Testing for Lyme Disease can be problematic at times. Many people with active Lyme Disease will be seronegative (i.e., have a negative blood test). There are many different theories for this. One test that is universally recommended by the ILADS (International Lyme And Related DIseases Society) is the Western Blot test. The preferred lab in the US for this test is Igenex.

This link helps explain how to interpret the results of the Western Blot test for Lyme Disease.
http://www.anapsid.org/lyme/wb.html

It is recommended that Lyme Disease be diagnosed by a Lyme Literate Doctor or a doctor who is Lyme-Aware. Lyme Disease is ultimately a clinical diagnosis.