What diagnostic tests and treatment should be ordered for a healthy adult patient with suspected knee bursitis?

Diagnostic Workup for Suspected Knee Bursitis

For a healthy adult with suspected knee bursitis, start with plain radiographs (AP and lateral views) to exclude fracture or other bony pathology, followed by ultrasound to confirm bursal fluid collection and guide aspiration if septic bursitis is suspected. 1

Initial Clinical Assessment

Key clinical features to identify:

• Location of tenderness and swelling - prepatellar (anterior knee), pes anserine (medial proximal tibia), infrapatellar (below patella), or MCL bursa (medial knee) 1, 2
• History of repetitive trauma or direct pressure (e.g., prolonged kneeling for prepatellar bursitis) as the most common cause 1
• Signs suggesting septic bursitis - fever, warmth, erythema, systemic toxicity, or acute onset without clear trauma 1
• Valgus stress test pain may indicate MCL bursitis rather than ligament injury 3

First-Line Imaging

Plain radiographs (AP and lateral views) should be obtained first to exclude:

• Fractures or bony pathology 4
• Underlying osteoarthritis that may predispose to bursitis 3
• Other structural abnormalities 2

Ultrasound is the key diagnostic modality for confirming bursitis:

• Distinguishes bursitis from cellulitis - critical for management decisions 1
• Identifies anechoic fluid-filled bursal distension with characteristic location 5
• Guides aspiration when infection is suspected 1, 5
• Detects associated pathology such as patellar tendinopathy 5

Laboratory Testing and Aspiration

If septic bursitis is suspected (fever, erythema, warmth, acute onset):

• Obtain serum inflammatory markers - ESR and CRP to assess for infection 6, 7
• Complete blood count with differential to evaluate for leukocytosis 6, 7
• Perform bursal aspiration under ultrasound guidance before starting antibiotics 1

Synovial fluid analysis must include:

• Gram stain and aerobic/anaerobic cultures - Staphylococcus aureus is the most common pathogen 1, 7
• Cell count with differential - elevated WBC with neutrophil predominance suggests infection 7
• Crystal analysis to exclude gout or pseudogout, which can coexist with infection 7, 1
• Glucose measurement 7

Advanced Imaging

MRI is reserved for specific scenarios:

• Chronic or recalcitrant bursitis not responding to conservative treatment 2
• Suspected concomitant pathology such as meniscal tears, ligament injuries, or tendinopathy 3, 5
• Distinguishing infectious from noninfectious causes when clinical picture is unclear 1
• Evaluating MCL bursitis - shows distended bursa with internal septations beneath superficial MCL 3

MRI findings in bursitis include fluid-filled bursal distension, and can demonstrate associated tendinopathy or ligamentous pathology 3, 5

Critical Pitfalls to Avoid

• Do not aspirate chronic microtraumatic bursitis routinely - this increases risk of iatrogenic septic bursitis 1
• Do not delay aspiration when infection is suspected - obtain cultures before starting antibiotics when clinically feasible 7, 1
• Do not confuse bursitis with cellulitis - ultrasound readily distinguishes these conditions 1
• Do not inject corticosteroids into retrocalcaneal bursa - may damage Achilles tendon biomechanics 2

Treatment Considerations Based on Etiology

Acute traumatic/hemorrhagic bursitis:

• Conservative management with ice, elevation, rest, and analgesics 1
• Aspiration may shorten symptom duration 1

Chronic microtraumatic bursitis:

• Address underlying cause (e.g., knee pads for prepatellar bursitis) 1
• NSAIDs and activity modification 2
• Avoid routine aspiration due to infection risk 1

Septic bursitis:

• Antibiotics effective against S. aureus as initial treatment 1
• Outpatient oral antibiotics if not acutely ill; hospitalization with IV antibiotics if systemically ill 1
• Surgery reserved for antibiotic-refractory or recurrent cases 1, 2

Inflammatory bursitis (gout, rheumatoid arthritis):

• Treat underlying condition 1
• Intrabursal corticosteroid injection often used 1, 3