The Coincidence Artifact: When Two Cells Count as One

The Bottom Line Up Front: Coincidence - multiple cells in the aperture simultaneously - causes two cells to be counted as one, corrupting both count and size data. Optimal aperture utilization means targeting 15-40% of aperture diameter so cells generate strong signals while avoiding coincidence artifacts. Match your cassette to your cell size, stay within concentration guidelines, and coincidence becomes a non-issue.

WHICH CASSETTES FOR YOUR INSTRUMENT

Moxi V and Moxi GO II use S+ and M+ cassettes. Moxi Z uses S and M cassettes. Same sizing principles, same selection logic — just match the cassette type to your instrument. All recommendations in this guide apply across the Moxi family.

Understanding Coincidence in Coulter Counting

The Coulter principle detects cells individually as they pass through the aperture. But when multiple cells pass through simultaneously, the system generates a single combined signal instead of separate signals. This creates a dual error: your count decreases (two events become one), and your size distribution is corrupted (the combined volume appears as an anomalously large cell).

Coincidence doesn't generate error messages. It silently biases your measurements, producing consistently low counts and size distributions with unexpected peaks at approximately double your normal cell size.

TL;DR - Coincidence Artifact Essentials

Two or more cells in aperture simultaneously generate one combined signal
Results in undercounting AND distorted size distributions (combined volumes look like giant cells)
Target cells at 15-40% of aperture diameter for optimal signal without coincidence
Cells should occupy 2-60% of aperture cross-sectional area for accurate measurements
Dilution test: if counts increase when you dilute, coincidence was present

Understanding and Preventing Coincidence

Learn the physics behind coincidence, how to detect it in your data, and protocols to prevent it from affecting your measurements.

The Physics: How Coincidence Happens

The Coulter principle detects cells individually as they pass through the aperture, measuring the electrical resistance change from each cell's volume displacement. But this elegant physics has a vulnerability: the sensing zone can only process one event at a time.

Multiple cells in the aperture simultaneously generate a single combined signal. The system has no way to distinguish "one 2000 fL cell" from "two 1000 fL cells arriving together." Both produce identical signals.

This creates a dual error: your count is reduced (two events become one), and your size distribution is corrupted (the combined volume appears as an anomalously large cell). Coincidence doesn't just affect a few cells - it systematically biases your entire measurement.

Silent Error

Unlike clogging or low-signal errors, coincidence produces no warnings. The only indication is systematic undercounting and artificial peaks at large cell sizes.

The 15-40% Optimization Range

Target cells should ideally be 15 to 40% of the aperture diameter for optimal sizing resolution. This range provides strong signals while avoiding coincidence. For accurate measurements, cells should occupy approximately 2 to 60% of the aperture's cross-sectional area.

Below 15% of diameter: Signals become weak and may be lost in noise. You're using more aperture volume than necessary, increasing the chance of coincidence.

15-40% of diameter: The sweet spot. Strong signals that are clearly detected, efficient use of aperture volume that minimizes coincidence probability.

Above 40% of diameter: Signals may saturate, clogging risk increases, and the cell dominates the sensing zone in ways that can still cause artifacts.

Rule of Thumb

If your cells are under 15 μm, use S+ or S cassettes. If over 15 μm, use M+ or M cassettes. This automatically puts you in the optimal range.

Cassette Selection to Minimize Coincidence

Matching cassette to cell size isn't just about signal strength - it directly affects coincidence probability. Using oversized apertures for small cells wastes sensing zone volume, allowing more cells to fit in the aperture simultaneously.

Small cells (under 15 μm) on S+ cassettes: The smaller aperture means your cells occupy a larger fraction of the sensing zone. Fewer cells can fit simultaneously, reducing coincidence even at higher concentrations.

Large cells (over 15 μm) on M+ cassettes: The larger aperture accommodates cell size while still optimizing the cell-to-aperture ratio for the detection physics.

The wrong cassette choice creates coincidence risk even at normal concentrations. A 6 μm lymphocyte in an M+ aperture occupies such a small fraction of the sensing zone that coincidence becomes probable well before reaching concentration limits.

Detecting Coincidence in Your Data

Coincidence is insidious because it doesn't generate error messages. Watch for these patterns:

Counts lower than expected: If your Moxi counts consistently underperform hemocytometer or other methods, coincidence may be systematically removing events.

Unusual peaks at large sizes: The combined volume of two coincident cells appears as one large cell. Look for unexpected peaks at approximately double your normal cell size.

Non-linear dilution behavior: The gold standard test. Dilute your sample 2x and measure again. If your concentration increases (rather than decreasing to half), coincidence was present in the original measurement.

Concentration-dependent accuracy: If accuracy degrades at higher concentrations, coincidence is the likely culprit. Measurements should be consistent across the working range.

Concentration Guidelines and Best Practices

Optimal aperture utilization combines cassette selection with concentration management:

Know your working range: Each cassette/cell type combination has a concentration range where coincidence remains negligible. Stay within it.

Dilute if necessary: High-concentration samples benefit from dilution before measurement. The dilution factor is precisely known; coincidence error is not.

Verify with serial dilutions: When validating a new cell type or protocol, run a serial dilution to confirm linear behavior across your working concentration range.

Match cassette to cells: Using the correctly sized cassette for your cells automatically optimizes the sensing zone utilization, providing coincidence protection built into your protocol.

Validation Protocol

For new cell types: prepare a 2x dilution series and run all samples. If counts scale linearly with dilution, your protocol is coincidence-free.

Troubleshooting Guide

Counts consistently lower than reference methods

Solution: Check for coincidence by diluting sample 2x and re-measuring. If the diluted count per mL increases, coincidence was present. Switch to size-matched cassettes and/or dilute samples.

Size distribution shows peaks at unexpected large sizes

Solution: These may be coincidence artifacts (two cells measured as one large cell). Verify by dilution: if peaks disappear at lower concentration, they were coincidence.

Accuracy varies with sample concentration

Solution: Concentration-dependent error patterns suggest coincidence at higher concentrations. Establish a standard working concentration that avoids the problematic range.

Small cells on M+ cassettes seem to undercount

Solution: Small cells in large apertures have higher coincidence probability. Switch to S+ or S cassettes - the smaller aperture reduces coincidence risk significantly.

Frequently Asked Questions

What is coincidence in Coulter counting?

Coincidence occurs when multiple cells pass through the aperture simultaneously, generating a single combined signal instead of separate signals for each cell. This results in undercounting (two cells counted as one) and distorted size measurements (combined volume appears as one large cell). Coincidence increases with cell concentration and undersized apertures.

How does aperture size affect coincidence?

Smaller apertures relative to cell size mean cells occupy a larger fraction of the sensing zone, reducing the probability of multiple cells fitting simultaneously. Using oversized apertures for small cells increases coincidence risk by allowing more cells to occupy the sensing zone at once. Matching cassette to cell size optimizes coincidence protection.

What is the optimal cell-to-aperture ratio to avoid coincidence?

For accurate measurements, cells should occupy approximately 2-60% of the aperture cross-sectional area, with optimal sizing resolution at 15-40% of the aperture diameter. This range provides strong signals while avoiding coincidence from multiple cells in the aperture simultaneously.

How do I detect coincidence in my measurements?

Signs of coincidence include unexpectedly low counts compared to other methods, size distributions with unusual peaks at approximately double normal cell size, poor reproducibility at higher concentrations, and counts that don't scale linearly with dilution. The gold standard test: if diluting your sample significantly increases apparent concentration, coincidence was occurring.

Key Takeaway

Coincidence is preventable, not inevitable. Match your cassette to your cell size to optimize the cell-to-aperture ratio. Target 15-40% of aperture diameter. Stay within concentration guidelines. When you get these fundamentals right, coincidence becomes a textbook phenomenon you never see in your own data - instead of a silent error corrupting every measurement you make.

Using an aperture much larger than necessary reduces sizing resolution by creating smaller signal differences between cell sizes. Cell populations that should be distinguishable appear merged when the aperture is too large for the cells being measured. Target 15-40% of aperture diameter for optimal resolution. If you're counting lymphocytes on M+ cassettes because it works, you're sacrificing the sizing resolution that S+ cassettes would provide.

The 15 μm boundary provides clear selection criterion: cells under 15 micrometers use S+ cassettes, cells over 15 micrometers use M+ cassettes. This boundary isn't arbitrary - it's where each aperture size achieves the optimal 15-40% cell-to-aperture ratio for signal quality and sizing resolution. Know your cell size, follow the boundary, and cassette selection becomes automatic.

When your sample contains both small and large cells, no single cassette optimizes measurement for both populations. The solution: run the same sample twice - once with S+ to get accurate small cell counts, once with M+ to get accurate large cell counts. This dual-cassette workflow delivers accurate data for both populations rather than compromised data for everyone.