Indium-111 White Blood Cell Scan

Indium-111 (In-111)–labeled white blood cell (WBC) scintigraphy is a nuclear medicine imaging modality used to diagnose occult infections, especially when other imaging techniques are contraindicated or uninformative. This technique leverages the fundamental biological process in which labeled leukocytes migrate to and accumulate at sites of inflammation, which can then be visualized through nuclear imaging. Clinicians commonly use this test to evaluate suspected infections, including osteomyelitis, prosthetic joint infections, vascular graft infections, and fever of unknown origin (FUO). The clinical utility of In-111–labeled WBC scintigraphy varies significantly depending on the indication, with scans being more useful for evaluating osteomyelitis and vascular access infections, but less so for fevers of unknown origin. Notably, while In-111–labeled WBC scans can identify localized inflammation, they cannot distinguish between infectious and sterile inflammatory processes definitively. The technique primarily localizes areas of neutrophilic inflammation, which is highly suggestive of infection. In contrast, conditions predominantly mediated by lymphocytes, such as tuberculosis, fungal infections, or sarcoidosis, often demonstrate little to no radiotracer accumulation. This distinction is critical, as false-negative results may occur in infections with minimal neutrophil recruitment. Multiple clinical series have reported wide sensitivities ranging from 43% to 100% and specificities ranging from 69% to 92% for infectious conditions. A positive predictive value (PPV) of 0.92 and a negative predictive value (NPV) of 0.37 indicate that positive scans hold significantly more diagnostic weight than negative ones. Although some studies report high sensitivity for certain infections, In-111–labeled WBC scans are generally used as adjunctive tools rather than standalone tests. They may not offer sufficient diagnostic clarity in complex cases, particularly when challenging clinical decisions are involved. The procedure involves obtaining a blood sample from the patient, isolating and labeling the WBCs with In-111 oxine, and then re-injecting the labeled cells intravenously. Subsequent imaging, typically performed 24 hours postinjection, reveals areas of In-111–labeled WBC accumulation, indicating sites of inflammation. Although the overall utilization has declined with the emergence of fluorodeoxyglucose–positron emission tomography/computed tomography (FDG-PET/CT), In-111 WBC scintigraphy maintains distinct advantages in specific clinical scenarios, such as in evaluating inflammatory bowel disease (IBD) and intra-abdominal infections. The minimal physiological bowel excretion of In-111 provides superior visualization of inflammatory foci within the abdomen compared to technetium-99m (Tc-99m) hexamethylpropyleneamine oxime (HMPAO) and FDG-PET. This characteristic makes In-111 particularly valuable for diagnosing active IBD, determining disease extent, and monitoring treatment response. Additionally, in cases of suspected intra-abdominal abscesses or postsurgical infections, the high specificity of In-111 WBC accumulation combined with minimal background interference often provides clearer diagnostic information than FDG-PET, where postsurgical changes and normal bowel uptake can confound interpretation. Although In-111 oxine has traditionally been used as a radiotracer for WBC labeling, Tc-99m HMPAO is also commonly used, each offering distinct advantages for specific clinical scenarios. Tc-99m HMPAO provides superior planar image quality, enables earlier imaging (0.5-4 hours postinjection), and reduces radiation exposure, making it particularly suitable for pediatric imaging. However, its 6-hour half-life can limit delayed imaging needed for indolent processes, and its normal activity in the gastrointestinal tract, urinary tract, and gallbladder may interfere with interpretation. Despite producing lower-quality planar and SPECT images, In-111 oxine offers distinct advantages, including higher labeling efficiency and minimal intestinal excretion, making it the preferred choice for abdominal infections and IBD. This is also compatible with concurrent Tc-99m nanocolloid bone marrow imaging due to different energy windows. Additionally, In-111's longer half-life (67 hours) allows for delayed imaging in chronic conditions. However, this benefit comes at the cost of higher radiation exposure to labeled cells, critical organs (particularly the spleen), and the whole body. FDG-PET has shown higher sensitivity than In-111 WBC scintigraphy in evaluating FUO. For prosthetic joint infections, FDG-PET has demonstrated excellent diagnostic accuracy, with sensitivity and specificity of 95% and 93%, respectively, outperforming the combination of Tc-99m bone scan and In-111 WBC scintigraphy (sensitivity of 50% and specificity of 95%). Although the high costs and occasional lack of insurance reimbursement for non-cancer indications have limited the widespread adoption of FDG-PET, it may eventually replace other nuclear imaging modalities in many clinical scenarios. For example, despite cost limitations, FDG-PET/CT has shown promise in assessing COVID-19, particularly for evaluating disease severity and monitoring treatment response. Nevertheless, in specific applications such as IBD and intra-abdominal infections, where physiological FDG uptake can complicate interpretation, In-111 WBC scintigraphy remains the preferred molecular imaging modality. In spine infections, where In-111 WBC scans exhibit characteristically low sensitivity and problematic photopenic defects, FDG-PET/CT is the preferred molecular imaging modality. Appropriate use criteria have been established to guide the selection of nuclear medicine procedures for musculoskeletal infection imaging, promoting their effective and judicious application.