A 67-year-old man with poorly controlled type 2 diabetes presents to the Emergency Department with a 24-hour history of rapidly worsening pain and swelling of his right lower leg. He sustained a small puncture wound to his shin three days ago while gardening. He reports that the pain is now "excruciating" and seems disproportionate to the visible inflammation. He feels generally unwell with fever and chills that began 12 hours ago.

On examination, there is diffuse erythema and oedema of the right lower leg with indistinct borders. The skin appears tense and shiny with scattered bullae containing serous fluid. There are areas of skin discolouration with dusky patches and early bruising not related to trauma. The patient experiences severe pain on light palpation of the affected area that extends beyond the visible margins of erythema. Crepitus is felt on deep palpation. There is no visible fluctuance or obvious abscess.

Which diagnostic approach is most appropriate to differentiate between necrotising fasciitis and cellulitis in this patient?