properties of human blood monocytes i cd91 expression and log orthogonal light scatter provide a robust method to identify monocytes that is more accurate than cd14 expression background this study was designed to improve identification of human blood monocytes by using antibodies to molecules that occur consistently on all stages of monocyte development and differentiationmethods we examined blood samples from 200 healthy adults without clinically diagnosed immunological abnormalities by flow cytometry fcm with multiple combinations of antibodies and with a hematology analyzer beckman lh750results cd91 2 macroglobulin receptor was expressed only by monocytes and to a consistent level among subjects mean median fluorescence intensity mfi16232 notably only 857582 of the cd91 monocytes expressed high levels of the classical monocyte marker cd14 with some cd91 cd16 cells having negligible cd14 indicating that substantial fcm undercounts will occur when monocytes are identified by high cd14 cd33 receptor for sialyl conjugates was coexpressed with cd91 on monocytes but cd33 expression varied by nearly tenfold among subjects mean mfi17477 in comparison to fcm analyses the hematology analyzer systematically overcounted monocytes and eosinophils while lymphocyte and neutrophil differential values generally agreed with fcm methodsconclusions cd91 is a better marker to identify monocytes than cd14 or cd33 furthermore fcm with anticd91 identifies monocytes better than a currently used clinical cbc instrument use of anticd91 together with anticd14 and anticd16 supports the identification of the diagnostically significant monocyte populations with variable expression of cd14 and cd16