>Not everyone will have the same reaction...

Is there even a single substance on earth this doesn't apply to?

Yes, think substances used for positive controls, e.g. histamine for skin-prick tests (allergen response).

>...substances used for positive controls, e.g. histamine for skin-prick tests...

Are you 100% certain that every single person, regardless of age, health, or any other criteria will always have the exact same response to these, under every conceivable circumstance and environment?

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.