Drugs Used as Diagnostic Agents

Introduction

• Definition — A diagnostic agent is given not to modify disease but to generate information — make a structure visible, unmask a latent functional abnormality, or quantify an organ's capacity. The recipient gains no therapeutic benefit from the exposure, so the risk–benefit calculus differs fundamentally from that of a therapeutic drug.
• Three mechanistic families — Contrast and tracer agents act physically — x-ray attenuation, proton relaxivity, gamma or positron emission, acoustic backscatter — dosed in grams or millicuries; provocative agents act at receptors or enzymes in ordinary pharmacological doses; clearance markers should ideally have no pharmacological action at all.
• Diagnostic and therapeutic doses differ by orders of magnitude — The most examinable feature of the topic. Radioiodine is the archetype — diagnostic 25–100 µCi (tracer only, no follicular damage) against therapeutic 3–6 mCi for hyperthyroidism: a 50–100-fold separation.
• Provocation is controlled iatrogenesis — Bronchial challenge demonstrates hyperresponsiveness by causing bronchoconstriction; stress testing demonstrates ischaemia by causing supply–demand mismatch; cosyntropin demonstrates adrenal reserve with a supraphysiological ACTH stimulus. Each test's contraindication list is simply the list of patients who cannot tolerate the induced abnormality. All radiation use follows ALARA.