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What percentage of total body surface area is involved? The answer to this question is necessary in computing severity scores, such as the Psoriasis Area Severity Index (PASI), the Atopic Dermatitis Severity Index (ADSI), the SCORing Atopic Dermatitis Index (SCORAD), and the mSWAT (Modified Severity Weighted Assessment Tool) for measuring cutaneous T-cell lymphoma.
Traditionally, calculations of percentage of body surface involvement are made by (1) the palmar method (surface area of the palm including fingers and thumb is considered roughly equal to 1% of body surface area); (2) the Rule of Nines (the body is divided into areas considered to equal 9%, or a multiple of 9%, of the total area; see ); or (3) estimation from a Lund and Browder chart. The palmar method is best for small calculations of disease in small body areas, but it can be incorrect for larger areas, because the area of the palm including fingers and thumb depends on sex, weight, age, and other factors.
The authors discuss various systems of measurement and published studies related to calculation of body area. They conclude that a modified palmar method is the most convenient reliable measure for small disease areas. In the modified version, the area of the palm not including the fingers and thumb is assumed to constitute 0.5% of total body area. The authors have developed an iPhone app that is especially useful for mSWAT scoring of cutaneous T-cell lymphomas.
Scarisbrick JJ and Morris S. How big is your hand and should you use it to skin score? Br J Dermatol 2013 Apr 29; [e-pub ahead of print]. (http://dx.doi.org/10.1111/bjd.12403)
Comment
Poor estimation by prescribers of needed amounts of topical agents commonly leads to poor outcomes. Given too little cream, patients skimp and therefore undertreat. Too much cream is wasteful and leads to prolonged, unsupervised therapy and excessive cost. By my calculations, it takes roughly 0.1 g of cream to cover an area the size of the palm not including fingers and thumb. For areas larger than 10% total body area, the Rule of Nines method works better for me. When prescribing cream, considerations include frequency of application, vehicle, and description of how much to apply. Interpretations of the latter can vary. I keep a jar of emollient cream at bedside to demonstrate how much to apply and also to see for myself just how much cream that takes.
P.S. — Patients really seem to like the educational effort and the “laying on of hands.”