The Molecules of Equivalent Fluorochrome (MEF) value is useful for flow cytometer users. However, the Quantum Yield of most fluorochromes usually changes upon binding to the cells. As a result, the actual number of the fluorochrome molecules binding to the cells may be different than the expected MEF value. In addition, the MEF value will vary depending upon the measurement method.

There are several methods for measuring the MEF value. Most commonly, a standard curve is generated with a fluorimeter using solutions of fluorochrome in various concentrations. The fluorescence of these solutions is then compared to the intensity of labeled cells or particles less the fluorescence of a blank particle or non-labeled cell. This provides the equivalent concentration of fluorochrome. The MEF value is then calculated by dividing the equivalent fluorochrome concentration by the number of cells or particles used.

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