An infection to the joint is called infectious arthritis. Many infectious organisms can affect the joint. Staphylococcus aureus, Neisseria gonorrhoeae, and other bacteria are the most common cause of infectious arthritis. Mycobacteria, spirochetes, fungi, and viruses also infect joints.
Acute bacterial infection can rapidly destroy articular cartilage, all inflamed joints must be evaluated without delay to exclude noninfectious processes and to determine appropriate antimicrobial therapy and drainage procedures.
Acute bacterial infection typically involves a single joint or a few joints. Subacute or chronic monarthritis or oligoarthritis suggests mycobacterial or fungal infection.
Episodic inflammation is seen in syphilis, Lyme disease, and the reactive arthritis that follows enteric infections and chlamydial urethritis.
Acute polyarticular inflammation occurs as an immunologic reaction during the course of endocarditis, rheumatic fever, disseminated neisserial infection, and acute hepatitis B.
Bacteria and viruses occasionally infect multiple joints, the former most commonly in persons with rheumatoid arthritis.
Aspiration of synovial fluid, an essential element in the evaluation of potentially infected joints, can be performed without difficulty in most cases by the insertion of a large-bore needle into the site of maximal fluctuation or tenderness or by the route of easiest access.
Ultrasonography or fluoroscopy may be used to guide aspirations of difficult-to-localize effusions of the hip and, occasionally, the shoulder and other joints.
Normal synovial fluid contains fewer than 180 cells (predominately mononuclear cells) per microliter. Synovial cells counts averaging 100,000/µL (range, 25,000 to 250,000/µL), with more than 90 percent neutrophils, are characteristic of acute bacterial infections.
Crystals induced, rheumatoid, and other noninfectious inflammatory arthritides are usually associated with fewer than 30,000 to 50,000 cells/µL.
Cell counts of 10,000 to 30,000/µL, with 50 to 70 percent neutrophils and the remainder lymphocytes, are common in mycobacterial and function of the pathogen from cultures of synovial fluid and blood, or detection of microbial nucleic acids and proteins by polymerase chain reaction based assays and immunologic techniques.
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