To count and how to count, that is the question: interobserver and intraobserver variability among pathologists in lymph node counting.

Optimal cancer staging requires retrieval of a minimal number of nodes. However, variability among pathologists in counting on a slide has not been studied. To study the differences in node counting among pathologists, 10 pathologists counted nodes on 15 slides on 2 occasions. They also opined on whether selected "structures" represented countable nodes. There was no slide on which all pathologists agreed on all occasions. The greatest variability was on slides on which the number of nodes exceeded 8. There was disagreement on the size of the smallest countable node, on how to count 2 closely related structures, and when the gross disagreed with the microscopic finding. With a mean count of 5.7 nodes per slide, the 95% confidence interval was +/- 2.6, which could be clinically significant when the count approaches the set minimum. Uniform criteria are necessary to allow for meaningful comparisons between studies on minimal nodal counts for cancer lymphadenectomies.