The Hidradenitis Suppurativa Physician's Global Assessment (HS-PGA) was developed for the phase 2 adalimumab programme and published with the trial in 2012 (Kimball et al. 2012). It is a six-grade global scale, from clear (0) to very severe (5), assigned once per patient after all affected regions have been examined. Unlike the Hurley classification, which is static and describes the architecture of scarring and sinus tracts, the HS-PGA is defined entirely by the counts of active lesions at the visit, so it can change from one visit to the next.

Four lesion types are counted across all regions: non-inflammatory nodules, inflammatory nodules, abscesses and draining fistulas. Abscesses and draining fistulas are pooled into a single count. A patient with no lesions is clear, and a patient with only non-inflammatory nodules is minimal. With no abscesses or fistulas, 1 to 4 inflammatory nodules is mild and 5 or more is moderate. One abscess or fistula is mild without inflammatory nodules and moderate with any. Two to five abscesses or fistulas is moderate with fewer than 10 inflammatory nodules and severe with 10 or more, and more than five abscesses or fistulas is very severe regardless of the nodule count. This calculator applies those definitions to the entered counts and highlights the grade that is met.

The primary endpoint of the phase 2 trial was the HS-PGA response: a grade of clear, minimal or mild with at least a 2-grade improvement relative to baseline at week 16, which was reached by 17.6% of patients on weekly adalimumab and 3.9% on placebo. A post hoc analysis of the same dataset derived the Hidradenitis Suppurativa Clinical Response (HiSCR), a 50% reduction in the abscess and inflammatory nodule count with no increase in abscesses or draining fistulas (Kimball et al. 2014). In a direct comparison, HiSCR was more responsive to change than the HS-PGA response, whose stringent threshold contributed to the modest phase 2 result (Kimball et al. 2016), and HiSCR replaced it as the primary endpoint of the PIONEER phase 3 trials and of subsequent biologic programmes.

The HS-PGA remains useful as a global severity descriptor and appears as a secondary endpoint and an eligibility criterion (moderate or worse) in current trials. Its limitations follow from its design: it collapses many lesion counts into six grades, so improvements within a grade are invisible and a floor of clear is rarely reached; the pooled abscess and fistula count treats a draining tunnel like an abscess; no correspondence to Hurley stage was defined, and the trial enrolled patients across Hurley stages I to III; and a systematic review of hidradenitis suppurativa outcome measures found that only HiSCR carried good-quality validation data (Ingram et al. 2016). The IHS4 is the preferred dynamic severity score, and Hurley staging remains the standard for surgical planning (Zouboulis et al. 2015).