The Severity of Alopecia Tool (SALT) was published in 2004 by a working group of investigators convened by the National Alopecia Areata Foundation (Olsen et al. 2004) as part of the second alopecia areata investigational assessment guidelines, which built on the baseline staging system of the 1999 guidelines. The scalp is assessed in four views, and the surface share of each view was determined by image analysis of a mannequin: the top of the scalp represents 40%, each side 18% and the back 24%. The visual aid published with the guidelines maps the views onto a diagram so that patches of hair loss can be drawn and compared between visits; the diagram is copyrighted material and is not reproduced here.

For each view the percentage of terminal hair loss is estimated and multiplied by the surface share of that view, and the SALT score is the sum of the four products, from 0 (no hair loss) to 100 (complete scalp hair loss). In the published example, 95% loss on the left side (17.1), 90% on the right side (16.2), 95% on the top (38.0) and 55% at the back (13.2) give a SALT score of 84.5. Only terminal hair is counted: vellus and intermediate hairs are not credited as coverage, although their presence may be recorded separately. Hair loss from androgenetic alopecia in the same areas cannot be separated from alopecia areata by the score, and diffuse thinning without discrete patches is captured poorly.

The guidelines subclassify the score into S0 (no hair loss), S1 (less than 25%), S2 (25% to 49%), S3 (50% to 74%), S4 (75% to 99%, divided into S4a at 75% to 95% and S4b at 96% to 99%) and S5 (100%, alopecia totalis). In the workshop that tested the tool, assigning the subclass by eye and then estimating the percentage produced discordant readings, so the recommended sequence is to determine the SALT score first and derive the subclass from it. Body hair (B0 to B2) and nail involvement (N0 to N1) are recorded alongside the score but do not enter it. SALT II (Olsen and Canfield 2016) refined the diagram into 1% segments so that small patches are easier to estimate; the arithmetic is unchanged.

Response is expressed as the absolute change or the percentage change from baseline, which the guidelines write as a subscript of the score: SALT50 denotes a 50% reduction from the baseline score, and the same convention gives SALT75 and SALT90. The guidelines suggested SALT50 as an acceptable endpoint for trials of systemic agents in extensive disease. Contemporary phase 3 trials of Janus kinase inhibitors instead define treatment success as an absolute SALT score of 20 or less, that is, at least 80% of the scalp covered by terminal hair, a threshold established through qualitative work with patients and clinicians (Wyrwich et al. 2020, 2023) and used as the primary endpoint of the baricitinib trials (King et al. 2022); a SALT score of 10 or less is a common secondary endpoint. A baseline SALT score of 50 or more defines severe disease for trial eligibility and is the threshold adopted in Canadian reimbursement recommendations for these agents.

The SALT score measures scalp hair loss only. It does not capture eyebrow, eyelash or body hair loss, nail changes or the psychological burden of the disease, and the multidimensional severity framework of King et al. (2022) supplements it with those factors, with disease duration and with the response to prior treatment. Estimates vary between observers, particularly in patchy disease, so consecutive visits are best scored by the same assessor with the four views photographed under standard conditions where possible.