Diagnostic Testing and Imaging
Core concepts
Know the indications, limitations, and normal values for the tests you order. Plain radiographs remain first-line for bony trauma, arthritis, and foreign bodies; obtain weight-bearing views for alignment and stress views when ligamentous injury is suspected. Understand when ultrasound is preferred (soft tissue, tendon, vascular, foreign body), when MRI is indicated (osteomyelitis, soft tissue mass, occult fracture, ligament/tendon pathology), and when CT is best (complex fractures, preoperative planning). Know the classic MRI appearance of osteomyelitis (low T1, high T2 marrow signal) and that MRI may be unreliable near hardware. Bone scan and labeled leukocyte scanning are alternatives when MRI is contraindicated or hardware is present. Laboratory testing-CBC, ESR, CRP, uric acid, HbA1c, cultures-supports but rarely confirms a diagnosis alone.
Traps in questions
A frequent trap is ordering advanced imaging before plain films in acute trauma. Another is relying on a single normal test-a normal radiograph does not exclude osteomyelitis, and a normal ultrasound does not exclude deep vein thrombosis. Know that elevated ESR/CRP are sensitive but nonspecific. Beware of questions that ask for the most appropriate initial test versus the definitive test.
Rule of thumb
Start with the least invasive, least costly test that answers the clinical question; escalate only if it will change management.
Worked example
A patient with a diabetic ulcer and exposed bone has a normal plain film. The next best step is MRI (or bone scan with labeled leukocytes if hardware is present) to evaluate for osteomyelitis-a normal radiograph does not exclude it.