Compartment syndrome is a clinical diagnosis. The earliest sign will be pain out of proportion to exam, and notably pain with passive flexion of a muscle that runs through the compartment. The compartments in the affected area may be tense, hard, or firm. If the diagnosis is suspected and requires confirmation prior to operative planning, measure the intra-compartment pressure.
Normal compartment pressure is 10mmHg. Levels >30mmHg or within 30mmHg of the diastolic blood pressure are suggestive of compartment syndrome.
How do you measure intra-compartment pressure? Please view this excellent procedures video with Dr. John Sarwark with assistance from Dr. Michael Macias: