What Actually Counts as an HS Flare? The New Consensus Definition

You probably know exactly what it feels like when your skin gets worse. Whether that feeling has a precise, shared definition is a different question — and for a surprisingly long time, dermatologyDermatology: The medical specialty concerned with diagnosing and treating skin conditions. Acne Inversa is often managed by dermatologists, although surgery and other specialties may also be involved.DermatologyThe medical specialty concerned with diagnosing and treating skin conditions. Acne Inversa is often managed by dermatologists, although surgery and other specialties may also be involved.Read more in the glossary did not really have a clear answer.

The word “flareFlare: A period of acute worsening in a chronic condition. In Acne Inversa, a flare may be triggered by stress, hormonal changes, friction, or other factors and can manifest as new nodules, abscesses, or increased pain.FlareA period of acute worsening in a chronic condition. In Acne Inversa, a flare may be triggered by stress, hormonal changes, friction, or other factors and can manifest as new nodules, abscesses, or increased pain.Read more in the glossary” is one of the most commonly used terms in daily HS life — in forums, in appointments, in trackers, on this blog itself. Yet research has used it very inconsistently for years — sometimes meaning a single new lesion, sometimes an entire bad week, sometimes simply “worse than usual.” The new European S2k guideline for hidradenitis suppurativa/acne inversa (Part 1, published 2026) dedicates a section to this term for the first time and assigns it a working definition.

This article explains what that definition says, what it deliberately leaves open, and what a flare does not automatically mean — because those exact misreadings are often what causes the most worry in daily HS life.

For information only. This article explains a working definition drawn from current guideline literature. It cannot establish whether your current symptoms represent a flare, and it does not replace an individual clinical assessment.

Key takeaways

• “Flare” gets used constantly in daily HS life, but research has long defined it inconsistently.
• The new European guideline (Part 1, 2026) sets out a working definition for the first time: a new or substantial worsening of signs or symptoms.
• Signs are what’s visible or measurable (a new lesion, more drainage); symptoms are what you feel yourself (mainly pain).
• A flare does not automatically mean permanent worsening, a changed Hurley stageHurley Stage: A regional, mainly structural classification system for Acne Inversa in one specific body region, in three stages: Stage I (single abscesses without sinus tracts), Stage II (recurrent abscesses with isolated, still-separated sinus tracts), Stage III (extensive, interconnected sinus tracts and scarring across an entire region). Named after Dr. Helen Hurley. Classic Hurley staging is static and isn't designed to reflect short-term inflammatory activity or response to medical treatment — IHS4 is used alongside it for that.Hurley StageA regional, mainly structural classification system for Acne Inversa in one specific body region, in three stages: Stage I (single abscesses without sinus tracts), Stage II (recurrent abscesses with isolated, still-separated sinus tracts), Stage III (extensive, interconnected sinus tracts and scarring across an entire region). Named after Dr. Helen Hurley. Classic Hurley staging is static and isn't designed to reflect short-term inflammatory activity or response to medical treatment — IHS4 is used alongside it for that.Read more in the glossary, treatment failure, or infection.
• The guideline sets no fixed threshold (how many days, how much of a pain increase) — that stays an individual judgement.
• Knowing what specifically counts as a flare also has practical consequences: some treatment decisions are tied to flare frequency.

Why “flare” has been so hard to pin down

The problem is not that people with HS don’t know when things get worse. The problem is that “worse” can mean many different things: a single new lesion, several at once, more pain without a new lesion, more drainage, or simply an overall worse feeling over several days.

In HS research, “flare” (or “exacerbation”) has been used differently across different studies for years, often without authors ever stating exactly what they meant by it. That is not a minor detail: when a study reports on “flare frequency” but doesn’t say what counted as a flare, its results are hard to compare with other studies — and even harder to apply to any one individual. That inconsistency is a reason the new guideline gives this term its own section at all, rather than continuing to assume everyone already agrees on what it means.

What the new working definition says

The new guideline describes a flare as a new or substantial worsening of signs or symptoms. Two categories sit inside that one sentence, and the distinction is more useful than it first sounds:

Signs — what is visible or measurable:

• a new inflammatory lesion (noduleNodule: A firm, palpable lump deep in tissue. Nodules can be painful and may progress to abscesses. They are among the typical early signs of Acne Inversa.NoduleA firm, palpable lump deep in tissue. Nodules can be painful and may progress to abscesses. They are among the typical early signs of Acne Inversa.Read more in the glossary, abscessAbscess: An inflamed cavity or swelling containing pus or fluid. In hidradenitis suppurativa, abscesses can form as part of the inflammatory disease process and do not automatically mean that a bacterial infection is present. Secondary bacterial infection can occur and may require separate medical assessment. In Acne Inversa, abscesses occur mainly in the armpits, groin, and other skin folds.AbscessAn inflamed cavity or swelling containing pus or fluid. In hidradenitis suppurativa, abscesses can form as part of the inflammatory disease process and do not automatically mean that a bacterial infection is present. Secondary bacterial infection can occur and may require separate medical assessment. In Acne Inversa, abscesses occur mainly in the armpits, groin, and other skin folds.Read more in the glossary);
• an existing lesion getting worse;
• increased drainage or swelling.

Symptoms — what you feel yourself, without it necessarily being visible from outside:

• pain;
• other meaningful worsening of symptoms.

Importantly, you don’t need both at once. A single new lesion can already meet the definition, just as a substantial increase in pain can, without any new visible change. That lines up fairly closely with what many people with HS already knew from experience — just without a shared word for it until now.

What the definition deliberately does not do: it sets no exact threshold — not how many days something needs to last, not how many points pain needs to rise by, not whether one lesion or several are required. That is not an oversight; it reflects how differently HS shows up from person to person. Where the line between “a bit worse” and “a flare” sits in your own case stays an individual judgement — ideally one you work out together with your care team.

What a flare does not automatically mean

This is exactly where most unnecessary worry comes from, so it is worth stating plainly. A flare does not automatically mean that:

• your disease has permanently worsened;
• your Hurley stage has changed;
• your treatment isn’t working;
• you caused it yourself;
• an infection is present.

Each of these deserves a short explanation, because the obvious-seeming conclusion is often the wrong one.

No automatic permanent worsening. A flare is, by definition, something new or worsened — it says nothing about whether that state persists. Most flares settle back down.

No automatic change in Hurley stage. Hurley staging describes your disease’s underlying pattern (isolated abscesses, tunnelsSinus Tract: A tunnel-like channel under the skin that forms between abscesses or nodules. Sinus tracts indicate more advanced disease and can chronically drain fluid. They are associated with Hurley Stages II and III.Sinus TractA tunnel-like channel under the skin that forms between abscesses or nodules. Sinus tracts indicate more advanced disease and can chronically drain fluid. They are associated with Hurley Stages II and III.Read more in the glossary with scarring, diffuse involvement) — not the activity on any single day. An acute flare doesn’t automatically change that staging.

No automatic proof of treatment failure. Individual flares occur even under effective treatment; most therapies reduce overall disease activity without ruling out every single flare. Only a pattern of repeated flares is a good reason to review treatment with your team — not one single episode.

No automatic blame. Some flares can be linked to a trigger (heat, friction, your cycle, stress), but many can’t. Blaming yourself for it is neither helpful nor supported by the evidence.

No automatic sign of infection. HS flares are usually a sterile inflammatory reaction rather than a bacterial infection — even though pain, redness and drainage can resemble infection. Fever, rapidly spreading redness, or feeling significantly unwell are different signals that warrant medical assessment.

Why this matters beyond terminology

What counts as a flare is not just an academic question — it has real treatment consequences. According to PubMed, a recent expert consensus (a Delphi process from the European Hidradenitis Suppurativa Foundation, 2025/2026) on criteria for earlier first-line biologicBiologic: A class of medications derived from living cells that target specific components of the inflammatory process. For Acne Inversa, biologics such as adalimumab and secukinumab are used when other treatments are insufficient.BiologicA class of medications derived from living cells that target specific components of the inflammatory process. For Acne Inversa, biologics such as adalimumab and secukinumab are used when other treatments are insufficient.Read more in the glossary therapy explicitly names frequent flares (≥3 in 12 weeks) as one criterion that can support moving to biologic treatment sooner (DOI). That shows that even without one single rigid clinical definition, flare frequency is already being used today as a concrete decision factor — one more reason it’s worth not just living through flares, but keeping a record you can actually follow.

How you can use this for your own tracking

The split between signs and symptoms gives you concrete language for next time something changes. Instead of “it’s worse,” you can note:

• Did a new lesion appear, or did an existing one change?
• Did drainage or swelling change?
• Did pain change — and if so, by how much?
• Did other symptoms appear or worsen?

That is more useful than a single number on a scale, because it describes what changed, not just that something got worse. The Symptom Tracker gives you a place to record that as it happens; the companion HS Flare Plan helps you note, ahead of time, what has already been agreed with your care team for the next flare.

One practical note on how to read this: the Symptom Tracker records these as two separate things, on purpose. It reports “days with pain ≥5” as a plain quantitative pain metric, and it asks you directly whether a day felt newly or substantially worse — the same working definition described above — rather than guessing at it from your pain score, drainage, or lesion count. The two numbers can differ, and that’s expected: a high-pain day is not automatically a flare, and a flare can happen on a low-pain day.

What the evidence does not show

Current evidence does not show that:

• there is one single, universally validated threshold at which something officially counts as a flare;
• flare frequency is distributed or predictable in the same way for everyone with HS;
• a single flare says anything about the long-term course of your disease;
• every flare has an identifiable, nameable cause.

The bottom line

“Flare” has been one of the most commonly used and least precisely defined words in daily HS life. The new European guideline doesn’t fix that with a rigid formula, but with a simple, broadly applicable idea: a new or substantial worsening of signs or symptoms. That’s not a checklist to tick off — it’s a vocabulary, one that describes what actually changed more precisely than “it’s worse” ever could. And that precision is exactly what makes a medical conversation, a tracker entry, or a treatment decision more useful.