Objective:  To analyze the clinical characteristics of multiple myeloma (MM) with malignant pleural effusion (MPE) diagnosed using multicolor flow cytometry (FCM).

  Methods:  A total of six patients confirmed with MM with MPE diagnosed using FCM in the period from July 2016 to May 2020 at the Northern Jiangsu People's Hospital were enrolled in this retrospective study. The clinical characteristics, laboratory indicators, and treatment outcomes of the patients were analyzed.

  Results:  The median age at the time of diagnosis of the six patients with MM was 62 years (range:49-72 years), while the male-to-female ratio was 2:1. The MM isotype was IgA in three patients, IgG in two patients, and light chain myeloma in one patient. Malignant cells were not detected with conventional cytology in the pleural effusion of any patient confirmed with MM, but plasma cells were detected with FCM. By the final follow-up, four patients had died, whereas two had survived. The ratio of plasma cells diagnosed using multicolor FCM positively correlated with the pleural effusion protein, albumin, globulin, lactate dehydrogenase (LDH), and adenosine deaminase (ADA) as well as the ratio of abnormal plasma cells in the bone marrow (P>0.05). There were no correlations among the pleural effusion red blood cell count, mononuclear cell count, white blood cell count, hemoglobin, platelets, serum total protein, serum albumin, serum LDH, serum ADA, and blood β2-macroglobulin (P>0.05).

  Conclusions:  Malignant pleural effusion in MM patients is rare, and routine pleural effusion pathological examinations have a low sensitivity. The positive rate in MM with MPE is significantly higher in the multicolor FCM assay than in conventional cytology. Multicolor FCM may be useful for improving the detection rate of MPE. Therefore, multicolor FCM can be used in routine pleural effusion examinations. MM patients with MPE have a shorter survival time and poor prognosis.