The positive enzyme-linked immunosorbent assay, or ELISA, test and Western blot test indicate an infection by the bacteria that causes Lyme disease. Blood tests confirm the diagnosis of Lyme disease when the characteristic bullseye rash is absent, which occurs in approximately one-forth of infected patients, states WebMD.
The ELISA test detects antibodies to B. burgdorferi but may not return a positive result in the early stage of the disease. ELISA tests are prone to false-positive results and are not used as a singular mode of diagnosis, according to Mayo Clinic. The Western blot test is performed when a positive ELISA test is present. The Western blot test detects several proteins found on B. burgdorferi. The combination of positive ELISA and Western blot tests confirms the diagnosis of Lyme disease. A polymerase chain reaction detects bacterial DNA in joint fluid from patients affected by chronic Lyme arthritis. PCR testing is not as effective in detecting infection in blood or urine.
Testing performed in the first weeks of infection and the administration of antibiotics adversely affect the detection of antibodies with blood testing, reports WebMD. Positive blood test results indicate previous infection but cannot indicate if the infection is active at the time of the test. A spinal tap detects the presence of antibodies to B. burgdorferi in the spinal fluid. It identifies inflammation or infection in the brain and spinal cord of patients who exhibit neurological symptoms.