S-100 protein immunohistochemistry and electron microscopy in the diagnosis of Langerhans cell proliferative disorders: a comparative assessment.

To better define the roles of S-100 protein immunohistochemistry and electron microscopy in the diagnosis of Langerhans cell proliferative disorders, a comparative assessment of the two techniques was performed using material from 39 cases of histiocytosis X and 2 cases of infantile self-healing reticulohistiocytoma. Both techniques proved highly reliable, but neither alone enabled diagnostic confirmation in all instances. The two techniques proved complementary and used together did enable identification of Langerhans-type histiocytes in all cases studied. Neither was judged clearly superior and both offered certain advantages. Electron microscopy was found overall to be a slightly less sensitive technique, but more specific and less subject to misinterpretation. The S-100 stain was found to be particularly useful in situations where sampling problems were likely to be encountered or where the available specimen was otherwise suboptimal for electron microscopic examination. Because the S-100 stain is the more cost-effective to employ, we now recommend it for the purpose of providing routine diagnostic confirmation. In an investigative setting, however, we continue to recommend electron microscopy as the primary technique.