Obtaining blood and synovial fluid cultures is the most appropriate next step in management for this patient with suspected prosthetic joint infection. Infections may occur early (within 3 months of surgery), have delayed onset (3-12 months), or have late onset (>12 months after surgery). Early-onset infections typically present with joint swelling, erythema, wound drainage, and/or fever. Delayed-onset infections present more insidiously with prolonged joint pain, often without fever. Late-onset infections present as acute pain and swelling, often in the setting of a nidus for hematogenous seeding such as a vascular catheter or other site of infection remote from the affected joint. This patient has acute onset of pain and swelling of the right knee, along with fevers, an elevated leukocyte count with a left shift, and elevated inflammatory markers, occurring 2 months after knee replacement. Urgent surgical consultation is warranted, and blood and synovial fluid cultures should be obtained before administration of antibiotics, whenever possible, to allow for more accurate culture data. Blood cultures are essential (even when fever is absent); although the infection likely arose locally at the surgical site in this patient, most cases of infectious arthritis arise from hematogenous spread. Blood cultures are also important because, even in cases in which the infection arises directly at the joint, the organism may occasionally be identified in the blood cultures. Before initiating antibiotics, synovial fluid cultures should be obtained via arthrocentesis or in the operating room if surgical intervention is imminent and the patient is stable enough to withhold antibiotics until surgery.

This patient will need antibiotics shortly, but she is stable enough to await surgical evaluation and collection of blood and synovial fluid cultures.

More advanced imaging such as a CT, bone scan, or MRI is generally not indicated in the preliminary evaluation and treatment of a suspected prosthetic joint infection because these studies delay the more urgent management of the patient and do not change the initial management.