How accurate are blood hCG tests?

I recently had a urine and blood test done in the hospital for unrelated reasons. Both came back negative. I wasn't surprised, but my husband was very disappointed. AF is currently 8 days late. I stopped tracking bbt this month because I was stressing myself out. However, I knew I ovulated about 4 or 5 days late. I don't feel AF coming on. Could the tests be wrong if they were taken 3 days ago when I was about 5 days late?