A 65-year-old man comes to the office due to a foot ulcer. The ulcer began 2 weeks ago, a few days after he scrubbed a callus off the bottom of his left foot. The patient tried over-the-counter antibiotic creams, but the ulcer gradually enlarged and started draining a foul discharge. The foot is not painful and he has had no fever. The patient has a history of hypertension, hyperlipidemia, and type 2 diabetes mellitus. His diabetes has been poorly controlled by oral medications, and he has refused to take insulin. The patient is a retired veteran and does not use tobacco, alcohol, or illicit drugs. Temperature is 37.2 C (99 F) , blood pressure is 130/86 mm Hg, and pulse is 80/min. BMI is 32 kg/m². Examination of the left foot shows a 2×2 cm plantar ulcer with a necrotic base. No surrounding erythema or induration is present, and metal probing does not contact bone. Lower extremity pulses are diminished but palpable. Sensation to light touch and vibration is reduced, and ankle reflexes are absent bilaterally. Leukocyte count is 13,000 cells/mm³ and erythrocyte sedimentation rate is 95 mm/hr. Blood cultures and wound swab cultures are obtained.
Which of the following is most helpful for ruling out bone infection in this patient?

Question

Quizplus · Accepted Answer

Explanation: 11ec2b69_c178_c29a_8f63_d96814f84b7f_MD0005_00 This patient with poorly controlled diabetes mellitus and a large, nonhealing foot ulcer requires evaluation for osteomyelitis. Initial screening is typically performed at the bedside with probe-to-bone testing. Contact with the bone is highly specific (>90%) for osteomyelitis, but sensitivity is much lower. As such, negative probe-to-bone testing necessitates imaging to rule out osteomyelitis. Plain radiographs are often ordered first due to low cost and low radiation profile. Evidence of cortical loss, bony destruction, and periosteal reaction can confirm osteomyelitis, but plain radiographs have a sensitivity of <55% and are often negative for >2 weeks after infection onset. As such, a negative plain radiograph cannot be relied on to definitively exclude osteomyelitis (Choice D). The diagnostic test of choice for osteomyelitis is MRI of the foot, which has a sensitivity >90% and a high negative predictive value. Osteomyelitis-related bone changes are present on MRI <5 days after infection onset; therefore, patients with symptoms for >1 week and a negative MRI are considered "ruled out" for osteomyelitis. (Choices A and E) Nuclear medicine scans, such as a 3-phase bone scan and a tagged white blood cell scan, are often used in the diagnosis of osteomyelitis if metal hardware prevents accurate images with MRI or CT scan. Diagnostic sensitivities of these tests are high, but, as any cause of inflammation results in a positive test, specificity is moderate. MRI has the greatest overall sensitivity and stronger negative predictive value. (Choice B) CT scans are useful in the diagnosis of osteomyelitis in patients who have contraindications (eg, pacemaker) to MRI. CT scans have greater sensitivity than plain radiographs but are less sensitive than MRI. Educational objective: MRI of the foot with contrast is the diagnostic test that has the greatest sensitivity and negative predictive value for the diagnosis or exclusion of osteomyelitis of the foot.

A 65-Year-Old Man Comes to the Office Due to a Foot