Antibodies are substances that the body itself produces in response to an infection.
Well, in the case of the severe lyme infection, the quantity of infecting spirochetes is so high, there are not enough antibodies to go around, leaving none of them available to be tested for in the common blood tests. This test seeks to find the specific DNA of the lyme organism. A positive result in this test is assuredly a confirmation that there is the presence of the lyme spirochete. There's just too little organism to be found. The lyme diagnosis is still a clinical one.