In fact, one of the hallmarks of small-fiber neuropathy of unknown or idiopathic cause is that often, all blood and other tests (i.e., nerve conduction/EMG of the larger fibers) are negative or normal, and that's why it takes specialized testing such as skin biopsy or qualitiative sensory testing to notice dysfunction of the small nerve fibres.

The doctor may mean that you show no blood test indicationos of vasculitis, toxicity, or impaired glucose tolerance, to name three common causes of small-fiber disruption--but that doesn't mean you can't have such disruption. The skin biopsy is really still the gold standard, as it directly looks for damage to the fibers (though there are other experimental technologies in the pipeline to try to detect such damage).