Background/objectives Pathology laboratories are required to determine or estimate the measurement uncertainty for all quantitative results, but there is no literature on the uncertainty in margin measurements for skin cancer excisions.

Methods Six pathologists measured 4–14 histological margins in each of 10 basal cell carcinoma.

Results The mean of measurements from all the margins from all the cases was 1.8 mm (range 0 and 6 mm). Regarding the overall variance in margin measurements across the ten cases, 25% was from variation within cases (differences in margin measurement for a given case, because of different margins and different pathologists measuring each margin, SD 0.7 mm). For a given case, we estimate that 95% of margin measurements would fall approximately within±1.4 mm of the mean measurement for that case. When only pathologists’ closest margin for each case were included (for the six cases with uninvolved margins), 6% of the overall variance was from differences within cases (because of different pathologists’ measurements of the closest margin, SD 0.2 mm). For a given case without an involved margin, 95% of closest margin measurements would fall approximately within±0.5 mm of the mean closest measurement for that case.

Conclusions Clinicians should be aware there is uncertainty in reported histological margins.