What does it mean when an f wave is absent on an emg/nvc?

Absence of F wave means there is a conduction abnormality when the subject has EMG-NCS done for the diagnosis of peripheral neuropathy.

The nerve being tested is not conducting an electrical impulse that results in a muscle action. It indicates a neuropathy. There are many causes of neuropathy: mechanical (carpal tunnel syndrome, etc.), metabolic (diabetes, etc.), toxic, and others.

Absent F wave, in the presence of normal nerve conduction velocity and amplitude is often a subtle sign of early peripheral neuropathy but will need clinical correlation and neurological follow-up to determine implications.