Abstract
Flow cytometry is widely used for high-throughput single-cell analysis. However, its data
analysis relies on either costly commercial software or programming-intensive open-source
tools. To bridge this gap, we developed FlowWeb, a freely accessible, web-based platform
that combines the flexibility of the R/Bioconductor ecosystem with an intuitive graphical
user interface. FlowWeb enables integrated workflows for data handling, quality control,
gating, visualization and statistical analysis within a unified environment.
FlowWeb integrates raw data, metadata, and analytical state within synchronized
Bioconductor structures, enabling coherent analysis and visualization workflows. FlowWeb
supports both manual and automated data-driven gating workflows. To evaluate its
performance, we applied FlowWeb to an in-house flow cytometry dataset and compared its
automated cell cycle and gating workflows to established commercial tools. FlowWeb’s
automated cell cycle workflow produced consistent and reproducible results across
replicates and demonstrated high concordance with reference analyses, highlighting the
platform’s robustness. FlowWeb’s advanced visualization tools include a wide range of fully
customizable individual, overlay, and statistical plots. To enhance usability and
reproducibility, the FlowWeb platform provides optional user-accounts that allow storage
of reusable configurations, including quality control presets, gating definitions, and plot
templates.
By lowering technical barriers without compromising analytical rigor, FlowWeb facilitates
accessible, reproducible, and scalable flow cytometry data analysis for a broad range of
users in research and clinical settings.
Introduction
Flow cytometry is a cornerstone technology in modern biomedical research and clinical
diagnostics, enabling rapid, high-throughput measurement of cellular phenotypes at the
single-cell level. It is used worldwide across diverse fields including immunology, oncology,
and hematology, where precise characterization of complex cell populations is essential. The
analysis of flow cytometry data is a critical step in the workflow, often requiring specialized
expertise and dedicated software environments.
Several commercial software packages, such as FlowJo (BD Biosciences), Kaluza (Beckman
Coulter), and FCS Express (De Novo Software), provide comprehensive solutions for data
visualization, gating, and statistical analysis. While these platforms are powerful and widely
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used, they are typically associated with substantial licensing costs and may impose limitations
on accessibility, reproducibility, or integration with modern data science workflows. In parallel,
a rich ecosystem of open-source, R-based tools (e.g., flowCore, flowWorkspace, openCyto)
provides highly flexible and reproducible alternatives. However, effective use of these
packages generally requires programming expertise, which remains a barrier for many
experimental researchers who rely on graphical user interfaces.
To address these challenges, we developed FlowWeb, an interactive, web-based platform for
flow cytometry data analysis that combines the flexibility of the R/Bioconductor ecosystem with
an intuitive graphical user interface.
FlowWeb enables reproducible, scalable,
and user-friendly analysis by integrating data
handling, visualization, gating, and statistical
evaluation within a unified environment. By
leveraging open-source infrastructure and a
modular architecture, it lowers the barrier to
advanced cytometric analysis while
maintaining analytical rigor and
transparency.
Data handling / architecture
FlowWeb is built on a layered data model that separates cytometry data, metadata, and
analytical state into distinct but synchronized components.
Uploaded FCS files are represented as flowFrame objects using the Bioconductor flowCore
infrastructure [1]. Each file is assigned both a stable internal identifier and a user-friendly label,
allowing reliable data handling while maintaining clarity in the interface.
Initially, uploaded FCS-files are grouped in flowSets based on their (shared) detector
configurations, which facilitates data-preprocessing (such as Transformations). For each
flowSet a corresponding GatingSet is created. The flowSet stores the data and derived
populations, while the GatingSet [2] captures the hierarchical gating structure and ensures
that analytical steps are applied consistently across samples. The software keeps the
GatingSet registry automatically synchronized with the flowSet registry, including creation of
new GatingSets for new flowSets and removal of obsolete GatingSets when flowSets are
deleted. This design facilitates the processing of multiple files with distinct antibody panels
and/or detector configurations in one session, reflecting the practical organization of flow
cytometry studies, where multiple staining panels are commonly analyzed in parallel.
Sample annotation data can be imported from spreadsheets and are integrated with cytometry
data in a central interactive table. This table links samples, metadata, and derived populations,
enabling intuitive navigation and analysis. The application is implemented in Shiny, which
manages user interactions and keeps all components synchronized.
Importantly, uploaded cytometry data and sample annotation files are processed within the
active session and are not persistently stored on the server. This session-based handling
minimizes long-term data retention while maintaining interactive performance.
Figure 1: Frontend–Backend Architecture
Overview: browser-based interface with server-
side analysis using open-source R packages from
CRAN, Bioconductor and GitHub.
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The FlowWeb Interface
FlowWeb is built as an interactive shiny environment embedded within a website, combining
a reactive analysis interface with the accessibility of a web-based platform. All computations
are performed on a secure, EU-based server, ensuring centralized processing and facilitating
compliance with data protection standards. In addition to the core analytical workspace, the
website provides tutorial material to guide users through the main features of the software,
with further tutorials expected as the platform develops. The site also includes a personalized
user environment, “My FlowWeb,” which enables storage and reuse of analysis-related
settings and will be discussed in more detail at the end of the manuscript. FlowWeb is hosted
as a web platform and is anticipated to expand over time, with additional functionality, interface
improvements, and supporting resources.
The main table
Architecture and Interaction
Uploaded FCS-files are integrated into a reactive table that serves as the main interface for
the dataset. Root rows correspond to imported FCS files, whereas child rows represent gated
subpopulations derived from a parent sample. Under the hood, derived populations store their
immediate parent in their metadata, enabling reconstruction of the full lineage tree. This
parent-child structure is used to calculate table indentation, build collapsible hierarchies in the
interface, propagate metadata, and support cascaded editing or deletion of (sub-) populations.
In addition, parent-child structure also serves as the substrate for summary statistics. Event
counts are taken directly from each flowFrame, and the number of events as percentages of
its parent (%Par) and its root sample (%Tot) are computed from the parent-child hierarchy.
Figure 2: Overview of the FlowWeb interface , showing the main reactive components of the web-
based analysis environment, including the main table, plot canvas, and associated control panels.
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The table is highly interactive and supports multiple levels of navigation, selection, and data
exploration. Hierarchical relationships between samples and derived populations can be
expanded or collapsed, while context menus provide access to advanced operations such as
file inspection, gate application, and data export. Users can directly visualize and edit gates
via dedicated controls, link table entries to corresponding plots, and display population
statistics within visualizations. In addition, flexible filtering options enable both global search
and column-specific subsetting to efficiently interrogate large datasets (see footnote 1).
Sample Info
FlowWeb supports importing spreadsheet-based sample annotation files. A template for the
spreadsheet-based sample annotation file is generated by, and can be downloaded from, the
FlowWeb interface. This template contains a column with the file names extracted from the
imported files and can be manually complemented with columns of sample information. Upon
uploading, the software automatically searches the uploaded spreadsheet for a column whose
values match the loaded rootfile identifiers. Once detected, the annotation sheet is joined to
the main table by rootfile. This join strategy is particularly important because it propagates
sample-level metadata not only to original files but, under the hood, also to all derived
populations belonging to that sample.
As with cytometry data, sample annotation data are processed within the active session and
are not stored on the server beyond the current session. Nevertheless, users are advised to
ensure that uploaded metadata do not contain sensitive or identifiable information.
Beyond sample-level annotations, the software provides direct access to FCS keyword
metadata. For a selected file, the full keyword list can be retrieved from the underlying
flowFrame via the context-menu. The application supports both direct keyword selection and
free-text search across keyword names and values, including atomic metadata fields and more
complex nested objects. This enables users to inspect acquisition metadata interactively
without leaving the web interface. Scientifically, this functionality is useful because FCS
Keywords
often contain acquisition settings, instrument details, compensation-related
metadata, and experiment descriptors that are relevant for quality control and tracking the
origin and acquisition conditions of the data.
The Interactive Main Table
Navigation: To keep a compact display of large datasets while preserving the parent-child relationships
between files and derived populations, Parent rows can be expanded or collapsed using a toggle symbol, both
of a single sample subtree or of the full table (via the context menu).
Context menus that provide advanced options can be accessed through right-click within the table.
Files: Population: %Par values:
- Select associated plots - Select associated files or plots - Edit the %Par label
- Select associated plots + children - Access file info Eye-icons:
- Export and download as fcs-file - Apply gate to other files - Show gate on all compatible plots
- Save gate
Eye- and Pencil- icons adjacent to the Population name allow the visualization of the corresponding gate in
the selected plots and direct access to gate editing, respectively.
%Par labels can be displayed in the corresponding plots by clicking the values in the table.
Filtering options support both global text search and column-specific filtering. Column filters can be accessed
via the column header and provide either categorical selection or numeric range filtering, depending on the
data type. Active filters are indicated visually in the column header.
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Functions
Data processing
Quality Control
Quality control (QC) ([Data Processing] > [Quality Control] is implemented in FlowWeb using
the R package flowAI[3], which provides automated detection and removal of anomalous
events in flow cytometry data. The QC pipeline operates directly on flowFrame objects and is
applied at the level of individual samples, while maintaining consistency with the higher-level
flowSet and GatingSet structures.
QC behavior is controlled through a parameter set that includes anomaly detection modes
(flow rate, signal acquisition, dynamic range), statistical thresholds, channel exclusions and
output format (filtered events, QC vector, or event indices). These parameters can be manually
set in the “Advanced QC parameters” modal ([Data Processing] > [Quality Control] >
[Settings]) and, when logged in, can be saved for future analyses.
To improve scalability, QC is optionally applied in a chunk-wise manner, where each
flowFrame is partitioned into subsets of events. This design avoids memory bottlenecks when
processing large cytometry files and allows incremental reconstruction of high-quality data.
These outputs are reassembled across chunks to form a cleaned dataset. The final high-
quality event matrix is used to construct a new flowFrame, preserving original parameter
metadata and keywords. Although chunk-wise QC improves computational scalability, its use
within the flowAI framework may reduce statistical power and can therefore lead to increased
event exclusion.
When QC has finished, the results appear in a new tab. The QC reports can optionally be
saved and downloaded as a zip-file, which allows visual inspection of the QC results. In
addition, the zip-file contains the cleaned fcs-files, which avoids repeated QC steps.
Cleaned flowFrame objects replace their corresponding entries in the flowSet and the reactive
main table is updated, including the percentage of retained events.
Transform
Data transformation is implemented using the transformation framework provided by flowCore,
which supports a wide range of mathematical transformations commonly used in flow
cytometry, including:
• logarithmic
• arcsinh
• linear and scaling transforms,
• quadratic transforms
• truncation
• biexponential
• logicle
Transformations are applied at the flowSet level, ensuring that all samples within a panel-
compatible group are processed consistently. Because the original (untransformed) flowSet
is stored in a separate reactive registry the transformations are fully reversible, which avoids
reloading files.
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Because transformations modify the underlying expression matrices, all downstream
analyses, including gating and visualization, operate on transformed data.
Plot
Visualization in FlowWeb is implemented using the R package ggplot2 [4] as the primary
rendering engine, combined with shiny for a dynamic user interface. Additional extensions and
base statistical layers within ggplot2 are used to support a wide range of visualization
modalities.
Individual Plots
Individual plots are generated from flowFrame objects retrieved from the internal flowSet
registry. For each selected sample, the expression matrix is extracted using exprs() and
converted into a standard data frame.
Each plot instance is represented by a unique identifier and associated configuration
parameters, including selected channels, plot type and visualization settings, axis scales and
ranges, and metadata display options.
FlowWeb supports multiple visualization modes commonly used in flow cytometry:
• Dot plots
• 1D density plots
• Density scatter plots
• Contour plots
• Box plots
All plots are constructed using layered ggplot2 grammar, allowing consistent application of
themes, scales, and annotations. Hence, a wide range of the different layers of ggplot2
variables can be adjusted by the users through [Edit].
Plot instances are tracked in a centralized table, which stores layout information (position,
size) and links each plot to its underlying data source. This enables multiple independent plots
to coexist and be interactively positioned within the interface.
A key feature of the plotting system is its integration with the gating framework. Gate
geometries are overlaid onto plots using polygon layers, ensuring that visualizations reflect
the current analytical state stored in GatingSet objects. These overlays are dynamically
recomputed based on the selected sample and channel configuration.
Data from the reactive metadata table allows users to display sample-level or population-level
information directly within the visualization. This creates a tight coupling between data,
metadata, and visual output.
Overlay plots
Overlay plots enable direct comparison of multiple samples within a single visualization. These
plots are constructed by combining data from multiple flowFrame objects into a unified data
frame, with an additional grouping variable indicating sample identity.
For each selected sample, expression values are extracted and concatenated into a single
long-format data frame, allowing ggplot2 to map color, fill, or grouping aesthetics across
samples.
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Overlay plots support the following plot types:
• Overlay density plots
• Frequency polygons
• Scatter overlays (dot plots)
• Box plots
• Ridge plots
Overlay plots can group and color samples based on metadata columns rather than raw file
identity. This is enabled by joining the combined expression data with the reactive metadata
table, allowing grouping by experimental condition, population, or user-defined annotations.
Statistical plots
Statistical visualization is implemented as a distinct plot family that operates on aggregated
tabular data rather than raw event-level measurements. These plots are generated using
ggplot2 for visualization and ggpubr for statistical testing and annotation.
Statistical plots are constructed from the reactive metadata table rather than directly from
flowFrame objects. For selected rows, a derived dataset is created containing sample
identifiers, population labels and summary metrics such as %Par or event counts. This dataset
is stored in a dedicated registry and serves as the input for all statistical visualizations. The
data are reshaped into a long format suitable for ggplot2, with allows the use of sample
metadata to customize the plots, such as grouping on the x-axis, fill and shape aesthetics.
Currently, two main statistical plot types are supported ; bar plots, showing mean values per
group with optional error bars (e.g., standard error), and stacked bar plots; hierarchical
representation of populations within each sample.
Statistics
Statistical information can be added to Statistical plots through their context menus ([Statistics]
> [Add statistics]). The “Statistics Settings” menu hosts a range of parameters that can be set
by the user to tailor the statistical comparisons performed by the ggpubr package. These
parameters include:
• Type of test (e.g., t-test, Wilcoxon)
• Paired vs. Unpaired design
• variance assumptions
• Multiple testing correction
When applied, the selected statistical test is automatically performed between the groups on
the x-axis and results are visualized as annotated brackets above the corresponding bars.
Label formatting is configurable (e.g., stars vs. exact p-values), and non-significant results can
be optionally suppressed. The system also includes logic to suggest appropriate statistical
tests based on data structure (e.g., number of groups, sample size).
The “Statistics summary” modal ([Statistics] > [Show summary]) allows the user to access a
summary interface that provides dataset structure (group counts), applied statistical
parameters, and an overview of the underlying data. This ensures transparency and
reproducibility of statistical results.
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Gating
Manual gating
Manual gating in FlowWeb is implemented as an interactive workflow that combines direct
graphical user input with formal cytometry data structures. The core gating machinery relies
on the Bioconductor packages flowCore and flowWorkspace, while Shiny provides the event-
driven user interface and ggplot2 supports graphical rendering of the gating canvas. In
practice, user-drawn gates are not treated as visual annotations but are converted into formal
gate objects and incorporated into sample-specific gating hierarchies.
Manual gating in FlowWeb is guided by the user’s current selection (sample, channels, and
gate type) and interactive input on the plot. Gates can be drawn and edited using standard
geometries such as thresholds, rectangles, polygons, ellipses, and quadrants. In all cases,
the drawn region is converted into a formal gate definition and added to the gating hierarchy.
FlowWeb supports the following gate geometries from the flowCore package:
• 1D density gates
• Rectangle gates
• Ellipse gates
• Polygon gates
• Quadrant gates
At the data level, cytometry data are handled as flowFrame objects, from which expression
values are accessed and gated subsets are derived. User-defined regions are translated into
formal gate objects (e.g. rectangular or polygonal gates), preserving the relationship between
geometric boundaries and measurement channels. These gates are embedded into a
GatingSet / GatingHierarchy structure, ensuring that population relationships are maintained
and consistently updated when gates are modified.
Each gated population is exported as a child population and represented both within the
hierarchical gating structure and in a metadata table that records parent–child relationships,
population names, event counts, and visual attributes such as gate colour. The tabular
representation and the gating hierarchy are updated in parallel, ensuring consistency between
views. When a gate is edited, its definition is retrieved, reconstructed in the editor, and updated
in the gating hierarchy. Changes are subsequently propagated to all dependent populations,
affected child populations are recomputed and their associated data and metadata are
updated accordingly. This ensures that manual adjustments are consistently reflected
throughout the analysis workflow.
Automated gating
Automated gating in FlowWeb complements manual gating by providing algorithmic methods
for identifying populations based on one- or two-dimensional distributions. This part of the
framework combines methods from openCyto and model-based packages such as flowClust,
mixtools, and mclust. These methods are wrapped by FlowWeb so that users can select them
from the interface, inspect their results in a preview panel, and then apply them to one or more
samples.
FlowWeb provides automated gating methods such as:
• Cell cycle analysis
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• Minimum-density thresholding
• Tail-based cutoff detection
• Quantile gating
• Two-dimensional model-based clustering
These methods are particularly appropriate for marker channels where one seeks to separate
negative and positive populations, exclude debris or extreme tails, or enforce percentile-based
thresholds across samples.
For two-dimensional model-based clustering, the implementation makes use of flowClust.
Here, the event cloud in a selected pair of channels is modelled as a mixture of elliptical
clusters. Each component is then expanded into an explicit polygonal contour corresponding
to a chosen confidence level, allowing both visualization and downstream export as child
populations. This approach is well-suited to cytometric populations that are approximately
ellipsoidal but overlap in complex ways. FlowWeb converts these ellipsoidal models into
polygonal gates for visualization and export, fitting them seamlessly into the broader gate
overlay and editing infrastructure.
For the one-dimensional automated cell-cycle analysis, FlowWeb uses a hybrid strategy. The
distribution is first characterized by a kernel density estimate, from which candidate peaks,
intervening valleys, peak prominence, and approximate basin mass are derived. A biologically
plausible G1 peak is selected from these KDE features, and a local two-component Gaussian
mixture is fit in a robust window around that peak using mixtools. The fitted mixture model is
used to identify the two principal DNA-content peaks and to assign G1 phase based on
posterior probability. The final gates are next constructed explicitly; G1 is centered on the
selected KDE peak and sized from high-confidence G1 events within its basin, whereas G2 is
placed at an approximate duplication multiple of the G1 center with related width, and S phase
is defined as the intervening interval between the two. Although this routine is not part of
openCyto, it follows the same design principle: model a population in a statistically principled
way, convert the result into explicit gates, and propagate those gates through the same export
pipeline as all other methods.
Results
To benchmark FlowWeb’s accuracy and reproducibility against established commercial
platforms, we performed automated cell cycle analysis on Propidium Iodide-stained samples.
To evaluate both performance and reproducibility, we analyzed samples from two conditions
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(Control and Treated), each with three
replicates, showing clearly distinct cell cycle
patterns (Figure 1). The distribution of cells
over G1, S and G2 as determined by
FlowWeb closely resembles the distribution
as reported by FCS-Express. The results
obtained by Kaluza deviated slightly,
assigning fewer cells to S-phase and more to
G2. Both FlowWeb as well as FCS-express
demonstrated high reproducibility across the
three replicates. When averaged across G1,
S and G2, FlowWeb results showed a
Standard Deviation of 6.69% and 3.74% in
Control and Treated condition, respectively,
which was comparable to that of FCS-
Express (6.52% and 4.22%) (Table 1).
Altogether, these results confirmed that the
performance of the FlowWeb software
closely matches that of commercially
available software packages.
As in the manual case, flowCore provides
the cytometry containers and gate classes,
flowWorkspace manages hierarchical
integration and recomputation, shiny
orchestrates the interactive workflow, and
ggplot2 renders previews and final overlays.
My FlowWeb
Flow Web provides a user-specific environment (“My FlowWeb”) that allows storage and reuse
of analysis configurations across sessions. This functionality is implemented through a hybrid
architecture combining a reactive R-based frontend through shiny, with a backend layer
implemented as a custom WordPress plugin.
The system enables users to store and retrieve three distinct classes of objects: (i) quality
control (QC) presets, (ii) plot templates, and (iii) gating definitions. All objects are associated
Figure 3: A dataset of 6 samples (3 x Control, 3 x
Treated) was first manually cleaned in FlowWeb by
excluding debris and doublets. The percentage of
cells in the three phases of the cell cycle was then
quantified using automated cell cycle analysis
pipelines from three different software packages.
Representative DNA content profiles are shown
along side the graphs. Error bars represent the
standard deviation of the mean.
Table 1: Overview of the
distribution of cells over
G1, S and G2 phases as
determined by the
automated cell cycle
analysis in three
software packages.
Note: For the Treated
samples, Kaluza was
only able to detect the
different phases in one
of three samples.
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with authenticated user accounts and are versioned and timestamped in a relational database.
Communication between the Shiny application and the backend is performed using the httr2
HTTP client, with authentication enforced through WordPress session cookies and nonce-
based request validation. The different objects are stored in a dedicated relational table within
WordPress, with each record linked to a specific user ID. This design allows different aspects
of the workflow to be treated as reusable configuration objects, decoupled from specific
datasets while remaining fully reproducible.
QC presets; a list of QC parameter configurations used during preprocessing and quality
control steps, allowing users to standardize workflows across datasets.
Plot templates; includes amongst others plot type and rendering parameters (e.g., density
adjustment, binning), aesthetics (colors, transparency, contour properties), axis configuration
(scales, labels, visibility),annotation settings (header and subheader formatting), and legend
and display options. Plot templates are portable across datasets and allow rapid replication of
complex visualization settings without reconfiguration.
Gates; encode explicit geometric and analytical gating objects that can be reapplied across
datasets. These objects integrate tightly with the cytometry infrastructure provided by flowCore
and flowWorkspace. Within the app, logged-in users can retrieve an overview of saved gates
from the reactive storage on demand. This overview includes information related to gate type,
associated channels and optional metadata (color, source file, versioning, edit history).
User-specific objects stored within “My FlowWeb,” including plot templates, QC presets, and
gating definitions, consist of analytical parameters such as channel selections, visualization
settings, thresholds, and gate geometries. These objects do not store raw cytometry data and
are not intended to contain patient-identifiable information. However, under frameworks such
as the GDPR, information may be considered personal if it can directly or indirectly identify an
individual. Users are therefore responsible for ensuring that no sensitive information is
embedded in user-defined labels, annotations, or metadata associated with these stored
configurations.
Competing interest
M.t.H. developed the web application described in this work and may pursue monetization of
the platform in the future.
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