Pregnancy and the Depo Shot

Patient: I am on the Depo vera shot. I thought I might have been pregnant. The last time I had sex was Dec 24, 2013. I have taken several home pregnancy test and had a blood test done at the Dr office all were negative. I have been on the Depo shot for almost 5 years. I have never been late or missed a shot. I took another HPT test April 1 and it was also negative. Could I be pregnant and all the tests missed it. How effective is the Depo shot?