What is the usefulness of a white blood cell (WBC) scan in detecting prosthetic valve endocarditis?

Usefulness of White Blood Cell Scan in Detecting Prosthetic Valve Endocarditis

White blood cell (WBC) scans have lower sensitivity but higher specificity compared to FDG-PET/CT for diagnosing prosthetic valve endocarditis (PVE), making them particularly valuable when echocardiography is inconclusive. 1

Diagnostic Performance of WBC Scans in PVE

• WBC scans demonstrate lower sensitivity (64%) but higher specificity (100%) compared to FDG-PET/CT (sensitivity 93%, specificity 71%) in diagnosing prosthetic valve endocarditis when echocardiography is inconclusive 1

• WBC-SPECT imaging provides excellent diagnostic performance against intra-operative findings with sensitivity of 87%, specificity of 100%, and accuracy of 92% 2

• WBCs may be labeled with different radioisotopes including indium-111 (In-111), technetium-99m (Tc-99m), or gallium-67 (Ga-67) for these scans 1

• The addition of WBC-SPECT results to the modified Duke score helps correctly reclassify approximately 25% of patients from possible to definite PVE 2

Clinical Applications and Limitations

• WBC scans are particularly useful for identifying and localizing infected vegetations and paravalvular abscesses when echocardiography results are inconclusive 1

• The intensity of WBC signal correlates with the severity of infection - patients with intense signal on WBC-SPECT have a high prevalence of abscesses (83%) compared to patients with only mild signal (12%) 2

• False-negative results may occur in patients who have received prolonged antibiotic treatment prior to imaging 2

• WBC scans have limited value in detecting native valve endocarditis, with one center reporting 0% sensitivity for detection of valvular vegetations by In-111 WBC in patients with known vegetations seen by TEE 1

Comparison with Other Imaging Modalities

• Transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE) remain the first-line imaging tests for suspected endocarditis but can be inconclusive in up to 30% of cases, especially with prosthetic valves 3

• FDG-PET/CT offers higher sensitivity but lower specificity than WBC scans for PVE diagnosis 4

• In a comparative study, leukocyte scintigraphy showed higher specificity (100% vs 71%) but lower sensitivity (64% vs 93%) than FDG-PET/CT 4

• FDG-PET/CT may yield false-positive results in the first two months after cardiac surgery, whereas WBC scans maintain high specificity during this period 4

Prognostic Value and Follow-up

• WBC signal intensity has prognostic value - patients with intense WBC signal treated conservatively have significantly worse outcomes (90% vs 11% composite endpoint of death, late cardiac surgery, or relapse) compared to those with mild signal 5

• WBC-SPECT can be used to monitor treatment response - the prevalence of positive WBC signal decreases progressively from 89% between 3-6 weeks to 8% after 9 weeks of antibiotic therapy 5

• Patient management is influenced by the results of leukocyte scintigraphy in approximately 29% of cases with suspected PVE and inconclusive echocardiography 6

Recommended Diagnostic Algorithm

• For suspected PVE, begin with TTE and TEE as first-line imaging tests 1, 3

• When echocardiography is inconclusive or equivocal, proceed with advanced imaging techniques 1

• Consider WBC scan particularly when:

  • Echocardiography is inconclusive 1, 6
  • Patient is within 2 months post-cardiac surgery (when FDG-PET/CT may yield false positives) 4
  • High specificity is needed to confirm the diagnosis 1, 4

• Interpret intense WBC uptake as highly suggestive of active infection with high likelihood of abscess formation 2, 5

• Use WBC-SPECT for follow-up imaging to monitor treatment response in patients managed conservatively 5