Laboratory Studies
Laboratory studies are not usually indicated to diagnose prepatellar bursitis. However, analysis of fluid aspirated from the bursa is important in differentiating septic bursitis (SB) from nonseptic bursitis (NSB). The fluid should be sent for analysis, since the prepatellar bursa is commonly a site of infection. (See the table below.)
Table 1. Characteristics of bursal fluid in patients with septic and nonseptic prepatellar bursitis (Open Table in a new window)
Characteristic |
Septic bursitis (SB) |
Nonseptic bursitis (NSB) |
Appearance |
Purulent |
Serosanguineous, straw colored, or bloody |
White blood cell count per µL |
1500-300,000; mean 75,000 |
50-10,000; usually < 3000 |
Differential count |
Polymorphonuclear (PMN) cells |
Predominantly mononuclear cells |
Bursal fluid–to–serum glucose ratio |
< 50% |
>50% |
Gram stain |
Positive in 70% |
Negative |
Culture |
Positive |
Negative |
Evaluate the aspirated fluid for WBC count, protein, lactate, glucose, crystals, and Gram stain results. Typical findings in septic bursitis include the following:
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WBC count of 50,000/µL or greater
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Elevated protein
-
Elevated lactate
-
Decreased glucose
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Gram stain results specific to bacteria
In crystal-induced diseases causing bursitis, findings include monosodium urate crystals (gout), calcium pyrophosphate crystals (pseudogout), and cholesterol crystals (rheumatoid arthritis).
Imaging Studies
Plain radiographs may show soft tissue swelling; however, radiographs are necessary only if other conditions are suggested (eg, fracture and/or dislocation).
CT scanning and MRI are reserved for cases that have been difficult to manage (eg, failure of initial treatment for septic prepatellar bursitis). [7, 8]
Ultrasonography may be a useful technique for evaluating synovial fluid in the knee.
Procedures
Aspiration of prepatellar bursa fluid may be indicated because sepsis is common. Consider injection of the prepatellar bursa with corticosteroids only when infection has been excluded. Select position of maximal fullness as the site for injection. Complications of injection include, but are not limited to, the following:
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Infection
-
Bleeding
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Postinjection inflammation and erythema
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Postinjection pain
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Tendon rupture
-
Subcutaneous atrophy
A compression dressing should be worn for 24-36 hours after the procedure, with avoidance of direct pressure to the knee.
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Prepatellar bursa in normal (left) and inflamed (right) state; trilaminar form.
Tables
Characteristic |
Septic bursitis (SB) |
Nonseptic bursitis (NSB) |
Appearance |
Purulent |
Serosanguineous, straw colored, or bloody |
White blood cell count per µL |
1500-300,000; mean 75,000 |
50-10,000; usually < 3000 |
Differential count |
Polymorphonuclear (PMN) cells |
Predominantly mononuclear cells |
Bursal fluid–to–serum glucose ratio |
< 50% |
>50% |
Gram stain |
Positive in 70% |
Negative |
Culture |
Positive |
Negative |
