A complete blood count (CBC) with differential is appropriate if an infectious etiology is suspected. Characteristics include the following:
The white blood cell (WBC) count may be elevated in the presence of proximal infection or systemic involvement
It is not elevated in nonsuppurative conditions and often is not elevated in immunocompromised patients
A left shift is frequently present in acute processes
Determination of the erythrocyte sedimentation rate (ESR) is appropriate if an infectious etiology is suspected in tenosynovitis, though the ESR is nonspecific for the disorder. Characteristics include the following:
The ESR typically is elevated in acute or chronic infections and may serve as a marker to follow resolution of an infection
The ESR may be elevated in cases of inflammatory flexor tenosynovitis (FT) as well
The ESR is not elevated in nonsuppurative conditions
Coagulation studies are needed in anticoagulated patients or in patients with known or suspected bleeding diathesis. In severe infection in which systemic sepsis is a concern, disseminated intravascular coagulation (DIC), though quite rare, must be ruled out.
Test for rheumatoid factor if rheumatoid arthritis is a consideration.
Gonococcal cultures of the urethra or cervix, rectum, and pharynx are appropriate if gonococcal tenosynovitis is suspected. One of these cultures is positive in approximately 80% of patients. In addition, transiently elevated liver function studies (LFTs) have been described with disseminated gonococcal infection.
Obtain standard anteroposterior and lateral radiographs to rule out bony involvement or a foreign body. 
Magnetic resonance imaging (MRI) has proven accurate in assisting in the diagnosis of tenosynovitis  ; however, it is expensive and generally unnecessary, in that the diagnosis is usually clinically evident.
Aspiration and Evaluation of Joint Fluid
If infection is suggested, culture of the suppurative synovial fluid is mandatory before definitive antimicrobial treatment is initiated. These cultures should include aerobic, anaerobic, fungal, acid-fast bacilli (AFB), and atypical AFB samples. In nonsuppurative conditions, synovial fluid may show nonbirefringent crystals (gout) or birefringent crystals (calcium pyrophosphate disease [CPPD], or pseudogout).
Diagnostic arthrocentesis is indicated when joint effusion is present with tenosynovitis because most patients with disseminated gonococcal infection have coexistent septic arthritis. (Most gonococcal arthritis is monoarticular; approximately 25% of cases are polyarticular.) Characteristics of the aspirated fluid can include the following:
Sterile fluid is common with gonococcal arthritis; cultures are negative in 50% of patients
Joint fluid glucose is usually normal.
White blood cell (WBC) counts are usually below 50,000/μL
A Gram stain is positive in only 25% of patients
Histopathologic examination of synovial biopsy specimens is helpful in diagnosing granulomatous changes observed in Mycobacterium infections and in cases of chronic processes.
Synovial biopsy may reveal acute or chronic inflammatory changes. Gram stains may reveal bacteria. A higher index of suspicion should be present for chronic infections or atypical presentations. These histologic findings help to confirm a diagnosis of inflammatory arthropathy. 
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