Explain how medical interventions are used in the diagnosis of disease.

I think the simplest intervention that used to diagnosis is "fluid challenge test".

Patient with too much water and lack water can give similar symptoms like a drop in blood pressure. Fluid challenge test is done by giving fluid to the patient with low blood pressure. If the intervention improves patient condition it means that he was losing blood volume. But if the condition worse then the patient was having too much water (overload).