A quantitative comparison of light and electron microscopic diagnoses in specimens obtained by fine-needle aspiration biopsy.

Because fine-needle aspiration biopsy (FNAB) is being increasingly used as a primary diagnostic tool, it is essential to obtain the maximum information from the aspirate. Electron microscopy is an adjunctive procedure that is readily applied to FNAB specimens. The value of electron microscopy to the cytopathologist was assessed by comparing the initial, tentative, or interim diagnosis based on light microscopic features with the diagnosis resulting from the ultrastructural characteristics of needle rinse material. Over a 3-year period, 279 FNAB specimens obtained under radiologic control were examined ultrastructurally; of these, 57 (20.4%) were considered inadequate specimens for diagnostic purposes. The remaining 222 FNAB specimens were segregated into four groups: 17 cases (7.7%) in which electron microscopy played a major role because the final diagnosis was unsuspected from light microscopy; 43 cases (19.4%) in which electron microscopy selected a specific diagnosis from a set of differential diagnoses or provided additional information that was clinically relevant; 45 cases (20.3%) in which additional diagnostic information was obtained by electron microscopy but was not clinically relevant; and 117 cases (52.6%) in which electron microscopy was not helpful because the light and electron microscopic diagnoses were the same. In this comparative study, electron microscopy provided a major contribution to the final cytologic diagnosis in 27.1% of the cases (groups 1 and 2). Descriptive examples illustrate how the architectural and cytologic features revealed by electron microscopy assist in establishing the final diagnosis. Examination of needle rinse specimens, particularly aspirates from lung and liver, indicates that most FNABs provide mini-surgical biopsy specimens that are well suited to ultrastructural examination.