What is compensation in flow cytometry?

Fluorophores emit light over a broad spectrum, so a dye meant for one detector also produces signal in others. Compensation subtracts that spillover so each channel reports only its true fluorophore. Getting it right is essential before any multi-colour panel can be interpreted.

How is spillover calculated?

From a single-stain control, spillover into another detector is the net positive signal seen in that detector divided by the net positive signal in the dye's own detector, as a percent. Net means the positive median minus the unstained negative median, which removes autofluorescence and background.

Why use the median and not the mean?

Fluorescence distributions are skewed and span several logs, so the arithmetic mean is pulled by bright outliers. The median fluorescence intensity, or MFI, is robust to that skew and is the standard statistic for compensation and for reporting populations.

What spillover value is too high?

Low single-digit spillover is trivial to compensate. Values into the tens of percent are common between adjacent dyes like FITC and PE and still work. Spillover much above 50 percent means heavy spectral overlap and large spreading error, so a different dye pairing is usually the better fix.

Do I subtract the unstained control?

Yes, always. Autofluorescence and instrument background add a baseline to every detector. The calculator subtracts the negative-population MFI you enter for each detector, so the spillover reflects only the dye-specific signal and not the cells' own glow.

Flow Cytometry Compensation Matrix Calculator

Before a multi-colour flow panel can be read, the spillover between detectors has to be measured and removed. This tool computes the spillover percentages for a two-fluorophore pair from single-stain control medians, the first step in building a compensation matrix.

How it works

Run one single-stain control per dye and record the median fluorescence (MFI) of the positive and negative populations in each detector. The spillover of fluorophore i into detector j is its net leak signal divided by its own net signal:

spillover%(i -> j) = 100 x (MFI_pos_j - MFI_neg_j) / (MFI_pos_i - MFI_neg_i)

Subtracting the negative population removes autofluorescence and instrument background in every detector. The diagonal, a dye into its own detector, is 100 percent by definition. The off-diagonal values form the spillover matrix the cytometer software inverts to compensate your data.

Tips and example

Suppose fluorophore A reads a net 19,800 MFI in its own detector and a net 2,250 in B’s detector. Its spillover into B is 100 x 2250 / 19800, about 11 percent. Keep controls as bright or brighter than the experimental samples, use the same voltages throughout, and prefer compensation beads when cells are rare or dim. If two dyes show very high mutual spillover, expect spreading error in those channels and consider re-designing the panel rather than relying on heavy compensation.