Comparability of naevus counts between and within examiners, and comparison with computer image analysis

In the course of an investigation of melanocytic naevus development in Queensland, Australia, whole-body naevus counts of 66 adolescents were performed separately by two nurse examiners on two occasions on average 4 weeks apart. There was good agreement between the two examiners for counts of total naevi on the whole body (intra-class correlation coefficient = 0.96) and at selected subsites (face, neck, back, upper arms, lower arms). Agreement was lower when raised naevi only were counted (0.83). Intra-examiner repeatability was high for both nurses, particularly for the more experienced examiner (intra-class correlation coefficients = 0.98 and 0.91 for total naevi on the whole body), and was consistently better when all naevi were counted rather than naevi of a particular size. Independent counts of naevi on the back using a computer imaging technique were reproducible (intra-class correlation coefficient = 0.92), but showed only moderate agreement with counts by the nurse examiners. Overall, these results demonstrate high comparability of naevus counts between and within similarly trained examiners. They do not support the common practice in epidemiological studies of restricting counts to naevi larger than 2 mm, or of counting raised naevi only.