FlowIO — Flow Cytometry File Handler

Overview

FlowIO is a lightweight Python library for reading and writing Flow Cytometry Standard (FCS) files. It parses FCS metadata, extracts event data as NumPy arrays, and creates new FCS files. Supports FCS versions 2.0, 3.0, and 3.1. Minimal dependencies — ideal for data pipelines and preprocessing before advanced analysis.

When to Use

• Parsing FCS files to extract event data as NumPy arrays
• Reading channel metadata (names, ranges, types) from FCS files
• Converting flow cytometry data to pandas DataFrames or CSV
• Creating new FCS files from NumPy arrays or processed data
• Handling multi-dataset FCS files (separating combined datasets)
• Batch processing directories of FCS files
• Preprocessing flow cytometry data before downstream analysis
• For compensation, gating, and FlowJo workspace support, use FlowKit instead
• For advanced cytometry visualization (density plots, gating plots), use matplotlib or plotly

Prerequisites

pip install flowio numpy pandas

Requires Python 3.9+. No compiled dependencies — installs on any platform.

Quick Start

from flowio import FlowData

flow = FlowData("experiment.fcs")
print(f"Events: {flow.event_count}, Channels: {flow.channel_count}")
print(f"Channels: {flow.pnn_labels}")

events = flow.as_array()  # Shape: (n_events, n_channels)
print(f"Data shape: {events.shape}")

Core API

1. Reading FCS Files

The FlowData class is the primary interface for reading FCS files.

from flowio import FlowData

# Standard reading
flow = FlowData("sample.fcs")
print(f"Version: {flow.version}")          # '3.0', '3.1', etc.
print(f"Events: {flow.event_count}")
print(f"Channels: {flow.channel_count}")

# Event data
events = flow.as_array()                   # Preprocessed (gain, log scaling)
raw = flow.as_array(preprocess=False)      # Raw values
print(f"Shape: {events.shape}")            # (n_events, n_channels)

# Memory-efficient: metadata only (skip DATA segment)
flow_meta = FlowData("sample.fcs", only_text=True)
print(f"Instrument: {flow_meta.text.get('$CYT', 'Unknown')}")

# Handle problematic files
flow = FlowData("bad.fcs", ignore_offset_discrepancy=True)
flow = FlowData("bad.fcs", use_header_offsets=True)

# Exclude null channels
flow = FlowData("sample.fcs", null_channel_list=["Time", "Null"])

2. Channel Metadata

Extract channel names, types, and ranges from FCS files.

flow = FlowData("sample.fcs")

# Channel names
pnn = flow.pnn_labels   # Short names: ['FSC-A', 'SSC-A', 'FL1-A', ...]
pns = flow.pns_labels   # Descriptive: ['Forward Scatter', 'Side Scatter', 'FITC', ...]
pnr = flow.pnr_values   # Range/max values per channel

# Channel type indices
scatter_idx = flow.scatter_indices   # [0, 1] — FSC, SSC
fluoro_idx = flow.fluoro_indices     # [2, 3, 4] — fluorescence channels
time_idx = flow.time_index           # Time channel index (or None)

# Access by type
events = flow.as_array()
scatter_data = events[:, scatter_idx]
fluoro_data = events[:, fluoro_idx]

# Full metadata (TEXT segment dictionary)
text = flow.text
print(f"Date: {text.get('$DATE', 'N/A')}")
print(f"Instrument: {text.get('$CYT', 'N/A')}")

3. Creating FCS Files

Generate new FCS files from NumPy arrays.

import numpy as np
from flowio import create_fcs

# Basic creation
events = np.random.rand(10000, 5) * 1000
channels = ["FSC-A", "SSC-A", "FL1-A", "FL2-A", "Time"]
create_fcs("output.fcs", events, channels)

# With descriptive names and metadata
create_fcs(
    "output.fcs",
    events,
    channels,
    opt_channel_names=["Forward Scatter", "Side Scatter", "FITC", "PE", "Time"],
    metadata={"$SRC": "Python pipeline", "$DATE": "17-FEB-2026", "$CYT": "Synthetic"},
)
# Output: FCS 3.1, single-precision float

4. Multi-Dataset FCS Files

Handle FCS files containing multiple datasets.

from flowio import FlowData, read_multiple_data_sets, MultipleDataSetsError

# Detect multi-dataset files
try:
    flow = FlowData("sample.fcs")
except MultipleDataSetsError:
    datasets = read_multiple_data_sets("sample.fcs")
    print(f"Found {len(datasets)} datasets")
    for i, ds in enumerate(datasets):
        print(f"Dataset {i}: {ds.event_count} events, {ds.channel_count} channels")
        events = ds.as_array()

# Read specific dataset by offset
first = FlowData("multi.fcs", nextdata_offset=0)
next_offset = int(first.text.get("$NEXTDATA", "0"))
if next_offset > 0:
    second = FlowData("multi.fcs", nextdata_offset=next_offset)

5. Modifying and Re-Exporting

Read, modify, and save FCS data.

from flowio import FlowData, create_fcs

# Read original
flow = FlowData("original.fcs")
events = flow.as_array(preprocess=False)  # Use raw for modification

# Filter events (e.g., threshold on FSC)
mask = events[:, 0] > 500
filtered = events[mask]
print(f"Before: {len(events)}, After: {len(filtered)}")

# Save filtered data as new FCS
create_fcs(
    "filtered.fcs",
    filtered,
    flow.pnn_labels,
    opt_channel_names=flow.pns_labels,
    metadata={**flow.text, "$SRC": "Filtered"},
)

# Or write with updated metadata (no event modification)
flow.write_fcs("updated.fcs", metadata={"$SRC": "Updated"})

Key Concepts

FCS File Structure

FCS files consist of four segments:

Segment	Content	FlowData attribute
HEADER	Version, byte offsets	flow.header
TEXT	Key-value metadata ($DATE, $CYT, channel names)	flow.text
DATA	Event data (binary/float)	flow.events (bytes), flow.as_array()
ANALYSIS	Optional processed results	flow.analysis

Preprocessing (as_array)

When preprocess=True (default), FlowIO applies:

1. Gain scaling: Multiply by PnG gain values
2. Log transform: Apply PnE exponential transform if present (value = a × 10^(b × raw))
3. Time scaling: Convert time channel to proper units

Use preprocess=False when you need raw values for modification or custom transforms.

Common Workflows

Workflow: Batch FCS Summary

from pathlib import Path
from flowio import FlowData
import pandas as pd

fcs_files = list(Path("data/").glob("*.fcs"))
summaries = []
for f in fcs_files:
    try:
        flow = FlowData(str(f), only_text=True)
        summaries.append({
            "file": f.name, "version": flow.version,
            "events": flow.event_count, "channels": flow.channel_count,
            "date": flow.text.get("$DATE", "N/A"),
        })
    except Exception as e:
        print(f"Error: {f.name}: {e}")

df = pd.DataFrame(summaries)
print(df)

Workflow: FCS to DataFrame with Channel Statistics

from flowio import FlowData
import pandas as pd
import numpy as np

flow = FlowData("sample.fcs")
df = pd.DataFrame(flow.as_array(), columns=flow.pnn_labels)

# Per-channel statistics
for col in df.columns:
    print(f"{col}: mean={df[col].mean():.1f}, median={df[col].median():.1f}, std={df[col].std():.1f}")

# Export
df.to_csv("output.csv", index=False)
print(f"Exported {len(df)} events, {len(df.columns)} channels")

Key Parameters

Parameter	Function	Default	Options	Effect
preprocess	as_array()	True	True/False	Apply gain/log scaling
only_text	FlowData()	False	True/False	Skip DATA segment (metadata only)
ignore_offset_discrepancy	FlowData()	False	True/False	Tolerate HEADER/TEXT offset mismatch
use_header_offsets	FlowData()	False	True/False	Prefer HEADER over TEXT offsets
ignore_offset_error	FlowData()	False	True/False	Skip all offset validation
null_channel_list	FlowData()	None	List of names	Exclude channels during parsing
nextdata_offset	FlowData()	None	byte offset	Read specific dataset in multi-dataset files
opt_channel_names	create_fcs()	None	List of names	Descriptive channel names (PnS)
metadata	create_fcs()	None	Dict	Custom TEXT segment key-value pairs

Best Practices

1. Use only_text=True for metadata scanning: When processing many files, skip DATA segment parsing for 10-100x speedup.

2. Use preprocess=False for data modification: Always work with raw values when filtering/modifying events, then re-export. Preprocessing is irreversible.

3. Anti-pattern — modifying flow.events directly: FlowIO does not support in-place event modification. Extract with as_array(), modify, then create_fcs() to save.

4. Preserve metadata on re-export: Pass flow.text as metadata to create_fcs() to retain original acquisition info.

5. Check for multi-dataset files: Catch MultipleDataSetsError and use read_multiple_data_sets() — some instruments write multiple acquisitions into one file.

Common Recipes

Recipe: Extract Fluorescence Channels Only

from flowio import FlowData
import numpy as np

flow = FlowData("sample.fcs")
events = flow.as_array()
fluoro = events[:, flow.fluoro_indices]
names = [flow.pnn_labels[i] for i in flow.fluoro_indices]
print(f"Fluorescence channels: {names}, shape: {fluoro.shape}")

Recipe: File Inspection Report

from flowio import FlowData

flow = FlowData("unknown.fcs")
print(f"Version: {flow.version} | Events: {flow.event_count:,} | Channels: {flow.channel_count}")
for i, (pnn, pns) in enumerate(zip(flow.pnn_labels, flow.pns_labels)):
    ctype = "scatter" if i in flow.scatter_indices else "fluoro" if i in flow.fluoro_indices else "time" if i == flow.time_index else "other"
    print(f"  [{i}] {pnn:10s} | {pns:30s} | {ctype}")
for key in ["$DATE", "$CYT", "$INST", "$SRC"]:
    print(f"  {key}: {flow.text.get(key, 'N/A')}")

Recipe: Normalize Events to [0, 1] Range

When to use: Prepare fluorescence channels for machine learning or cross-sample comparison.

from flowio import FlowData
import numpy as np

flow = FlowData("sample.fcs")
events = flow.as_array()

# Normalize each fluorescence channel to [0, 1]
fluoro_idx = flow.fluoro_indices
fluoro = events[:, fluoro_idx]
pnr = np.array(flow.pnr_values)[fluoro_idx]  # Per-channel max range
normalized = fluoro / pnr
print(f"Normalized shape: {normalized.shape}, range: [{normalized.min():.3f}, {normalized.max():.3f}]")

Troubleshooting

Problem	Cause	Solution
DataOffsetDiscrepancyError	HEADER/TEXT offset mismatch	Use ignore_offset_discrepancy=True
MultipleDataSetsError	File contains multiple datasets	Use read_multiple_data_sets() instead
FCSParsingError	Corrupt or non-standard FCS file	Try ignore_offset_error=True; verify file is valid FCS
Out of memory on large files	Millions of events loaded at once	Use only_text=True for metadata; process in chunks by channel
Unexpected channel count	Null/padding channels in file	Use null_channel_list=["Time", "Null"] to exclude
Modified data has wrong values	Applied preprocessing before modification	Use preprocess=False for raw data when modifying events
Channel names missing (empty PnS)	Instrument didn't set descriptive names	Use pnn_labels (short names) instead; PnS is optional in FCS spec

Related Skills

• matplotlib-scientific-plotting — create scatter plots, density plots, and histograms from extracted cytometry data
• scikit-learn-machine-learning — clustering and dimensionality reduction on cytometry event data

References

• FlowIO documentation — official GitHub repository and API
• FCS file format specification — ISAC data standards for flow cytometry
• Spidlen et al. (2010) "Data File Standard for Flow Cytometry, Version FCS 3.1" — Cytometry Part A