Contraindications
Overlying cellulitis, severe coagulopathy.
Applications
Diagnosis of unknown effusion, septic arthritis, gout, or pseudogout. Diagnosis and relief of hemarthrosis.
Equipment necessary
30-mL syringe
20-gauge, 1.5-inch needle
Hemostat
Sterile gloves
Sterile container
Antiseptic solution of your choice
Procedure
Landmark the knee for needle insertion. There are many approaches to knee aspiration.1 For the lateral approach, have the patient hang his or her knee off the table at 90° of flexion (as with any joint, the general principle is distracting the joint to open it in order to increase the chances of a successful tap), and find the soft tissue triangle bordered by the lateral edge of the patella and the superior and inferior joint lines. Mark it clearly with a marker or with a physical indentation.
Cleanse the area thoroughly.
Attach the needle to the syringe and, using sterile technique, enter the joint at the landmark perpendicular to the skin, aiming for the centre of the joint. Entry should be smooth, aspirating throughout the insertion.
Once in the joint space, the effusion should be easily aspirated into the syringe. Note colour, viscosity, and presence of blood, fat globules, or pus.
Take care to hold the needle in place with the hemostat when removing it in order to express contents.
Analyze a sample under a microscope to assess for crystal identification, or send it to a laboratory for confirmation.
Send a sample for cell counts, Gram stain, and culture and sensitivity to evaluate for infection.
Evidence
The greater the white blood cell (WBC) and neutrophil counts in aspirate, the more likely septic arthritis is (eg, in one study,2 the likelihood ratio for infection in a prosthetic knee was very high [48.5] with a WBC count of only greater than 1700/mm3 and neutrophils more than 65%, but most authors use a cutoff of 100 000/mm3 for WBCs and 90% for neutrophils). Unfortunately, there is overlap between infectious and inflammatory cases (including crystal-induced inflammation, especially readily confused with sepsis for high WBC counts produced), thus necessitating Gram stain before culture to argue for empiric treatment. The likelihood ratio for gout is a convincing 567 with identification of urate crystals.3
Diagnostic confirmation
Gram stain and culture for septic arthritis. Crystal microscopy is diagnostic for gout and pseudogout.
The physical examination is facing extinction in modern medicine. The Top Ten Forgotten Diagnostic Procedures series was developed as a teaching tool for residents in family medicine to reaffirm the most important examination-based diagnostic procedures, once commonly used in everyday practice. For a complete pdF of the Top Ten Forgotten diagnostic procedures, go to http://dl.dropbox.com/u/24988253/bookpreview%5B1%5D.pdf.
References
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