Many clinical studies have confirmed that ELISPOT can be up to 200 times more sensitive than ELISA. T-SPOT.TB requires just a few spots to give a positive result (one spot = one cell).
However on their USA site they say that
Borderline (equivocal) results are those where the maximum of the two (Panel minus Nil) spot count results are within ±1 spots from the ROC-determined assay cutoff of ≥6 spots. Borderline (equivocal) results, although valid, are less reliable than results where the spot count is further from the cut-off. Retesting of the patient, using a new sample, is therefore recommended.
If the result is still Borderline (equivocal) on retesting, then other diagnostic tests and/or epidemiologic information should be used to help determine TB infection status of the patient.
By their own measure a positive T-Spot TB test can be borderline (equivocal).
