Are pregnancy tests ever wrong?

Q: My periods are always regular. I have missed my period the past two months. I have taken three pregnancy tests, but they are show a negative result. Could I still be pregnant? And if I'm not pregnant, then why are my periods not coming?

A: Urine pregnancy tests may be wrong.  Irregular menses can and should be evaluated with blood tests to determine the cause - a serum pregnancy test should be also run at that time to determine if pregnancy is the reason despite negative urine tests.