How to diagnose a femoral arteriovenous (AV) graft infection?

Diagnosis of Femoral Arteriovenous Graft Infection

Femoral AV graft infection should be diagnosed primarily through clinical examination combined with ultrasound imaging, with blood cultures and CT angiography reserved for cases requiring surgical planning or when the diagnosis remains uncertain. 1

Clinical Presentation

The clinical diagnosis of femoral AV graft infection relies heavily on recognizing specific physical findings:

• Painful erythematous swelling in the groin is highly suggestive of underlying graft infection 1
• Draining sinus tract is the most obvious sign of graft infection and strongly indicates infection 1, 2
• Purulent drainage from the graft site occurs in approximately 55% of cases 3
• Palpable, pulsatile mass may indicate pseudoaneurysm formation at the anastomotic site 1
• Fever and systemic sepsis occur in approximately 36-57% of patients, though absence does not exclude infection 1, 3
• Bleeding at the graft site presents in approximately 32% of infected grafts 3

Important caveat: Fever is absent in 50-60% of peripheral graft infections and should not be relied upon as a diagnostic criterion 1

Laboratory Evaluation

Blood Cultures

• Obtain blood cultures in all suspected cases, as they are positive in 30-60% of patients with femoral graft infections 1
• Prior antibiotic therapy frequently renders blood cultures negative, limiting their utility 1
• Staphylococcus aureus (including MRSA) is the most common pathogen, isolated in approximately 53% of cases 3, 4
• Coagulase-negative staphylococci account for approximately 11% of infections 3, 4
• Pseudomonas aeruginosa represents approximately 8-10% of cases 1, 3

Inflammatory Markers

• Elevated white blood cell count, erythrocyte sedimentation rate, and C-reactive protein occur commonly but are nonspecific 1
• Normal inflammatory markers are uncommon in graft infections, making them useful for ruling out infection 1

Imaging Studies

First-Line: Ultrasound

Ultrasound should be the initial imaging modality for suspected femoral AV graft infection because it is:

• Inexpensive and can be performed at bedside 1
• Highly sensitive (94%) and specific (97%) for detecting pseudoaneurysm 1
• Useful for distinguishing cellulitis or abscess from graft infection 1
• Capable of identifying fluid collections for aspiration and culture 1

Limitation: Ultrasound cannot reliably differentiate seroma, hematoma, and abscess 1

CT Angiography (CTA)

CTA should be obtained when:

• Ultrasound findings are nondiagnostic 1, 5
• Surgical planning requires detailed vascular anatomy 1
• Deep infection or anastomotic involvement is suspected 5

CTA provides:

• Sensitivity of 85-100% and specificity of 85-94% for graft infection 1, 5
• Rapid results and precise anatomic localization 5
• Detection of perigraft fluid collections and gas 5

Caveat: Contrast-associated kidney injury is a concern in dialysis patients 1

Advanced Imaging (When Initial Studies Are Nondiagnostic)

• MRI: Sensitivity 68-85%, specificity 97-100%; useful when CT is nondiagnostic and no contrast kidney injury risk 1, 5
• Indium-labeled white blood cell scan: Sensitivity 67-73%, specificity 87%; requires ≥24 hours for results 1, 5
• PET/CT: Sensitivity 78-96%, specificity 70-93%; useful for indeterminate cases but expensive 1, 5

Microbiological Confirmation

Aspiration and Culture

Obtain cultures from ultrasound- or CT-guided aspiration of perigraft fluid collections whenever possible, as these provide the most accurate microbiological diagnosis 1, 4

• Bacterial growth is present in approximately 85% of specimens 3
• Avoid relying on wound swab cultures from draining sinuses, as these may represent skin flora or colonization rather than the true causative organism 1
• Genetic typing (PFGE) demonstrates that isolates from skin site and vascular prosthesis are identical in most cases 4

Intraoperative Specimens

• Graft tissue cultures obtained during surgical exploration provide definitive microbiological diagnosis 1
• Multiple specimens should be sent for aerobic, anaerobic, and fungal cultures 1

Diagnostic Algorithm

1. Clinical examination for groin erythema, swelling, draining sinus, pulsatile mass, or purulent drainage 1
2. Blood cultures (two sets from separate sites) 1
3. Ultrasound as initial imaging to detect pseudoaneurysm and fluid collections 1
4. Aspiration and culture of any identified fluid collection under ultrasound guidance 1, 4
5. CTA if ultrasound is nondiagnostic or surgical planning is needed 1, 5
6. Advanced imaging (MRI, indium scan, or PET/CT) only if diagnosis remains uncertain after above steps 1, 5

Common Pitfalls

• Do not wait for fever or positive blood cultures to make the diagnosis, as these are absent in many cases 1
• Do not rely on wound swab cultures alone, as they may not reflect the true pathogen 1
• Do not delay imaging in patients with suspected infection, as anastomotic rupture can be life-threatening 1
• Recognize that the graft may still be patent at presentation in approximately 62% of infected cases 3