This article first reviews scientific meanings of single-cell analysis by highlighting two key scientific problems: landscape reconstruction of cellular identities during dynamic immune processes and mechanisms of tumor origin and evolution. Secondly, the article reviews clinical demands of single-cell analysis, which are complete blood counting enabled by optoelectronic flow cytometry and diagnosis of hematologic malignancies enabled by multicolor fluorescent flow cytometry. Then, this article focuses on the developments of optoelectronic flow cytometry for the complete blood counting by comparing conventional counterparts of hematology analyzers (e.g., DxH 900 of Beckman Coulter, XN-1000 of Sysmex, ADVIA 2120i of Siemens, and CELL-DYN Ruby of Abbott) and microfluidic counterparts (e.g., microfluidic impedance and imaging flow cytometry). Future directions of optoelectronic flow cytometry are indicated where intrinsic rather than dependent biophysical parameters of blood cells must be measured, and they can replace blood smears as the gold standard of blood analysis in the near future.

As label‐free biomarkers, bioelectrical properties of single cells have been widely used in hematology analyzers for 3‐part differential of leukocytes, in which, however, instrument dependent bioelectrical parameters (e.g., DC/AC impedance values) rather than inherent bioelectrical parameters (e.g., diameter Dc, specific membrane capacitance Csm and cytoplasmic conductivity σcy) were used, leading to poor comparisons among different instruments. In order to address this issue, this study collected inherent bioelectrical parameters from hundreds of thousands of white blood cells based on a home‐developed impedance flow cytometry with corresponding 3‐part differential of leukocytes realized. More specifically, leukocytes were separated into three major subtypes of granulocytes, monocytes and lymphocytes based on density gradient centrifugation. Then these separated cells were aspirated through a constriction‐microchannel based impedance flow cytometry where inherent bioelectrical parameters of Dc, Csm and σcy were quantified as 9.8 ± 0.7 μm, 2.06 ± 0.26 μF/cm2, and 0.34 ± 0.05 S/m for granulocytes (ncell = 134,829); 10.4 ± 1.0 μm, 2.45 ± 0.48 μF/cm2, and 0.42 ± 0.08 S/m for monocytes (ncell = 40,226); 8.0 ± 0.5 μm, 2.23 ± 0.34 μF/cm2, and 0.35 ± 0.08 S/m for lymphocytes (ncell = 129,193). Based on these inherent bioelectrical parameters, neural pattern recognition was conducted, producing a high “classification accuracy” of 93.5% in classifying these three subtypes of leukocytes. These results indicate that as inherent bioelectrical parameters, Dc, Csm, and σcy can be used to electrically phenotype white blood cells in a label‐free manner.