immunoglobulin light chains commonly seen in chronic lymphocytic leukemia. Flow cytometry is particularly good at identifying clonality in B-cell populations. Although T-cell neoplasms may exhibit a predominance of antigens CD4 or CD8, these antigens should not be considered as surrogate markers of clonality. The use of antibodies to the T-cell receptor family may occasionally be helpful in a small percentage of cases; however, many reactive processes can show expansion of particular T-cell receptor clones (66–69). Antigen deletions are common in T-cell lymphomas and may suggest neoplasia, but the only way to definitively diagnose T-cell clonality is by molecular methods. Flow cytometry can be used for lymphoma phenotyping of fine needle aspirates, and is a powerful adjunct to cytologic diagnosis (70). The high sensitivity and capacity for simultaneous analysis of multiple characteristics make flow cytometry useful for the detection of minimal residual disease, especially if abnormal patterns of antigen expression are present (71–75). Flow cytometry is not recommended for the diagnosis of Hodgkin lymphoma, chronic myelogenous leukemia, or myelodysplastic syndrome, although disease progression in the latter two conditions can often be monitored. Neutropenia may be immune or nonimmune in nature. The work-up frequently entails a bone marrow examination. Immune neutropenia may result from granulocytespecific autoantibodies, granulocyte-specific alloantibodies, or transfusion-related anti-HLA antibodies. Flow cytometry can readily identify anti-neutrophil antibodies that are either bound to granulocytes or free in plasma (76). Autoimmune neutropenias may develop in patients with autoimmune disorders such as Felty syndrome, systemic lupus erythematosus, and Hashimoto thyroiditis. When immune-related, flow cytometry can detect anti-neutrophil antibodies and confirm the origin of neutropenia, possibly eliminating the need for a bone marrow procedure. Conversely, the absence of anti-neutrophil antibodies narrows the differential diagnosis to nonimmune causes such as bone marrow failure, myelodysplasia, or marrow-infiltrative processes. Fig. 4. Immunophenotyping of acute myeloid leukemia. (Left), CD45/side scatter gating allows discrimination between multiple populations of leukocytes in peripheral blood. Region A, mature lymphocytes; region B, mature monocytes; region C, mature granulocytes; region D, immature myeloid cells. (Right), the population in region D from the left panel expresses CD13, a myeloid marker, and CD34, a stem cell marker. The co-expression of these two markers in the peripheral blood is highly suggestive of an acute myeloid leukemia. Table 2. Common phenotypes of B-cell lymphoproliferative disorders. Diagnosis CD5 small lymphocytic lymphoma/chronic lymphocytic leukemia; 1, positive; 2, negative; 1/2, often positive; 2/1, occasionally positive; w, weak expression. Clinical Chemistry 46, No. 8(B), 2000 1225 Functional deficiencies of leukocytes can be assessed by flow cytometry. Assays for oxidative burst, phagocytosis, opsonization, adhesion, and structure are available (77). One clinical example measures neutrophil adhesion molecules central to a diagnosis of leukocyte adhesion deficiency syndrome type I (78). This syndrome is characterized by an immunodeficiency related to defective neutrophil and monocyte migration to sites of inflammation. The disorder is caused by a congenital deficiency of the leukocyte b2 integrin receptor complex (CD11/CD18 antigen complex) on the myeloid cell surface. This receptor complex binds endothelial cell ligands such as intercellular adhesion molecule-1 (CD54 antigen), which is necessary for neutrophil adherence and transendothelial migration (78, 79). Flow cytometry can be used to identify neutrophils that lack the CD11/CD18 antigen complex to establish a diagnosis that is otherwise difficult to make. platelet analysis The analysis of platelets by flow cytometry is becoming more common in both research and clinical laboratories. Platelet-associated immunoglobulin assays by flow cytometry can be direct or indirect assays, similar to other platelet-associated immunoglobulin