To have no reaction on a histamine test you would have to be on antihistamines OR have a malfunctioning innate immune system _and_ malfunctioning vasculature an

To have no reaction on a histamine test you would have to be on antihistamines OR have a malfunctioning innate immune system _and_ malfunctioning vasculature and/or severe malnutrition to the point where you're at deaths door (possibly not even then, those extreme cases aren't usually considered when administering allergen tests). Assuming the dose is controlled the result is a small, well pronounced welt ~0.8mm diameter. It's very consistent, you know that it will occur (because it's the direct primary mediator), that why you use it as a positive control (also you control for it, thats the point of a positive control). In answer to you question, barring the use of antihistamines, pretty much.