Joint infection, also called septic arthritis, is relatively common in patients with swollen, painful joints and must be a priority consideration. Almost any infectious agent can cause septic arthritis, including fungi and viruses, but the vast majority are caused by bacteria. Bacterial infections are generally referred to as gonococcal or nongonococcal, since Neisseria gonorrhoeae is a causative agent among young sexually active adults. Staph. and Strep. species are the most common gram-positive agents and cause up to 90 % of septic arthritis cases [4]. Gram-negative organisms are more common in older patients and in the immunocompromised.

Several risk factors for septic arthritis have been identified in recent studies. Those with the highest positive likelihood ratios (LR) were hip and knee prosthesis and skin infection (LR 15), recent joint surgery (LR 6.9), age >80 years (LR 3.5), and hip or knee prosthesis (LR 3.1) [5]. Diabetes and rheumatoid arthritis also increase the risk.

Septic arthritis typically presents as a hot, swollen, tender joint with a reduced range of motion. Fever occurs in approximately 50 % of patients with septic arthritis and does not distinguish this diagnosis from other inflammatory causes of joint pain and swelling [5]. An elevated peripheral WBC count, an elevated ESR, or an elevated CRP were found to increase the likelihood of septic arthritis minimally, so a high index of suspicion needs to be maintained.

Progressively higher WBC counts in the synovial fluid increase the likelihood of septic arthritis. In one systematic review and one meta-analysis, the likelihood ratio (LR) of septic arthritis with a synovial WBC count of <25,000/mm³ (25 × 10⁹/L) was 0.32 [5], the LR for a WBC count of ≥ 25,000/mm³ was 3.2, for a WBC count > 50,000/mm³ (50 × 10⁹/L) the LR was 4.7, and the LR for a synovial WBC count > 100,000/mm³ was 13.3 [6]. If 90 % or more of the cells in the synovial fluid are polymorphonuclear cells (PMNs), then the risk of septic arthritis is increased threefold (LR 3.4). Other markers in the synovial fluid, such as glucose, protein, and lactic acid, have not been found to be helpful [5, 6].

Gram staining of synovial fluid is very helpful when positive, but is not sensitive enough to rule out infection [4]. Culture results can take several days, so treatment should be instituted if infection is suspected. Blood cultures should always be obtained when septic arthritis is suspected or diagnosed, and they can be very helpful for guiding therapy. Unfortunately, blood cultures are only positive in 10–50 % of cases [1, 7].