The ‘handprint’ approximates to 1% of the total body surface area whereas the ‘palm minus the fingers’ does not

Conflicts of interest: none declared. Sir, We read with interest the study by Berth‐Jones et al.1 that investigated the reliability of three methods of assessing the severity of patients’ psoriasis. The authors give details in the methods section of how the ‘raters’ were given training on how to estimate the percentage of patients’ body surface area (BSA) affected by psoriasis. They define the ‘palm’ as the ‘subject’s flat hand and thumb together, fingers not included’ and say that this represents 1% of the total BSA. However, this is a significant overestimation, as a palm plus the thumb is likely to represent only about 50–60% of the total BSA. We have previously shown, in a sample of 50 adults, that the area of the whole palmar surface of the patient’s hand (the palm plus the palmar surface of the five digits, with no spaces between the fingers and thumb) equals 0·76% (SD ± 0·10) of their total BSA for male patients and 0·70% (SD ± 0·07) of total BSA for female patients.2 This finding is similar to that of another group, working independently, who carried out an almost identical study on 70 adults.3 They found that the mean percentage of the total BSA represented by the whole palmar surface of a man’s hand was 0·81% (range 0·61–0·91), and of a woman’s hand was 0·67% (range 0·51–0·76). Similar calculations were performed in a study of 91 children aged 1–13 years.4 The entire palmar surface of a child’s hand was found to be, on average, 0·94% [95% confidence interval (CI) 0·93–0·97] of their total BSA.