Detect and remove doublets from flow and mass cytometry data. Covers FSC/SSC gating and computational doublet detection methods...
Reference examples tested with: flowCore 2.14+, CATALYST 1.26+, ggplot2 3.5+.
Before using code patterns, verify installed versions match. If versions differ:
packageVersion('<pkg>') then ?function_name to verify parametersIf code throws an error, introspect the installed package and adapt rather than retrying.
"Remove doublets from my cytometry data" -> Discriminate single cells from aggregates using pulse geometry (flow) or ion-cloud parameters (CyTOF), before any clustering or quantification.
flowCore gate on the FSC-A vs FSC-H diagonal (+ FSC-W/SSC-W)A doublet has roughly double the pulse AREA of a singlet but NOT double the Height, and a longer Width/transit time - so singlets fall on a tight FSC-A vs FSC-H diagonal and doublets deflect above it. A 1D area histogram therefore does NOT remove doublets; the discriminating signal is the Area-Height relationship (plus Width). This matters because an unremoved doublet of a CD3+ and a CD19+ cell reads as an artifactual CD3+CD19+ "double-positive," and clustering will faithfully (and wrongly) carve it out as a real population. Crucially, scatter gating is necessary but NOT sufficient: heterotypic conjugates (e.g. a CD3+CD14+ T:monocyte) survive standard FSC-A/H gates and present as genuine double-positives whose lineage-marker levels look COMPARABLE to true single-positives - the tell is an ELEVATED shared marker (e.g. CD45) and a high bright-field aspect ratio, so the definitive resolver is imaging flow cytometry, not a lineage-intensity check (Stadinski 2020 Cytometry A 97:1102). On CyTOF there is no scatter at all - doublets are removed by ion-cloud Gaussian parameters and DNA intercalator content (Bagwell 2020 Cytometry A 97:184).
| Method | Instrument | Principle | Caveat |
|---|---|---|---|
| FSC-A vs FSC-H | flow/spectral | singlets on the A-H diagonal | the standard; the discriminator is non-proportionality, not area |
| FSC-W / SSC-W | flow/spectral | doublets have longer pulse Width | complementary to A-vs-H |
| DNA intercalator (Ir191/193) | CyTOF | doublets show ~2N+ DNA | also separates cells from beads/debris |
| Gaussian params + Event_length | CyTOF | ion-cloud fit residual/length flags fusions | catches fusions DNA alone misses (Bagwell 2020) |
| imaging cytometry | imaging flow | bright-field aspect ratio | the only clean resolver of heterotypic conjugates |
Note: cytometry doublet removal is GATING-based. DoubletFinder/Scrublet/scDblFinder are scRNA-seq DROPLET methods (they simulate artificial doublets) - limited transfer, because cytometry has direct physical doublet signals.
Goal: Keep events on the singlet diagonal.
Approach: A polygon along the A=H diagonal (preferred over a rectangle, which keeps off-diagonal doublets); visualize with the gate overlaid.
library(flowCore); library(ggcyto)
# matrix dimnames preserve 'FSC-A'/'FSC-H'; data.frame() would mangle them to FSC.A
singlet <- polygonGate(filterId = 'singlets', .gate = matrix(
c(20000, 10000, 250000, 200000, 250000, 260000, 20000, 40000), ncol = 2, byrow = TRUE,
dimnames = list(NULL, c('FSC-A', 'FSC-H'))))
singlets <- Subset(fs, singlet)
autoplot(fs[[1]], 'FSC-A', 'FSC-H') + ggcyto::geom_gate(singlet)
Goal: Keep intercalator-positive single ion clouds.
Approach: Gate DNA intercalator (nucleated, ~2N) and Event_length/Gaussian residual; CATALYST exposes these as channels in the SCE.
library(CATALYST)
# prepData moves Time/Event_length to int_colData by default - keep them in the assay with FACS=TRUE
sce <- prepData(fs, panel, md, transform = TRUE, cofactor = 5, FACS = TRUE)
e <- assay(sce, 'exprs')
dna <- e['DNA1', ] # intercalator-positive = nucleated single cells
keep <- dna > quantile(dna, 0.05) & dna < quantile(dna, 0.95)
if ('Event_length' %in% rownames(sce)) # retained by FACS=TRUE (now on the arcsinh scale)
keep <- keep & e['Event_length', ] <= quantile(e['Event_length', ], 0.99) # quantile-relative, so scale is fine
sce_singlets <- sce[, keep]
Trigger: gating only FSC-A. Mechanism: doublets overlap singlets in area. Symptom: double-positive clusters persist. Fix: gate the FSC-A vs FSC-H diagonal (+ Width).
Trigger: a surprising double-positive between two single-positive clusters. Mechanism: T:monocyte conjugate is scatter-normal, lineage markers comparable to singlets. Symptom: "novel" DP population with an elevated shared marker (e.g. CD45). Fix: treat as suspected doublet; check the shared-marker signal; confirm/resolve by imaging flow (bright-field aspect ratio) when load-bearing.
Trigger: porting flow logic to CyTOF. Mechanism: no FSC/SSC exists. Symptom: no scatter channels. Fix: use DNA + Gaussian/Event_length.
| Threshold | Source | Rationale |
|---|---|---|
| expected doublet rate ~1-5% (PBMC), higher in tissue | community | flag samples far above as prep issues - not a removal cutoff |
| Gaussian + DNA gating improves CV (3.45 -> ~2.04) | Bagwell 2020 Cytometry A 97:184 | combined DNA + Gaussian over baseline (Gaussian alone ~2.41) |
Note: a fixed "95th-percentile residual" cutoff is arbitrary; prefer a visual diagonal gate or the instrument's Gaussian parameters over an unjustified quantile.
| Error / symptom | Cause | Solution |
|---|---|---|
| double-positive cluster that "shouldn't" exist | residual heterotypic doublets | check for an elevated shared marker (CD45); confirm by imaging flow |
| no FSC/SSC channels (CyTOF) | mass data has no scatter | use DNA/Gaussian/Event_length |
| over-removal of large cells | rectangle gate clips real large singlets | use a diagonal polygon, not a box |
Workflow order (CyTOF): EQ-bead drift normalization (raw, FIRST) -> cytometry-qc -> doublet-detection -> clustering -> CytoNorm cross-batch (LAST)
