musculoskeletal and connective tissue disorders
Acute Infectious Arthritis
Diagnosis of Acute Infectious Arthritis
Arthrocentesis with synovial fluid examination and culture Blood culture Usually complete blood count and erythrocyte sedimentation rate (or C-reactive protein) Molecular testing (eg, polymerase chain reaction) Sometimes imaging studies Infectious arthritis is suspected in patients with acute monarticular or oligoarticular arthritis and in patients with other combinations of symptoms characteristic of particular infectious arthritis syndromes (eg, migratory polyarthritis, tenosynovitis, and skin lesions typical of Symptoms and Signs ; erythema migrans or other symptoms and signs of Even mild monarticular or oligoarticular joint symptoms should arouse suspicion in patients taking immunosuppressive therapy (eg, corticosteroids, tumor necrosis factor or interleukin 6 inhibitors) with Risk factors (eg, rheumatoid arthritis), a prosthetic joint, or an extra-articular infection capable of spreading to a joint (eg, genital gonococcal infection, pneumonia, bacteremia, any anaerobic infection). Do an arthrocentesis and culture synovial fluid to exclude joint infection in patients with acute monarticular or oligoarticular joint effusion and findings compatible with bacterial infectious arthritis, even if a known joint disorder (eg, rheumatoid arthritis) seems like a more likely cause. Synovial fluid examinationis the cornerstone of diagnosis of acute joint infection. Fluid is examined grossly and sent for cell count and differential, Gram stain, aerobic and anaerobic culture, and crystals. Foul-smelling synovial fluid suggests anaerobic infection. Fluid from an acutely infected joint usually reveals a white blood cell (WBC) count>20,000/mcL (sometimes>100,000/mcL) consisting of>95% polymorphonuclear leukocytes. WBC counts tend to be higher in nongonococcal bacterial than in gonococcal infectious arthritis. WBC counts may also be lower in early or partially treated infections. Gram stain reveals organisms in only 50 to 75% of joints with acute bacterial arthritis, most often with staphylococci. If positive, Gram stain is suggestive, but cultures are definitive. The presence of crystals does not exclude coexisting infectious arthritis. Initial synovial fluid analysis often cannot differentiate between infectious and other inflammatory synovial fluid. There is significant overlap between the synovial cell counts in fluids from patients with gout and infection. If differentiation is impossible by clinical means or synovial fluid examination, infectious arthritis is assumed, pending culture results. Inoculation of synovial fluid into aerobic blood culture bottles can improve detection ofKingella kingae. Blood tests,such as blood cultures, complete blood count, and erythrocyte sedimentation rate (or C-reactive protein), are usually obtained. However, normal results do not exclude infection. Likewise, WBC count, erythrocyte sedimentation rate, or C-reactive protein may be increased in noninfectious joint inflammation (including gout) as well as infectious joint inflammation. The serum urate level should not be used to diagnose or exclude gout as the cause of the arthritis, because the level can be normal or even low in gout, and may be high, although unrelated to gout, with an acute bacterial infection. Molecular testing(eg, polymerase chain reaction) may be used to directly detect organisms in clinical specimens. Gonococci may be detected with nucleic acid amplification testing (NAAT) of specimens from the cervix, urethra, oropharynx, or rectum. Some difficult to cultivate organisms, such asMycobacterium tuberculosisandTropheryma whipplei, may be directly detected in synovial fluid using NAAT. Plain x-raysof the involved joint are not diagnostic of acute infection but can exclude other conditions sometimes under consideration (eg, fractures). Abnormalities in early acute bacterial arthritis are limited to soft-tissue swelling and signs of synovial effusions. After 10 to 14 days of untreated bacterial infection, destructive changes of joint space narrowing (reflecting cartilage destruction) and erosions or foci of subchondral osteomyelitis may appear. Gas visible within the joints suggests infection withEscherichia colior anaerobes. is considered if the joint is not easily accessible for examination and aspiration (eg, an axial joint). MRI or ultrasonography can identify sites of effusion or abscess that can be aspirated or drained for both diagnosis and therapy. MRI can provide early suggestion of associated osteomyelitis. Bone scans using technetium-99m can be falsely negative in infectious arthritis. Also, because they show increased uptake with increased blood flow in inflamed synovial membranes and in metabolically active bone, they can be falsely positive in noninfectious inflammatory arthritis such as gout. Nuclear imaging and MRI do not distinguish infection from Overview of Crystal-Induced Arthritides . If gonococcal arthritis is suspected, blood and synovial fluid samples should beimmediatelyplated on nonselective chocolate agar, and specimens from the urethra, endocervix, rectum, and pharynx should be plated on selective Thayer-Martin medium. The nucleic acid–based tests often used to diagnose genital gonococcal infection are done on synovial fluid only in specialized laboratories. Genital cultures or DNA testing is also done. Blood cultures may be positive during the first week and may assist in microbiologic diagnosis. Synovial fluid cultures from joints with frank purulent arthritis are usually positive, and fluid from skin lesions may be positive. If disseminated gonococcal infection is suspected based on clinical criteria, it is assumed to be present even if all gonococcal cultures are negative. Clinical response to antibiotics (anticipated within 5 to 7 days) can help confirm the Diagnosis . 