90% to 95% of patients In areas endemic for Lyme disease, serum for Lyme titers (IgM and IgG; two-tiered testing) should be obtained Plain radiographs may demonstrate signs of an effusion ranging from subtle blurring or displacement of fascial planes to complete dislocation of the joint The main role of the radiograph in the evaluation is to exclude fractures or other bony abnormalities that may mimic septic arthritis Ultrasound is most useful in evaluating the hip It is much more sensitive than the radiograph in detecting a joint effusion Some have suggested that the absence of an effusion on an ultrasound scan effectively excludes the diagnosis of septic arthritis The ultrasound cannot, however, distinguish between infected and sterile inflammatory effusions Ultrasound guidance is useful in performance of a needle aspiration of the hip joint MRI is not routinely indicated but can be useful when there is concern for concomitant osteomyelitis The isolation of a bacterial pathogen provides a definitive diagnosis and directs subsequent management Cultures of joint fluid and blood should therefore be performed on all patients with a possible septic joint The yield of organisms can be increased by directly inoculating joint fluid into a blood culture bottle When indicated, cultures from additional sites should be obtained to increase the potential isolation of a pathogen Cultures of the joint fluid demonstrate the highest yield and are positive in 50% to 80% of cases PCR testing of synovial fluid can identify difficult to culture organisms such as K kingae and is a useful adjunct in the setting of negative cultures Blood cultures identify an organism in 15% to 46% of patients with septic arthritis and are positive in many cases in which the organism is not isolated from the joint fluid Cervical or urethral cultures or urine PCR testing in sexually active adolescents with septic arthritis may identify Neisseria gonorrhoeae as the responsible organism In 20% of cases, a causative organism is not recovered Improved application of PCR techniques may increase the ability to identify pathogens in the future A Gram stain should be performed on joint fluid, and it occasionally provides additional assistance in identifying both the presence of an infection and the infecting organism Although elevation of the WBC count more than 100,000/mm3 in the synovial fluid is considered strong evidence of infection, the actual counts are often much lower and there is considerable overlap with other causes of joint inflammation Presence of purulent fluid, a positive Gram stain, and a highly elevated WBC count with a left shift in the synovial fluid are often used as indications for operative intervention when there is a concern of a septic hip (See Septic Arthritis Pathway here: http://www.chop.edu/clinicalpathway/septic-arthritis-suspected-clinical-pathway )