Label-free cell differentiation

The blood count is a compilation of data on the quantity and the shape of cells. From this, the physician can draw conclusions concerning a patient's state of health. The corresponding measurements also include flow cytometric procedures in which optical or impedance counting devices are used. In both cases, the red blood cells are first destroyed by hemolysis to facilitate the measurement of the concentration of white blood cells which, in healthy patients, are less abundant than red blood cells by a factor of approx. 1000. In the case of certain diseases such as leukemia, the white blood cells, however, may either be modified or destroyed by hemolysis procedures. In other blood samples, e.g. from newborns or from anemic patients, hemolysis-resistant erythrocytes may be observed which impede the correct differentiation and concentration measurement of leukocytes. To ensure reliable cell differentiation, such samples require demanding microscopic investigations or staining of the DNA or of the cell membrane.

The procedure developed at PTB allows blood samples, diluted to a suitable concentration, to be analyzed by means of flow cytometry, without modifying them by hemolysis or by using staining procedures. This method is based on measuring the change which occurs in the complex resistance (impedance) when a blood cell passes through the electrode configuration of a microfluidic assembly. In order to identify the cells unequivocally, two alternating voltages are applied with different frequencies and each of the complex AC impedance signals is measured, i.e. the effective resistance, the reactance and the absolute value of the impedance are determined. By choosing suitable frequencies and measurands, it is possible to differentiate in the micro flow cytometer between the cells attributed to the full blood count including the 3-part differential white blood count, i.e. the red blood cells, the platelets, the granulocytes, the monocytes and the lymphocytes.

With this new method, it is possible to facilitate the examination of blood samples from certain groups of patients and, at the same time, to prevent systematic measurement errors. This reduces the number of subsequent microscopic differentiations and/or of cell-specific staining. Moreover, the procedure, which is based on a microfluidic assembly, offers potential for developing a point of care device utilizing a flow cytometric sensor.