About 30% of patients with daily pots symptoms have a negative tilt table test. There is more on this subject on pubmed, but the gist is: (quote)
Chronic day-to-day symptoms of orthostatic intolerance are the most notable features of postural orthostatic tachycardia syndrome (POTS). However, we have encountered patients with such symptoms and excessive tachycardia but with no symptoms during the tilt-table test (TTT)
Significant patients with POTS did not present orthostatic symptoms during the TTT despite having chronic daily symptoms. This finding may be important for establishing definitive diagnostic criteria for POTS.
POTS diagnosis may be difficult in clinical practice because interpretation of the phrase "orthostasis associated with symptoms of orthostatic intolerance" varies in the literature.