Psoriasis is an inflammatory disease that affects 2%–3% of the adult population in Europe and the United States,1 with a considerable negative impact on quality of life.2,3 Currently, there is vigorous debate about whether the severity of psoriasis should be classified in 2 categories (mild and moderate-severe) or 3 categories (mild, moderate, and severe).4–6 Different scales have been used to provide an objective assessment of disease severity, although none of these fully satisfy the international community.7,8 The Psoriasis Area and Severity Index (PASI), with a high degree of inter- and intraobserver reliability,9 is the most widely used and accepted, despite having known limitations.5 The Dermatology Life Quality Index (DLQI) takes into account the view the patients themselves have of their disease. However, the lack of transcultural equivalence limits the validity of this quality-of-life questionnaire. The third most widely used scale is the Body Surface Area (BSA). The classic (or traditional) method of assessment consists of visual determination of the affected body area, assuming that the area of the palm of the patient accounts for 1% of the body surface area. Therefore, this scale can range from 0% to 100%. The body is subdivided into head and neck (9%), each arm (9%), anterior face (18%) and posterior face (18%) of the trunk, each leg (18%), and perineum (1%).10 In agreement with other international groups and expert recommendations,4,11,12 the Psoriasis Group of the Spanish Academy of Dermatology and Venereology reached a consensus to define severity of psoriasis in function of different criteria, which include PASI, BSA, and DLQI assessed by the same person.2

BSA is subjective, tends to overestimate severity, is not particularly useful for some clinical forms (for example, guttate psoriasis), is difficult to assess in patients with residual dyschromia, and the 1% rule is controversial.12

The objective of this study was to validate an application for BSA assessment (with an optical pencil [OP]), comparing it with the traditional method ™. The study hypothesis was that the application could represent a more objective assessment of BSA, thus decreasing inter- and intraobserver variability.

We conducted a multicenter, prospective, study, which enrolled adult patients with plaque psoriasis who were consecutively seen in different hospitals in Spain and Portugal (Hospital Universitario de la Princesa in Madrid, Hospital General in Valencia, Hospital de Bellvitge in Barcelona, Complexo Hospitalario Universitario in Santiago de Compostela, Hospital Reina Sofía in Cordoba, all in Spain, and Hospital Santa María in Lisbon, Portugal). The study was approved by the ethics committee of each hospital, and written informed consent was obtained from each patient.

Each patient was assessed at 2 visits, separated by 3±1 days. In the first visit, sociodemographic and clinical variables were recorded. At each visit, 2 dermatologists at the same hospital, blinded to the results of the other dermatologist, performed an assessment of the BSA performed with the TM and the OP application, designed by Serono. Not more than 1h could elapse between the assessment of the investigators. Fig. 1 shows design of the study.

Figure 1.

Study design. Abbreviations: OP, optical pencil; TM, traditional method.

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To use the OP application, the investigator applied a capacitive stylus to a touchscreen showing a representation of a human figure, and shaded the areas affected to obtain a complete planimetry of involved body area (Fig. 2); this information was stored by the application. The order of application of the methods of evaluation of BSA (TM→OP or OP→TM) was randomized at the first visit and reversed for the second.

Figure 2.

Details of the optical pencil method.

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Fifty-six patients were included, 34 men (60.7%) and 22 women (39.3%) with a mean (SD) age of 49.93 (16.76) years. The mean duration of psoriasis from diagnosis was 18.77 (14.28) years. The mean baseline PASI was 11.4 (8.7). Treatments for psoriasis administered were topical therapy in 24 patients (42.9%), phototherapy in 17 patients (30.4%), conventional systemic therapy in 14 patients (25%), and biologic therapy in 16 patients (28.6%).

Concordance between measurement methods

With TM, the mean (SD) BSA obtained in 224 measurements (4 measurements in 56 patients) was 25.63 (20.90), significantly greater than with OP, with a mean value of 19.36 (19.5) (P<.001), and a similar trend was observed in the different areas analyzed (Table 1).

Table 1.

Mean Body Surface Area (BSA) of 4 Measurements in 56 Patients (N=224) With the Traditional Method and Optical Pencil Method for the Entire Body and by Body Areas Analyzed: Intraclass Correlation Coefficient (ICC) Between Mements in 56 patients (N=224) with the traditional method and optical pencil method for the entire body and by body areas analyzed: intraclass correlation coefficient (ICC) between methods.

	Method
	Traditional	Optical Pencil	Difference
Area	Mean (TM)	Mean (TM)	Mean (TM)	P	ICC (95% CI)
Body as a whole	25.63 (20.90)	19.36 (19.50)	6.27 (8.86)	<.001	0.87 (0.83; 0.90)
Head	1.49 (2.15)	0.79 (1.19)	0.70 (1.49)	<.001	0.57 (0.47; 0.65)
Trunk	8.29 (8.37)	6.97 (7.99)	1.32 (4.18)	.011	0.86 (0.82; 0.89)
Arms	4.42 (4.21)	3.13 (3.63)	1.29 (2.55)	<.001	0.74 (0.68; 0.80)
Legs	9.41 (8.24)	7.94 (7.93)	1.47 (4.23)	.006	0.85 (0.81; 0.88)

Data presented as mean and standard deviation (SD).

The ICC between the 2 methods was 0.87 (95% CI, 0.83–0.90). By body area analyzed, the highest ICCs were found for the trunk and legs, and were lower for the arms and even lower for the head.

The concordance between methods was studied using the Bland–Altman graphical method. The mean difference, 5.85, and the corresponding 95% CI (3.94–8.59) (indicated as d and shaded area around this marker in Fig. 3) remained entirely above 0, thereby confirming the presence of a systematic bias, consistent with higher BSA values with TM compared with OP. Moreover, the distribution of the differences suggests that the bias arises with small and median sized BSA values, where BSA with TM is often greater than the value obtained with OP, whereas at very high values, BSA>70, the tendency is inverted and the values measured with OP are greater than with TM. On the other hand, the limits of agreement, lower limit −11.10 and upper limit 23.55 (indicated with d −1.96s and d +1.96s with corresponding 95% CIs [shaded areas] in Fig. 3), are somewhat larger than desired, suggesting a moderate precision in some of the methods.

Figure 3.

Graphical concordance between the 2 methods. Abbreviations: d, mean difference; OP, optical pencil method; s, standard deviation of the difference; TM, traditional method.

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Although there is agreement among dermatologists about the need to use measurement scales in clinical practice, in reality, it is rarely used routinely. The traditional assessment of BSA is based on the subjective assumption of the 1% rule,13 reducing interobserver reliability, and introducing the possibility of measurement errors.4,8 Although much progress has been made to automate measurement of PASI,16–19 few studies have aimed to achieve a more objective measurement of BSA.

The OP method consisted of a system in which the affected surface is shaded on a graphical representation of the body on a touchscreen with a stylus; the application then calculates the percentage of area affected.

Acceptable concordance was observed between TM and OP, although over a wide range of BSA values, TM tends to return lower values than OP, a finding consistent with those of the metaanalysis by Rhodes et al.,12 demonstrating that BSA based on the 1% rule overestimates the true BSA in adults. On the other hand, the upper limit of agreement of 23.55% estimated using the Bland–Altman graphic method, is high, suggesting a lack of precision in one of the methods,14 a finding in line with some authors,15,20 who suggest that TM is not very precise or reproducible. Both methods show greater concordance in large areas such as the trunk and legs compared with smaller areas such as the head and arms, where concordance is lower.

Given that at each visit, the second measurement of each observer was not blinded to the first measurement, a carry-over effect cannot be ruled out. For this reason, the sequence of application of the methods was randomized at the first visit, and reversed for the second. The ICCs obtained in both sequences were similar to one another, suggesting that a carry-over effect was not present.

With regards the test–retest reliability, concordance was measured between the assessments at the first and second visit, showing ICCs greater than 0.95 for the entire body. The test–retest reliability for OP was also excellent for the trunk, legs, and arms, and was slightly lower for the head. These results indicate excellent test–retest reliability.

Ramsay and Lawrence21 reported BSA measured in a sample of 10 patients ranging from 14% to 33% depending on the observer. More recently, other authors have found modest interobserver reliability for BSA.8,22,23 In our study, we found significant differences between observers in the mean BSA obtained by the 2 methods. However, the interobserver reliability was good with both methods, with higher ICCs for OP than for TM, both for the body as a whole and for different areas studied, although the ICC values dropped notably when only the head was considered.

Finally, in terms of the opinions of the participating dermatologists, most considered that the OP method was a tool that was easy to use, could be readily standardized to assess the affected body area, and potentially useful in clinical research. Additionally, OP could be useful for monitoring disease progression in patients, given that readings can be readily stored, and these could be incorporated into electronic medical histories.

An important limitation of this study is that there was no photographic archive to establish a gold standard with which to compare the findings with each of the techniques used. Another limitation is the possible underestimation of BSA with the OP method on transforming a 3D topography onto a 2D projection, in comparison with TM. Likewise, there was no formal measurement of the time used for each of the methods, although the opinion of the investigators was that the time employed for each method was approximately the same or lower for OP method as the investigator was spared the mental calculation required with the TM.

In conclusion, the present study validates the OP method for measurement of BSA in patients with psoriasis. Concordance with TM is not as good as could be expected for interchangeable methods, possibly reflecting inherent limitations of TM. On the other hand, the rest-retest reliability was excellent and the interobserver reliability improved discretely compared with TM. OP is a reliable method that, although potentially applicable in clinical practice, is aimed particularly for use in clinical trials.

The authors declare that they have no conflicts of interest.