Diagnosing a chemical pregnancy?

I was at the end of my 1st 50mg cycle of Clomid. On CD 28, my ER drew blood (1 sample) to do a beta test and a serum test. On CD 28, she called back to say I had a faint positive on my beta test. Then, on CD 29, she called back to say I had a negative on my serum test and that I had a chemical pregnancy. How could she detect a chemical pregnancy using 1 sample. From the few websites I read, it sounds like she would have needed to compare blood samples from 2 different days to detect a chemical pregnancy...