Topic 4: Use IHS4 to Measure Inflammatory Activity
The International Hidradenitis Suppurativa Severity Score System, or IHS4, is a validated dynamic measure of inflammatory disease activity. It uses three lesion types:7
- Inflammatory nodule: 1 point each
- Abscess: 2 points each
- Draining tunnel: 4 points each
IHS4 formula
| IHS4 = number of inflammatory nodules + (number of abscesses × 2) + (number of draining tunnels × 4) |
Interpreting the score
- Mild inflammatory disease: 3 points or fewer
- Moderate inflammatory disease: 4–10 points
- Severe inflammatory disease: 11 points or more
Different paths to an IHS4 score of 4
- Four inflammatory nodules
- Two abscesses
- One draining tunnel
- Two inflammatory nodules and one abscess
These examples illustrate why lesion morphology matters. A draining tunnel is weighted more heavily than an inflammatory nodule because it represents a more complex manifestation of disease.
Important limitations
- IHS4 does not count nondraining tunnels.
- It does not directly measure scarring or other permanent structural damage.
- It does not incorporate pain, flare frequency, drainage burden, or quality of life.
- A low score on a quiet examination day may not reflect usual disease activity.
- The same score may represent different lesion combinations with different clinical implications.
IHS4 is best used as one component of a broader assessment, particularly for tracking inflammatory activity over time.4
| Clinical pearl: While lesions carry weighted scores on the IHS4, a score of 4 is the threshold for moderate disease, so any patient who has had 4 active lesions at any given time has moderate disease. |