What distinguishes FCM from other analytical and preparative techniques is that it combines the following five essential characteristics: quantitative analysis, detection sensitivity, speed, multiparameter analysis cell by cell, sorting.

It is a major advantage compared to current optical microscopy to be able to quantify the observed parameters. Indeed, with a microscope, it is difficult to classify cells in more than four categories according to their fluorescence: "negative", "weak", "medium", "strong". A cytometer with a logarithmic amplifier allows the rigorous quantification of each optical criterion over a range of 1 to several million arbitrary fluorescence units. But the number of parameters involved in any FCM analysis (optical settings, fluorochromes, markers) imposes, within the framework of an absolute quantification, the use of calibrated standards (fluorescent beads for example).

In immunofluorescence, it is possible to discern from the background a population of lymphoid cells carrying about 1000 antigenic determinants per cell.

The average analysis speed of a cytometer is 10,000 cells per second, although it is possible on modern instruments to reliably analyze up to 100,000 events per second on several parameters. In a few seconds, the statistical significance of the count is much higher than that obtained conventionally by an optical microscope, even for a subpopulation of cells in the minority.

CMF offers the possibility to work simultaneously on several parameters, which allows to measure, for example, ten or twenty parameters simultaneously on a lymphocyte population of blood or marrow. No other method, either physico-chemical (gradient centrifugation, elutriation) or immunological (panning, columns, rosettes) offers this versatility.
The new spectral analyzers are capable of measuring up to 40 simultaneous fluorescences in addition to 3 morphology parameters.

Cells can be isolated with purity levels above 99%. These cells can be recultured. However, one should not forget the relative slowness of the sorting: to obtain 10⁶ cells from a population initially representing 1% of the starting population, it would take a long time (9 hours and 15 minutes) if the cells are very fragile, for example, whereas it takes only one hour and 10 minutes if they are small and resistant (Table 3). The high purity of populations sorted by CMF can therefore only be achieved at the cost of a serious limitation of the number of cells collected and constant critical monitoring during separation.

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