An object of the present invention is to provide a method for quantifying cells of interest potentially contained in a blood-derived sample in cases of their quantification after separation from the blood-derived sample, which method enables accurate quantification of the cells without causing underestimation of the cell number due to loss of the cells of interest during the process from immediately after blood collection to the separation or causing erroneous quantification of the cell number due to inaccurate calibration of the apparatus for detecting the cells of interest. The quantification method of the present invention is a method for quantifying cells of interest having a specific gravity larger than a specific gravity of blood plasma but smaller than a specific gravity of erythrocytes, the method includes the steps of : (A) separating a blood-derived sample containing a known number (P0) of resin particles P having a specific gravity larger than a specific gravity of blood plasma but smaller than a specific gravity of erythrocytes by density gradient centrifugation into at least two layers including a layer of erythrocytes and a layer of cells other than erythrocytes; (B) extracting the layer of cells other than erythrocytes and counting the number of cells of interest and the number (P1) of the resin particles therein; and (C) correcting the number of cells of interest by multiplying the number of cells of interest by P0/P1.
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