Inter‐observer variability in reading of phototest reactions with sharply or diffusely delineated borders

Background: In both clinical and experimental phototesting, naked eye assessment of erythema has been the main assessment parameter. As with all subjective assessment, variability in recorded results due to variable circumstances around the performance and reading of tests influences reliability and utility of data whether they be interpreted for an individual patient or for a group of research subjects. Methods: In the present study, variability in the reporting of diameter of ultraviolet B (UVB) erythema has been studied. The erythematous reactions were assessed by the naked eye and with the help of a millimetre‐graded ruler by a group of dermatologists and dermatological trainees. Reaction size, objectively quantified by means of laser Doppler perfusion imaging (LDPI) using thresholding of the reaction perfusion, and known size of UVB provocation were used as yardsticks in order to quantify this variability. Results: Agreement between observers, against known size, was excellent for reactions with a sharp border, but for reactions with a diffuse or indistinct border there was a substantial inter‐observer variability. This was also true for the comparison between naked‐eye reading and LDPI assessment of the reaction size. Conclusion: It is concluded that if naked‐eye readings are to be the outcome measurement, then provocations/protocols producing distinct borders are an advantage. If borders between provoked and unprovoked skin can be expected to be diffuse, i.e. part of a continuum of response, the use of objective, bioengineering techniques such as LDPI is required. Quantitative methods are also the basis for more detailed presentation and interpretation of test results including information on dose response above the minimal erythema dose.