Detects and removes doublets/aggregates from flow, spectral, and mass cytometry before clustering or quantification. Covers FSC-A vs FSC-H singlet discrimination (the Area-Height non-proportionality, not a 1D area gate), FSC-W/SSC width gating, CyTOF Gaussian discrimination parameters (Center/Offset/Width/Residual/Event_length) and DNA intercalator gating, and the residual heterotypic conjugates that survive scatter gating and masquerade as double-positive populations. Use when filtering aggregates before phenotyping, choosing a doublet method for flow vs CyTOF, or diagnosing a suspicious double-positive cluster.
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Detects and removes doublets/aggregates from flow, spectral, and mass cytometry before clustering or quantification. Covers FSC-A vs FSC-H singlet discrimination (the Area-Height non-proportionality, not a 1D area gate), FSC-W/SSC width gating, CyTOF Gaussian discrimination parameters (Center/Offset/Width/Residual/Event_length) and DNA intercalator gating, and the residual heterotypic conjugates that survive scatter gating and masquerade as double-positive populations. Use when filtering aggregates before phenotyping, choosing a doublet method for flow vs CyTOF, or diagnosing a suspicious double-positive cluster.
Before using code patterns, verify installed versions match. If versions differ:
R: packageVersion('<pkg>') then ?function_name to verify parameters
If code throws an error, introspect the installed package and adapt rather than retrying.
Doublet Detection
"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.
R (flow/spectral): flowCore gate on the FSC-A vs FSC-H diagonal (+ FSC-W/SSC-W)
R (mass/CyTOF): gate on DNA intercalator + Gaussian/Event_length parameters
The Single Most Important Modern Insight -- Doublets Are Caught by Area-vs-Height Non-Proportionality, and Scatter Gating Is Necessary but Not Sufficient
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 Taxonomy
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.
FSC-A vs FSC-H Singlet Gating (flow/spectral)
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)
CyTOF Doublet Removal
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]
Per-Method Failure Modes
1D area gate leaves doublets
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).
Heterotypic conjugate survives scatter gating
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.
CyTOF "doublet gate" using scatter
Trigger: porting flow logic to CyTOF. Mechanism: no FSC/SSC exists. Symptom: no scatter channels. Fix: use DNA + Gaussian/Event_length.
Quantitative Thresholds
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.
Common Errors
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
