Osteomyelitis is an infection that causes inflammation and destruction of bone tissue. This condition can arise when bacteria or fungi spread to the bone from an infection elsewhere in the body or from a direct entry point, such as a wound. Because osteomyelitis can lead to severe complications, including permanent bone damage, prompt and accurate diagnosis is necessary to guide effective treatment and prevent long-term disability.
Initial Clinical Assessment
Diagnosis begins with a detailed patient history to identify potential sources of infection and predisposing factors. Physicians inquire about recent surgeries, traumatic injuries, infections, and intravenous drug use. Medical conditions like diabetes and peripheral vascular disease increase the risk of bone infection.
The physical examination focuses on the site of suspected infection, looking for signs of inflammation. Common findings include localized pain, swelling, and redness over the affected bone. Physicians also check for systemic symptoms, such as fever, and assess difficulty bearing weight on the limb.
For patients with foot ulcers, especially those with diabetes, a specific screening tool called the probe-to-bone test may be used. This involves gently probing the ulcer with a sterile metal instrument to see if bone is felt, which is highly suggestive of underlying osteomyelitis. However, a definitive diagnosis requires more objective testing beyond the initial clinical findings.
Laboratory Markers of Infection
Initial blood tests look for general signs of an active inflammatory or infectious process. These markers are not specific to bone infection but help establish the likelihood of a systemic problem. Two primary indicators measured are the Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP).
The CRP level rises rapidly within six to eight hours of infection onset and is useful for monitoring treatment response, as it quickly decreases with effective therapy. ESR measures how quickly red blood cells settle, and while elevated in acute osteomyelitis, it increases and decreases more slowly than CRP. A White Blood Cell (WBC) count is also checked; while it may be elevated in acute infection, a normal count does not rule out chronic osteomyelitis.
Elevated markers like CRP, often reaching 50 to 100 mg/L in acute cases, and ESR, frequently exceeding 70 mm/hr, support a suspicion of osteomyelitis. Blood cultures are also drawn to identify the causative organism if the infection has spread through the bloodstream. However, these cultures are only positive in about half of all osteomyelitis cases.
Diagnostic Imaging Techniques
Imaging techniques localize the infection and visualize the extent of bone and soft tissue damage. Plain film X-rays are usually the first study performed due to their accessibility and low cost. X-rays help rule out other conditions like fractures, but changes suggestive of osteomyelitis may not be visible until 50% to 75% of the bone matrix is destroyed, which can take 10 to 14 days in adults.
Magnetic Resonance Imaging (MRI) is the most sensitive and preferred modality for detecting early bone infection. MRI’s high resolution visualizes bone marrow edema, one of the earliest signs of osteomyelitis, sometimes appearing within one to two days of infection onset. It is also excellent for showing the spread of infection into surrounding soft tissues and detecting abscesses.
Computed Tomography (CT) scans offer superior detail of the bony structure compared to MRI and are particularly useful for surgical planning. CT is better at detecting cortical bone destruction, the presence of dead bone fragments known as sequestra, and gas within the soft tissues.
Nuclear medicine scans, such as a technetium-99m bone scan, may be used when the infection site is unclear or when the patient cannot undergo an MRI. These scans involve injecting a radioactive tracer that accumulates in areas of increased bone metabolism and blood flow, indicating a problem, though not specifically an infection.
Confirming the Diagnosis
The definitive diagnosis of osteomyelitis and identification of the specific pathogen requires a bone biopsy. This procedure is considered the gold standard because it provides tissue for both histopathology (microscopic examination) and microbial culture.
The biopsy can be performed using a long needle guided by fluoroscopy or CT, or through an open surgical procedure. It is important that the tissue sample is taken from the infected bone itself, not just from an overlying wound or sinus tract, as superficial cultures often do not match the deep bone infection.
The bone sample is sent for culture to determine the exact species of bacteria or fungus causing the infection. Identifying the specific pathogen dictates the choice and duration of targeted antibiotic therapy necessary to clear the infection and prevent recurrence. Delaying antibiotics until the biopsy is performed is often recommended to ensure the most accurate culture results.