthy spine/brain. So the earlier you know, the better it is for both mother and child.

If you get a negative but are still worried, wait a little bit (the next day early in the morning as luna suggested, or a week later, etc...). If you are still really worried, make an appointment with your doctor. Better safe than sorry.

well one of the tests could have been defective

Missed period is usually the earliest sign of pregnancy, can't do much better then that.

As for the tests, it's not going to read that it found HcG if there was none and you don't make it unless you are/were pregnant. If we made it at random then otc tests--well any tests as they look for the same thing, blood or urine, blood tests are just much more precise and can detect smaller amounts of hormone--would be ineffectual and useless. The tests work by reacting with the hormone, so no hormone, no positive result...or not enough hormone to register a positive. You are much more likely to get a false negative. "False" positives, as it's been mentioned, usually mean ectopic pregnancies or you were pregnant and your body already miscarried it, leaving traces of the hormone behind.

You could just go in and have them do a urine test for you as well. [Different clinics have different guidelines in regards to the use of blood tests to determine pregnancy.] It's basically the same as the OTC one but it eliminates operator error from the equation as lab techs process it instead.

I really don't think there's much else you can do at this point besides go in and get the results checked by a clinic, or else wait the two weeks for your period. Which may seem like a long time, but it's about all you can do.