Tests for Diagnosing Chronic Osteomyelitis in the Diabetic Foot

In Spain, amputations due to osteomyelitis are performed each year in 46.1 of every 100,000 individuals with diabetes. In diabetic patients, approximately two-thirds of all amputations are preceded by the infection of a foot ulcer. However, the diagnosis of chronic osteomyelitis in the diabetic foot continues to be a challenge, and we believe that there is a need for more studies validating the different diagnostic methods available. There is also a clear need for a safe, rapid, and efficient diagnostic protocol designed to optimize the therapeutic approach and improve the prognosis in these patients.

The early suspicion of osteomyelitis is essentially clinical and is based on detection of the presence of signs and symptoms of infection, although many patients have no typical local signs. Even in the absence of clinical signs, most infected chronic ulcers of the diabetic foot have underlying osteomyelitis. A lack of clinical signs can lead to a delay in diagnosing the initial stages of infection.

The plain X-ray is not useful for detecting bone infection in the first 2 weeks. Bone abnormalities can generally not be seen until bone mineral density drops to 35–50% of that of normal adjacent bone. Moreover, subtle changes at the onset of osteomyelitis are not easily distinguished from other changes that occur in the feet of these patients due to their neuropathy or peripheral vascular disease.

The probe-to-bone (PTB) test is routinely performed to detect, using a blunt instrument, palpable bone through the ulcer, indicating osteomyelitis. However, this test has been validated only in a few previous studies, and there is currently no consensus on a standardized protocol for the clinical diagnosis of osteomyelitis. The present study provides data on the validity of the clinical tests most frequently used to detect this disease.

This observational, descriptive study with prospective collection of data was conducted in patients with type 1 or 2 diabetes who attended the Diabetic Foot Clinic of the University Podology Clinic, Universidad Complutense de Madrid (Spain) because of a foot ulcer.

Over the period May 2006 to November 2008, we treated 210 foot lesions in diabetic patients. Of these lesions, 132 with clinical suspicion of infection were selected as the study sample, of which 105 (79.5%) were finally diagnosed as osteomyelitis. Infection was recorded according to the presence of clinical signs and symptoms and a positive soft tissue culture result. Once infection of the ulcer had been established, presumptive osteomyelitis was diagnosed by plain radiography and a clinical examination.

Patients were enrolled if they had a single ulcer of neuropathic or neuroischemic etiology below the ankle, there was suspicion of bone infection according to clinical signs and symptoms and the diagnostic tests standardized in the protocol used at our center (ulcer specimen culture, radiography, and PTB), if they had undergone surgery for acute osteomyelitis, or if after adequate local or antibiotic treatment and rest, the ulcer did not resolve. Patients were excluded if they had critical ischemia according to the classification of Fontaine et al. or were due for an operation that was unrelated to a diagnosis of osteomyelitis. The study protocol was approved by our institutional review board.