Diagnostic agents
Products given to make a disease visible or measurable: dyes, X-ray contrast media and skin-test antigens. They do not treat anything.
What they are
A mixed group from the WHO Essential Medicines List: ophthalmic dyes (fluorescein), radiocontrast media (iohexol, barium sulfate, amidotrizoate) and skin-test antigens (tuberculin PPD).
Mechanism of action
Each one works physically or immunologically, not on a drug target: fluorescein glows under blue light, iodine and barium block X-rays, and tuberculin calls up a delayed T-cell reaction in sensitised people.
Target: None: optical, radiological or immunological effects
Drugs in this class
- FluoresceinAn orange dye that glows green under blue light. On the eye it shows corneal scratches; injected, it outlines the…No receptor: a fluorescent dye (absorbs blue light, emits yellow-green light)
- IohexolA non-ionic iodinated contrast medium for CT, angiography and other X-ray studies. Leaves unchanged through the kidneys.No pharmacological target: its iodine atoms block X-rays
- Tuberculin purified protein derivative (PPD)A protein extract of tuberculosis bacteria injected into the skin. A firm swelling 2–3 days later suggests infection…Memory T cells sensitised to Mycobacterium tuberculosis (delayed, type IV hypersensitivity)
- TetracosactideSynthetic ACTH (its first 24 amino acids) used for the ACTH stimulation test: if cortisol does not rise, the adrenal…Melanocortin-2 receptor (ACTH receptor) on adrenal cortex cells
Others in the class
- Barium sulfate
- Amidotrizoate
- Iodixanol
Test yourself
Think of the answer, then tap the question to see it.
Which diagnostic agent stains corneal abrasions?
Fluorescein.
Which kidney-related risk links iodinated contrast and metformin?
Contrast can injure the kidney, metformin then accumulates and can cause lactic acidosis.
What type of hypersensitivity does the tuberculin test rely on?
Delayed (type IV), T-cell mediated hypersensitivity.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - FLUORESCEIN injection (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - IOHEXOL injection, solution (DailyMed (U.S. National Library of Medicine), 2026); Clinical Testing Guidance for Tuberculosis: Tuberculin Skin Test | Tuberculosis (TB) | CDC (U.S. Centers for Disease Control and Prevention (CDC), 2026).
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Representative model, for educational purposes only.