Burnham RS, Burnham TR. Effect of hand warming on electrodiagnostic testing results and diagnosis in patients with suspected carpal tunnel syndrome. Objectives To evaluate the effects of hand warming on electrodiagnostic (EDX) parameters of carpal tunnel syndrome (CTS). Specifically, to evaluate the type and magnitude of change in EDX values; to determine whether warming can change the EDX diagnosis of CTS and, if so, to determine whether specific EDX tests are more vulnerable to the effects of warming. Design Interventional before-after trial. Setting Community EDX laboratory. Participants Consecutive patients (N=33) referred for EDX evaluation of suspected CTS. Intervention Each upper limb was tested at room temperature (average hand temperature, 31.9°C) and retested after 20 minutes of heating pad warming (average hand temperature, 33.5°C). Main Outcome Measures Median and ulnar nerve motor and sensory latency, conduction velocity, and amplitude. Results Warming resulted in significantly faster transcarpal tunnel sensory and motor conduction (6%–10%), reduced median: ulnar sensory latency to digit 4 difference (23%), and reduced sensory nerve and compound motor action potential amplitude (10% and 13%, respectively). The changes resulted in 15% fewer limbs meeting the EDX conduction velocity criteria of CTS and 9% fewer by median: ulnar sensory latency to digit 4 difference. Waveform amplitudes became abnormally low after warming in 12% of limbs. Conclusions Hand warming using a heating pad for 20 minutes increases median nerve transcarpal tunnel conduction velocity and reduces amplitude. These phenomena introduce potential sources of diagnostic error, particularly in borderline cases. Generally, within hand, between nerve, and within nerve comparison techniques are less susceptible to the conduction velocity effects of warming.