Is it possible for pregnancy tests to not work on someone even if they are pregnant?

Pregnancy tests: Tests to determine possibility of pregnancy are done on urine or blood samples. Another method is ultrasound imaging. The blood tests are very accurate and rarely are falsely positive. The urine tests we have today are somewhat sensitive, best done on a first morning urine sample and are more accurate with pregnancies that are further along,because the level of hormone(HCG) is higher.

Depends: If the test is done too early, it may be negative even though the woman is pregnant. The other issue to keep in mind the technique for the test. Follow the instructions for the test carefully and read it within the prescribed time frame. If you had sex since your last period, missed your period by 4-5 days and have any doubt, get a blood test for HCG level.