Is there a randomized controlled trial (RCT) comparing Indium (White Blood Cell) WBC scans to other targeted White Blood Cell (WBC) scans?

Comparison of Indium WBC Scans with Other Targeted WBC Scans

There are no randomized controlled trials directly comparing Indium-111 WBC scans with other targeted WBC scans, but comparative evidence exists in observational studies showing different performance characteristics between imaging modalities.

Available WBC Labeling Techniques

Indium-111 (In-111) WBC Scans

• Introduced in the 1980s for infection detection 1
• Requires in vitro labeling of patient's WBCs with In-111 oxine
• Imaging performed 18-24 hours after reinjection of labeled WBCs
• Sensitivity ranges from 67-73%, specificity around 87% for vascular graft infections 1
• For intra-abdominal abscesses, sensitivity of 95% and specificity of 91% has been reported 1

Technetium-99m (Tc-99m) HMPAO WBC Scans

• Uses Tc-99m hexamethylpropyleneamine oxime (HMPAO) for labeling
• Similar in-vitro labeling process as Indium scans
• For pelvic inflammatory disease: sensitivity of 100%, specificity of 90% in a small study 1
• For knee prosthesis infections: more sensitive (84%) than for hip prostheses (57%) but less specific (52% vs 75%) 1

Comparative Performance

While not direct head-to-head RCTs, several studies provide insight into the relative performance:

1. Abdominal/Pelvic Infections:

   • In-111 WBC scan correctly identified intra-abdominal abscesses with 95% sensitivity and 91% specificity
   • Tc-99m-HMPAO WBC scan showed 100% sensitivity and 90% specificity for pelvic inflammatory disease 1
   • When compared to ultrasound for intra-abdominal abscesses, In-111 WBC was more sensitive (95% vs 45%) but less specific (91% vs 100%) 1

2. Vascular Graft Infections:

   • In-111 WBC scans show sensitivity of 67-73% and specificity of 87% 1
   • Both In-111 and Tc-99m WBC scans are rated similarly in appropriateness criteria (both considered "usually not appropriate" as first-line imaging) 1

3. Joint Prosthesis Infections:

   • Combined leukocyte/marrow imaging showed 95% accuracy for diagnosing prosthetic knee infection 1
   • Tc-99m-HMPAO labeled leucocyte scintigraphy was more sensitive for knee (84%) than hip prosthesis (57%) but less specific for knee (52% versus 75%) 1

Clinical Utility by Indication

A 2014 study evaluating the clinical utility of In-111 WBC scans found:

• Overall clinical utility in only 48% of cases
• Higher utility for osteomyelitis (70% useful) and vascular access infections (67% useful)
• Lower utility for fever of unknown origin (34% useful) 2

Limitations of WBC Scans

1. Technical Considerations:

   • Requires blood drawing, WBC isolation, and radiolabeling
   • 18-24 hour delay between injection and imaging for In-111 scans
   • Decreased sensitivity with recent antimicrobial therapy 1
   • False-positive results in early postoperative period 1

2. Diagnostic Challenges:

   • Cannot provide exact anatomic localization comparable to CT scans 1
   • Decreased sensitivity with low-grade infections 1
   • WBCs accumulate in both infections and normal bone marrow, requiring additional scans to differentiate 1
   • Major cause of false positives is non-infectious bowel activity 3

Current Recommendations

Current guidelines suggest:

1. For sepsis evaluation:

   • WBC scans should only be used after source localization with CT has failed 1
   • Both In-111 and Tc-99m WBC scans are rated as "usually not appropriate" (score of 3) for initial evaluation of acute abdominal pain and fever 1

2. For vascular graft infections:

   • Indium scan should be used in combination with blood culture results and echocardiography findings 1
   • CT, MRI, indium scanning, and PET/CT are all options for diagnosing intra-abdominal VGI 1

3. For joint prosthesis evaluation:

   • WBC scans have limited utility for differentiating mechanical failure from occult infection in painful loose total knee prostheses 1
   • SPECT/CT with labeled leukocytes may eliminate the need for correlative bone scans 1

Emerging Alternatives

• PET/CT imaging is increasingly replacing conventional WBC scans due to:
  • Higher resolution images
  • Better sensitivity for low-grade infections
  • Improved accuracy for central skeleton problems 1
  • No need to handle potentially infected blood products 1

While there are no direct RCTs comparing different WBC labeling techniques, the available evidence suggests that the choice between In-111 and Tc-99m HMPAO WBC scans should be based on the specific clinical scenario, with consideration of the anatomical location of suspected infection and the availability of newer imaging modalities like SPECT/CT or PET/CT.