Immunocytochemical staining of cytologic specimens. How helpful is it?

The contribution of immunocytochemical (ICC) staining to the diagnosis of a range of cytologic specimens reported over a 20-month period was retrospectively assessed. A total of 194 cases (1.6% of total workload) were stained for diagnostic purposes in this period. Immunocytochemical staining was determined to be helpful in 75.8% of cases. In body fluid cases (105), staining was most often performed to assist with the discrimination of mesothelial cells and metastatic malignancy. Immunocytochemical staining was helpful in 82% of fluid specimens, with the preliminary diagnosis confirmed in 64%, refined in 8%, and revised in 10% of cases. In fine-needle aspiration (FNA) specimens, staining was helpful in 69%, resulting in a refinement of diagnosis in 55%, and confirming the preliminary diagnosis in 14%. The preliminary diagnosis was revised in a single FNA case. Immunocytochemical staining was particularly valuable in assisting the subclassification of poorly differentiated malignancies. A more precise diagnosis was possible following consideration of ICC results in 68% of cases with a preliminary diagnosis of poorly differentiated malignancy. Immunocytochemical staining was also helpful in identifying the primary site of metastatic carcinoma in six FNA cases. It is concluded that the selective use of well-chosen panels of antibodies can be very helpful in resolving diagnostic difficulties in cytologic specimens. In particular, ICC may be an invaluable aid to the diagnosis of difficult serous effusion specimens and in the sub-typing of poorly differentiated malignancy.