Short report: Quantification of Plasmodium falciparum gametocytes by magnetic fractionation.

A method of gametocyte quantitation in human blood was developed based on magnetic fractionation using commercially available magnetic fractionation columns (MFCs) and exploiting the magnetic susceptibility of mature Plasmodium falciparum gametocytes. The technique uses magnetic microspheres as a calibration standard. Microspheres are added to each blood sample to a known concentration. When exposed to a magnetic field, gametocytes and magnetic microspheres are preferentially captured inside MFCs. After removal of the magnetizing field, the magnetically captured material can be eluted, placed on a microscope slide that is stained, and counted by using conventional methods. The limits of quantitation for P. falciparum gametocytes were determined from serial dilutions of blood samples with known gametocyte density. The upper limit was 1,000 gametocytes/μL. Quantitative analysis above this threshold is difficult because of an over-abundance of gametocytes. The lower limit was 0.1 gametocytes/μL, and there is a significant probability of a false-negative result below this level.