Laboratory testing for Borrelia Burgdorferi infection supports a physician's clinical judgment about the diagnosis of Lyme disease in the context of epidemiological and clinical facts of the case. Direct demonstration of Borrelia Burgdorferi or its DNA in clinical samples by cultivation or polymerase chain reaction is desirable but is still expensive and lacks clinical sensitivity. Detection of antibodies to Borrelia Burgdorferi in a two-tier approach remains the most practical and common for laboratory testing for suspected Lyme disease. In as much as serologic assays fall short of 100% sensitivity and specificity, testing is recommended only when there is at least 20% a priori probability that the patient has the disease.