Diagnosis and triage of a patient with an extra-osseous fat fluid level.

Extra-osseous fat fluid level is rarely seen in osteomyelitis, with only three magnetic resonance imaging (MRI) cases previously reported in the literature. The rarity of this finding is probably secondary to the extensive necrosis of bone marrow that needs to occur at a rapid phase for the fat to accumulate. However, an extra-osseous fat fluid level is a specific diagnostic sign of osteomyelitis in cases with otherwise equivocal imaging findings and should be reported as such, especially when associated with medullary bone destruction and in the absence of trauma.