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# 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.

Updated Nov 1, 2026

## 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

## Key points

- Fluorescein + blue light shows corneal abrasions in green.
- Iodinated contrast: watch kidneys, metformin, thyroid and allergy.
- Only presentations approved for intrathecal use go into the spinal canal.
- Tuberculin is read by induration at 48–72 hours; BCG causes false positives and live vaccines false negatives.

## Drugs in this class

- [**Fluorescein** An 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)](https://durgical.com/en/pharmacology/fluorescein/)
- [**Iohexol** A 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](https://durgical.com/en/pharmacology/iohexol/)
- [**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)](https://durgical.com/en/pharmacology/tuberculin-purified-protein-derivative/)
- [**Tetracosactide** Synthetic 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](https://durgical.com/en/pharmacology/tetracosactide/)

## 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.

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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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0a2a65b1-dc88-445b-bfb7-53b2fd7bea2c) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - IOHEXOL injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ff9456ef-c45a-450d-9004-a684af2bcc59) (DailyMed (U.S. National Library of Medicine), 2026); [Clinical Testing Guidance for Tuberculosis: Tuberculin Skin Test | Tuberculosis (TB) | CDC](https://www.cdc.gov/tb/hcp/testing-diagnosis/tuberculin-skin-test.html) (U.S. Centers for Disease Control and Prevention (CDC), 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/classes/diagnostic-agents/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/classes/diagnostic-agents/