Chemical Pregnancy?!

Dawn

If i had a chemical pregnancy, how long after the start of af would i no longer be able to detect a positive pregnancy by blood test?

I had a faint positive test last Wednesday and got AF Thursday morning and had blood work done midday on Friday, which came back negative. Just wondering if it was a false positive or possible chemical pregnancy. I had a heavier period and cramped for 2 1/2 days rather than just morning of.