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DR3.2 | Grattage Test Technique — SDL Guide

Learning Objectives

• State the clinical indications and contraindications for performing the Grattage test.
• Explain the structural basis (parakeratotic scale, thinned suprapapillary epidermis, dilated dermal capillaries) underlying each grattage sign.
• Demonstrate the graded scraping technique that sequentially elicits the candle-grease sign, the last (glistening) membrane, and the Auspitz sign.
• Interpret positive, partial, and negative grattage findings and use them to support or refute a diagnosis of psoriasis.
• Apply the test to worked clinical scenarios and recognise common technical errors.

INSTRUCTIONS

The Grattage test (Brocq's method) is a simple, instrument-light bedside skill that converts a vague 'scaly red plaque' into a confident diagnosis of psoriasis. Where the recognition module (DR3.1) taught you to read the plaque, this module teaches you to perform a defined procedure correctly — to scrape in a graded way, to observe three sequential endpoints, and to interpret what they mean. Done well it is diagnostic and reassuring; done carelessly (with a sharp blade, on the wrong skin, or stopping too early) it misleads. This is a hands-on skill SDL: master the sequence, the instrument, and the interpretation.

References

• IADVL Textbook of Dermatology, Section on Clinical Methods — Grattage / Brocq's method (textbook)
• Neena Khanna, Illustrated Synopsis of Dermatology & STD, Ch. on Psoriasis (bedside signs) (textbook)

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Version 2.0 | NMC CBME 2024

Clinical Indications for Grattage Testing

Clinical Indications and Cautions for Grattage Testing

The Grattage test earns its place in clinical practice as a quick, low-cost way to resolve diagnostic uncertainty at the bedside, and knowing when to reach for it — and when not to — is the first part of competence. Its principal indication is the differentiation of suspected psoriasis from its erythematosquamous mimics, most often seborrhoeic dermatitis, but also from eczema or other scaly plaques where the diagnosis is not visually obvious. It is especially useful in the outpatient setting before committing a patient to a treatment pathway, and as a teaching demonstration of the elegant link between psoriatic histology and clinical signs. Just as important is recognising when the test is unsafe or unhelpful, because applying it indiscriminately can harm the patient or mislead you.

Indications:

• Suspected psoriasis where the plaque morphology is atypical or overlaps with seborrhoeic dermatitis or eczema.
• Confirmation of a clinical impression before starting treatment in the OPD.

Contraindications and cautions:

• Acute inflamed, pustular, or erythrodermic psoriasis — do not scrape inflamed or unstable skin, as it may aggravate the disease and provoke Koebnerisation.
• Active secondary infection of the lesion.
• Bleeding tendency or anticoagulant therapy (the Auspitz endpoint involves capillary bleeding).
• Already excoriated or treated plaques, where prior trauma or therapy invalidates the result.

Principle and Materials for Grattage

Principle of Grattage in Psoriasis

The whole test is an applied lesson in pathology, and understanding the principle is what lets you interpret the findings rather than merely produce them. In psoriasis, the surface scale is parakeratotic — keratinocytes have raced to the surface so fast that they retain their nuclei and pile up as loose, silvery, weakly adherent layers; these strip away cleanly to give the candle-grease sign. Beneath the scale lies a thin film, and below that the suprapapillary epidermis is reduced to just two or three cell layers, so that the dilated capillaries within the elongated dermal papillae sit abnormally close to the surface. Gentle removal of that final film exposes these vessels, which bleed as discrete points — the Auspitz sign. The principle therefore demands a graded, controlled scrape with a blunt instrument, because the diagnostic information lies in the sequence of layers, not in how hard you can dig.

Psoriatic Plaque Cross-Section and Grattage Signs

Materials required:

• A blunt instrument — a glass microscope-slide edge, a wooden spatula, or a blunt curette. A sharp blade is forbidden: it lacerates skin and produces traumatic bleeding that mimics — but is not — a true Auspitz sign.
• Good lighting and a magnifying lens if available.
• Clean gloves and gauze.
• A fresh, well-developed, untreated plaque to test on.

Step-by-Step Grattage Technique

Step-by-Step Grattage Technique in Psoriasis

Performing the Grattage test well is about graded, patient scraping and disciplined observation at each layer, because the diagnosis is read from a sequence of three endpoints that must each be looked for in turn. Stopping after the first sign — as inexperienced examiners often do — misses the most specific finding, while scraping too hard skips the orderly layers and produces a wound rather than a sign. Select a mature, untreated plaque on an accessible extensor site such as the elbow or knee, position the patient comfortably, explain in plain language what you are about to do and obtain consent, and put on gloves. Hold your blunt instrument tangentially to the skin and decide in advance that you will pause and observe after each pass rather than scraping continuously to the point of injury. The discipline of advancing exactly one layer at a time, and looking before you scrape again, is what makes the test both diagnostic and safe.

The graded sequence:

1. With the blunt edge held tangentially, scrape the scale in one direction using light, even pressure. Silvery-white flakes lift off like shavings of wax — this is the candle-grease sign (the first endpoint).
2. Continue gentle scraping until the loose scale is fully removed, revealing a thin, smooth, glistening last membrane (also called the terminal film or Büchner sign) — the second endpoint.
3. Now scrape this membrane very gently. Discrete pinpoint bleeding spots appear from the exposed dilated dermal papillary capillaries — the Auspitz sign, the third and most specific endpoint.

Look for all three, in order. Use only the pressure needed to advance one layer at a time; force converts a diagnostic test into a wound.

Grattage Test in Psoriasis: Candle-Grease, Büchner, and Auspitz Signs