What Are Delphi Criteria and PARACELSUS Scores for PG?

Pyoderma gangrenosum (PG) is notoriously difficult to diagnose because no single blood test or skin swab can definitively prove you have it. Instead, doctors must act like detectives, gathering clues from your medical history, symptoms, and how your skin reacts to treatment ^[1].

To make this process more structured, medical experts developed two commonly used clinical tools: the Delphi criteria and the PARACELSUS score. These are not definitive pass/fail tests. Instead, they are frameworks that help your care team organize evidence, assess how compatible your wound is with PG, and distinguish it from lookalike conditions ^[2].

(Note: These frameworks were primarily designed and validated for classic ulcerative PG. They may not apply equally to other subtypes, such as bullous or pustular PG, which require specialist clinical judgment.)

The Delphi Criteria

The Delphi criteria were created by an international panel of experts. To meet the threshold for a classic ulcerative PG diagnosis under this framework, the clinical picture must include one “major” feature and at least four “minor” features ^[2].

The Major Criterion:

• A supportive biopsy result: A small sample of the ulcer edge (histopathology) shows a presence of neutrophils (a type of white blood cell involved in inflammation) ^[2]. (Note: this type of inflammation is not unique to PG and can be seen in other conditions).

The 8 Minor Criteria:

1. Infection is not the primary cause: Clinical assessment determines that an infection does not better explain the ulcer ^[2]. (PG ulcers can have surface bacteria, but a deep infection isn’t the root cause).
2. Pathergy: Ulcers form or worsen at sites of minor skin trauma, like a bump, surgical cut, or needle prick ^[2].
3. Related diseases: You have a history of an inflammatory disease, such as inflammatory bowel disease (Crohn’s or ulcerative colitis) or inflammatory arthritis ^[2].
4. Rapid start: A small bump, blister, or pimple rapidly turned into an ulcer within 4 days ^[2].
5. Classic appearance: The ulcer is painful and has a red border that is “undermined” (the skin at the edge forms a lip or overhang above the wound base) ^[2].
6. Location and number: You have multiple ulcers, with at least one located on the front of your lower leg ^[2].
7. Specific scarring: Old, healed PG ulcers left behind “cribriform” scars (scars that look indented or like wrinkled paper) ^[2].
8. Treatment response: The ulcer shrinks within one month of starting immune-suppressing medication ^[2].

What the numbers mean: When tested against a specific group of patients, requiring the major criterion plus four minor criteria had an 86% sensitivity (meaning the tool correctly caught 86% of true PG cases) and a 90% specificity (meaning it correctly identified 90% of non-PG cases) ^[2]. However, these are study statistics, not a guarantee of your individual probability.

The PARACELSUS Score

The PARACELSUS score is a point-based system ^[3][4]. Doctors assign points for each symptom you have and add them up.

Major Criteria (3 points each):

• Rapid progression: The disease and ulcer are growing quickly ^[3][4].
• Assessment of mimics: Your doctor has actively assessed for and evaluated other relevant causes ^[3][4].
• Distinct color: The wound border is a reddish-violaceous (red-purple) color ^[3][4].

Minor Criteria (2 points each):

• Medication response: The wound improves (amelioration) when treated with immune-suppressing drugs ^[3][4].
• Irregular shape: The ulcer has a highly irregular outline (described in the criteria as “bizarre”) ^[3][4].
• Extreme pain: You rate your pain as greater than a 4 out of 10 ^[3][4].
• Pathergy: The lesion is located exactly where your skin was traumatized ^[3][4].

Additional Criteria (1 point each):

• Biopsy results: Histopathology shows suppurative (pus-forming) inflammation ^[3][4].
• Undermined borders: The deep wound edges overhang the base ^[3][4].
• Associated diseases: You have a linked systemic condition (like inflammatory bowel disease) ^[3][4]. (Note: Not having these diseases does not rule out PG).

Scoring the Results:
In the original study, a total score of 10 or higher indicated a high likelihood of PG ^[3][4]. A later, larger international study involving over 1,400 wounds found that using a slightly stricter cutoff of more than 10 points (meaning 11 or higher) improved the tool’s accuracy in that specific study group, correctly classifying 96.8% of other wound types (specificity) and capturing 98.3% of PG cases (sensitivity) ^[5].

Why Clinical Judgment Still Matters

The Delphi and PARACELSUS frameworks are excellent aids, but they are not standalone answers. They are designed to guide clinical judgment.

1. Ruling Out Lookalikes is Mandatory

Excluding lookalike conditions (mimics)—such as bacterial or fungal infections, blood vessel disorders, or skin cancer—is the most critical step before confirming PG ^[6][1]. If a deep infection is misdiagnosed as PG, treating it with heavy immune-suppressing drugs will cause the infection to rapidly and dangerously worsen ^[7][6].

2. The Role and Risks of a Biopsy

While both frameworks look at skin biopsies for clues, a biopsy alone cannot confirm PG ^[8]. The neutrophilic inflammation seen in a PG biopsy is supportive, but it can look exactly like the inflammation seen in other diseases ^[8].
Additionally, because of pathergy, the physical trauma of taking a biopsy can sometimes cause a PG ulcer to grow larger ^[2][8]. However, you should not refuse a necessary biopsy just because of pathergy. Your care team carefully weighs this risk because a biopsy is often the only way to rule out dangerous mimics like cancer or severe infections.

3. Treatment Response is Not a “Test”

Both frameworks give points if the ulcer improves on immune-suppressing drugs. However, some non-PG inflammatory ulcers also improve with these medications. Never start, stop, or change medications just to “test” your score. Response to medication is only one piece of the puzzle and should be evaluated by your doctor over time.

⚠️ When to Seek Urgent Care

Rapid progression is a symptom of PG, but it is also a sign of a serious infection or emergency. Seek urgent clinical assessment if your ulcer rapidly spreads, if you develop a fever or confusion, if the area becomes suddenly swollen or significantly more painful, or if you feel very unwell.