Malignant plasma cells can be detected in the blood of patients with multiple myeloma (MM) using flow cytometry (FC), immunofluorescence microscopy (IM), or a variety of molecular techniques. Increased numbers of light chain-restricted blood plasma cells as detected by IM is associated with a diagnosis of overt MM and a decreased overall survival. The IM technique is time consuming; therefore, a prospective study was designed to test whether CD38 CD45 FC could simplify the procedure. Blood samples from 769 patients with plasma cell proliferative disorders were studied prospectively by FC and IM over a one-year period. The FC technique was performed on 1 ml of whole blood after ammonium chloride red blood cell lysis and utilized anti-CD38PE and anti-CD45PerCP. The number of CD38+ 45- events were enumerated and compared to the number of light chain-restricted plasma cells detected by the standard IM technique. In 46% (353/769) of cases > or = 1 CD38+ CD45- events were detected by FC whereas IM was positive for light chain restricted plasma cells in 33%; there was concordance between FC and IM in 73% of cases. In 20% of cases FC was positive and IM was negative; however, in 7% of cases FC was negative yet light chain-restricted plasma cells were detected by IM. FC was positive in 88% (134/153) of cases where the IM technique showed a high number of circulating plasma cells. This study demonstrates that two-color CD38/45 FC identifies most cases with a high IM result and reduces the workload in the clinical laboratory. The prognostic implications of a positive FC screen but a negative IM will require long-term patient follow-up.