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Ruler is better than eye for pigment measure


Physicians should measure pigmented lesion diameter with a ruler rather than estimating via landmarks or the naked eye, a study shows.

Because recommendations for skin cancer treatment depend on lesion size, it is crucial to begin with an accurate measurement. A recent study suggests that although dermatologists appear to estimate pigmented lesion sizes without using rulers or landmarks more accurately than medical students and internal medicine residents, using a ruler provides significantly more accurate measurements.

Previous studies have examined the relative prowess of dermatologists and general practitioners in assessing and triaging pigmented lesions without a ruler. In perhaps the first study of its kind, researchers led by June K. Robinson, M.D., of Northwestern University Feinberg School of Medicine, pitted dermatologists against internal medicine residents and medical students in various testing scenarios. The research letter appeared in the December 27, 2017, online version of JAMA Dermatology.

Study participants included 39 medical students, 27 medical residents and 12 dermatology residents, fellows and faculty at Northwestern. Using anonymous online or paper surveys, participants characterized their use of rulers in clinic and their confidence in estimating the size of skin lesions. At baseline, dermatologists were more likely to carry rulers (92%) than were medical students (44%) and internal medicine (22%) cohorts. On a scale of 1 to 5, dermatologists reported a mean confidence level of 3.9, versus 2.7 and 3.1 for the other cohorts, respectively.

Participants also used multiple-choice answers to estimate the diameter of 4 lesions photographed in 3 ways each:  With a ruler in the frame, with an anatomical landmark and with neither ruler nor landmark.

"All participants in this study were most accurate in assessing diameter in images with a ruler, and most participants were accurate estimating the diameter in images with an anatomic landmark and no ruler," the authors write. Regarding images containing a ruler, the medical students correctly estimated the diameter of 107 of 156 lesions (69%) within 1 mm. The corresponding figures for internal medicine residents and dermatology participants were 79 of 108 lesions (73%) and 34 of 48 lesions (71%), respectively.1

For images with an anatomic landmark, medical students gauged 71 of 156 lesions (46%) correctly within 1 mm. Internal medicine and dermatology participants did so with 55 of 108 lesions (51%) and 23 of 48 lesions (48%), respectively.

While investigators found no statistically significant differences between study cohorts in the above analyses, statistical significance did emerge with respect to images having no ruler or landmark. Here, dermatologists scored 25 of 48 lesions (52%) correctly within 1 mm, versus 36 of 156 (23%) and 20 of 108 (19%) for medical students and internists, respectively (P = 0.003). Pairwise post hoc analysis showed the same degree of statistical significance between the dermatology and internal medicine cohorts (P = 0.003), and slightly less significance (P = 0.02) between the dermatology and student cohorts.

"The dermatology cohort was most accurate in assessing diameter in images with no landmark and no ruler but only estimated 52% of lesions correctly within 1 mm," write Robinson et al. Within the well-established ABCDE guidelines for evaluating suspicious lesions, they add, a 6 mm diameter is a clinically useful yardstick for assessing malignant potential of pigmented lesions.2 Accurate size estimation also supports the ability of clinicians, regardless of specialty, to both triage lesions and track their evolution.

"This study shows that an accurate clinical assessment of lesion size requires the use of a ruler for all physicians, including dermatologists, and that simply increasing physician awareness of this potential pitfall is sufficient to compel change." After receiving the correct responses, participants said they were more likely to measure suspicious lesions with a ruler (mean/SD Likert scale score 3.8/0.9, P = 0.04).

Although authors of the brief research letter did not explore the study's limitations, the relatively small size of the dermatology cohort, and the fact that it contained an unspecified number of fully credentialed dermatologists, may have skewed findings.



Dr. Robinson is the editor of JAMA Dermatology but was not involved in any of the decisions regarding review or acceptance of the manuscript.


1.  Mannam H, Choi A, Jacob S, Kwa M, Xu S, Robinson JK. Assessment of the diameter of pigmented skin lesions with and without a ruler.JAMA Dermatol. 2017 Dec 27. doi: 10.1001/jamadermatol.2017.5083. [Epub ahead of print]

2.  Abbasi NR, Yancovitz M, Gutkowicz-Krusin D, et al. Utility of lesion diameter in the clinical diagnosis of cutaneous melanoma.Arch Dermatol. 2008 Apr;144(4):469-74.


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