Does an indium (Indium-111) White Blood Cell (WBC) scan lead to a meaningful impact on clinical care or outcomes in patients with an unidentified primary source of infection?

Indium-111 WBC Scan Has Limited Clinical Utility in Patients with Unidentified Primary Source of Infection

Indium-111 WBC scans are not recommended as a routine diagnostic tool for patients with unidentified primary source of infection due to their limited clinical utility, with studies showing they are helpful in less than half of cases. 1

Diagnostic Performance and Limitations

The paper by Vyas et al. (2014) directly addresses the clinical utility of indium-111 WBC scans for evaluation of suspected infection. Their findings reveal:

• Only 48% of indium WBC scans were deemed clinically useful across all indications 1
• The utility varied significantly by indication:
  • More helpful for osteomyelitis (70% useful) and vascular access infections (67% useful)
  • Less helpful for fever of unknown origin (34% useful) 1

These findings align with guidelines from the American College of Radiology, which note several limitations:

• WBC scans have sensitivities ranging from 40% to 96% and specificities of 76% to 100% for joint prosthesis infections 2
• The procedure is complex, requiring blood drawing, WBC isolation, and radiolabeling 2
• There's an 18-24 hour delay between injection and imaging 2
• Decreased sensitivity with low-grade infections and limited neutrophilic component 2
• High false-positive rates due to leukocyte accumulation in reactive bone marrow 2

Specific Clinical Scenarios

For Unidentified Sources of Infection:

• WBC scans should only be used after source localization with CT has failed 3
• Annual usage has decreased over time (from 26 in 2005 to 13 in 2011 at one tertiary center) 1

For Spinal Infections:

• Extremely poor performance with sensitivity as low as 17% 4
• Prior antibiotic therapy correlates with high false-negative rates 4

For Prosthetic Joint Infections:

• Not recommended as routine for differentiating mechanical failure from occult infection 2
• A negative indium leukocyte scan might support the absence of infection in equivocal cases 2

Impact on Clinical Management

While the scan has limitations, it can occasionally provide valuable information:

• In one study, 16% of patients had unexpected sites of infection outside the abdomen 5
• Of patients with proven infection, 34% had alterations in diagnostic evaluations and therapeutic regimens due to such findings 5
• Can be useful in detecting systemic fungal infections before lesions become clinically apparent 6

Modern 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 3

Conclusion

The paper by Vyas et al. demonstrates that indium-111 WBC scans have limited overall clinical utility in patients with unidentified sources of infection, being helpful in less than half of cases. Their use should be targeted to specific indications where they show greater utility, such as osteomyelitis and vascular access infections, rather than as a general tool for identifying unknown sources of infection.