Arthritis and Lyme Disease
How Is Lyme Disease Diagnosed?
Lyme disease may be difficult to diagnose because many of its symptoms mimic those of other disorders. Although a tick bite is an important clue for diagnosis, many patients cannot recall having been bitten by a tick. This is not surprising because the tick is tiny, and a tick bite is usually painless.
The easiest way for a doctor to diagnose Lyme disease is to see the unique bull's-eye rash. If there is no visible rash (as is the case in about one-fourth of those infected), the doctor might order a blood test three to four weeks after the onset of the suspected infection to look for antibodies against the bacteria. Unfortunately, the Lyme disease bacterium itself is difficult to isolate or culture from body tissues or fluids. These blood tests are:
ELISA. This blood test measures the levels of antibodies against the Lyme disease bacteria that are present in the body. Antibodies are molecules or small substances tailor-made by the immune system to lock onto and destroy specific microbial invaders.
Western blot. This blood test identifies antibodies directed against a panel of proteins found on the Lyme bacteria. The test is ordered when the ELISA result is either positive or uncertain.
The presence of antibodies, however, does not prove that the bacterium is the cause of a patient's symptoms. The presence of specific antibodies suggests a prior infection, which may or may not still be active.
Note: In the first few weeks following infection (when the rash first appears), antibody tests are not reliable because a patient's immune system has not produced enough antibodies to be detected. Antibiotics given to a patient early during infection may also prevent antibodies from reaching detectable levels, even though the Lyme disease bacterium is the cause of the patient's symptoms.
Other tests. Some patients experiencing nervous system symptoms may also undergo a spinal tap. A spinal tap is a procedure in which spinal fluid is removed from the spinal canal for the purpose of diagnosis in a laboratory. Through this procedure, doctors can detect brain and spinal cord inflammation and can look for antibodies against the Lyme disease bacterium in the spinal fluid.