A 54-year-old man presents to the clinic with 4 weeks of persistently worsening back pain. The pain is localized to 1 spot in his lower back and is worse with physical activity. His medical history is pertinent for intravenous drug use. He has no past surgical history. His temperature is 36.8°C (98.2°F), blood pressure is 118/90 mmHg, pulse is 92/min, respirations are 13/min, and oxygen saturation is 99% on room air. On physical exam, he has midline back tenderness to palpation at L2-L3. Laboratory workup reveals an erythrocyte sedimentation rate of 112 mm/h and C-reactive protein of 10 mg/dL. Which of the following is the most likely diagnosis?
Ankylosing spondylitis
Osteomyelitis
Degenerative spine disease
Metastatic tumor
Osteomyelitis
This patient with a history of intravenous drug use who presents with worsening focal back pain and elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) most likely has vertebral osteomyelitis.

Vertebral osteomyelitis typically occurs via three routes: hematogenous spread from a distant site (the most common mechanism), direct inoculation from spinal surgery or trauma, or contiguous spread from adjacent soft tissue infection. Patients usually present with back or neck pain, with or without fever, and the most common clinical finding is local tenderness to percussion over the involved posterior spinous process. Risk factors include injection drug use, degenerative spine disease, prior spinal surgery, infective endocarditis, diabetes mellitus, corticosteroid therapy, and other immunocompromised states. Vital signs may be normal, and laboratory evaluation commonly reveals an elevated white blood cell count, ESR, and CRP. An MRI of the spine is the most appropriate diagnostic test. In stable patients, antimicrobial therapy should be withheld until a microbiological diagnosis is confirmed; once the diagnosis is confirmed, pathogen-directed therapy should then be administered.

Beronius et al. conducted a retrospective study on vertebral osteomyelitis in pursuit of diagnostic criteria to simplify the diagnosis and classification of vertebral osteomyelitis. The authors found that elevated ESR and CRP are found in most patients with vertebral osteomyelitis. The authors recommend clinicians have a high index of suspicion for vertebral osteomyelitis in order to avoid delayed diagnosis.

Incorrect Answers:
Answer A: Ankylosing spondylitis is an inflammatory arthritis of the spine and is often associated with one or more articular or periarticular extraspinal features, including synovitis, dactylitis, and enthesitis. Patients typically present with chronic back pain and loss of mobility before the age of 45.

Answer B: Degenerative spine disease involves osteoarthritis of the spine and is associated with increasing age, presenting commonly as sharp or chronic pain in the neck or back. It typically does not result in radiculopathy, defined as irritation or injury of a nerve root, causing pain, weakness, numbness, or tingling in specific distributions depending on the location of the affected nerve root. Though it is certainly on the differential for this patient, his elevated ESR and CRP make osteomyelitis a more likely diagnosis.

Answer C: Herniated disc can cause a variety of symptoms depending on the position and size of the herniation. If the herniated disk impinges on a nerve, patients may have radiculopathy.

Answer D: Metastatic tumor is typically suspected in patients with back pain in the setting of a known malignancy. The most common primary cancers that metastasize to bone include breast, prostate, thyroid, lung, and renal cancer.

Bullet Summary:
Vertebral osteomyelitis typically presents as focal back pain, with or without fever, with an elevated erythrocyte sedimentation rate and C-reactive protein.