From the Guidelines
Three-phase bone scans have a sensitivity of 83% but a specificity of less than 50% for detecting osteomyelitis at a 2-month-old transmetatarsal amputation site, making them less reliable for definitive diagnosis in this setting. This imaging modality is useful for ruling out osteomyelitis, but a positive result is nonspecific and may be influenced by post-surgical changes and increased bone turnover at the amputation site 1. The three phases (blood flow, blood pool, and delayed bone uptake) help differentiate between soft tissue infection and bone infection, though interpretation can be challenging due to osseous remodeling and radiotracer uptake in the setting of prior surgery or trauma 1.
Some of the key limitations of three-phase bone scans in this context include:
- False positives due to reactive bone formation, fracture healing, or Charcot arthropathy
- False negatives with chronic infections or when patients are on antibiotics
- Lower specificity in the presence of underlying conditions such as osteoarthritis, recent fracture, or recent hardware implantation 1
- Equivocal findings even with the addition of single-photon emission CT (SPECT)/CT for anatomic localization of infection 1
To improve diagnostic accuracy, combining bone scan with other imaging modalities like MRI (for better soft tissue contrast) or CT (for bone detail) is recommended, along with clinical correlation with physical examination findings, laboratory markers of inflammation, and wound characteristics 1. White blood cell-labeled scans may also provide better specificity in this setting. Ultimately, the diagnosis of osteomyelitis at amputation sites requires a comprehensive approach that incorporates multiple diagnostic tools and clinical judgment.
From the Research
Sensitivity and Specificity of 3-Phase Bone Scan
- The sensitivity of a 3-phase bone scan in detecting osteomyelitis is reported to be around 68% 2.
- The specificity of the 3-phase bone scan is not explicitly stated in the provided studies, but it is mentioned that the test is very sensitive and is the study of choice in the evaluation of patients with suspected osteomyelitis and normal radiographs 3.
- A study comparing the sensitivity of MRI, 3-phase bone scanning, and indium scanning found that MRI and 3-phase bone scanning had similar sensitivities (72% and 68%, respectively) in diagnosing osteomyelitis 2.
Factors Affecting Sensitivity and Specificity
- The presence of underlying bone pathology, such as a healing fracture or degenerative disease, can affect the accuracy of the 3-phase bone scan 3.
- Other diseases of bone, such as Paget's disease or metastatic bone disease, can also produce similar results to osteomyelitis on a 3-phase bone scan, making it difficult to differentiate between these conditions 4.
Diagnosis of Osteomyelitis at 2-Month Old Transmetatarsal Amputation Site
- There is limited information available on the sensitivity and specificity of 3-phase bone scans in detecting osteomyelitis at a specific site, such as a 2-month old transmetatarsal amputation site.
- However, it is recommended that magnetic resonance imaging (MRI) be used as the diagnostic imaging modality of choice for osteomyelitis, given its high sensitivity and specificity 5.