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A Coulter counter is an instrument that counts and sizes particles suspended in a conductive liquid by detecting the electrical pulse each particle produces as it passes through a small opening. The method is called the Coulter principle. It is the foundation of automated hematology and is also used to analyse subvisible particles in protein formulations.
Short answer: what does a Coulter counter do?
A Coulter counter counts particles and measures their size. Each particle passing through the measurement aperture in an electric field causes a discrete electrical pulse whose magnitude is proportional to the particle’s volume, so the number of pulses gives the count and the pulse height gives the size (AAPS J 2010 minireview, PMC3032098, read 2026-11-07).
Definition and scope
Three names are often mixed up. The Coulter principle is the measurement method. A Coulter counter is an instrument built on that method. Beckman Coulter is the company name that followed Beckman Instruments’ acquisition of Coulter Corporation in October 1997; it was renamed Beckman Coulter, Inc. in April 1998 (Beckman Coulter, Wallace H. Coulter page, read 2026-11-07).
Who invented the Coulter counter?
Wallace H. Coulter developed the Coulter Principle in 1948, which Beckman Coulter describes as the foundation of automated hematology. The documented timeline is:
| Year | Event | Source |
|---|---|---|
| 1948 | Wallace Coulter develops the Coulter Principle | Beckman Coulter |
| 1953 | US patent 2,656,508 (W. H. Coulter) | PMC3032098 |
| 1956 | Coulter paper on a high-speed automatic blood cell counter | PMC3032098 |
| 1958 | Wallace and Joseph Coulter found Coulter Corporation and launch the Coulter Counter | Beckman Coulter |
| 1997-1998 | Beckman Instruments acquires Coulter Corporation; renamed Beckman Coulter, Inc. | Beckman Coulter |
What is the principle of a cell counter? How the Coulter principle works
Particles are suspended in a conductive liquid and drawn through a small aperture while an electric field is applied. The aperture physically constricts the current path, so a particle passing through changes the current. The sample must be dilute enough that only one particle passes the detection area at a time. Each passage produces one pulse; the number of pulses is the count and the pulse magnitude, proportional to volume, is the size.
Because the signal does not depend on light, the technique does not rely on the optical properties of the sample. The AAPS J review calls it an orthogonal technique for that reason.
Limits and a practical checklist
These points are stated in the AAPS J review for protein-particle work. They are a useful checklist for any Coulter-type measurement; follow your own instrument manual for model-specific limits.
- Conductive liquid: the buffer must be conductive enough. Low conductivity raises noise and reduces sensitivity at the low end, and smaller apertures are more affected.
- Aperture sizing: the dynamic range is about 2-80% of the aperture diameter, so a 100 um aperture covers roughly 2-80 um. Smaller apertures are used below 2 um; protein aggregates down to about 0.4 um have been measured.
- Dilution: samples usually need dilution so particles pass singly. Commercial sample holders take about 10 mL.
- Count precision: results below about 10,000 counts are much more variable; the review reports a coefficient of variation of 9% at about 100 counts.
- Cleaning and blanks: the aperture tube needs regular cleaning, and a high blank count indicates contamination.
Coulter principle compared with related methods
| Method | What the source says |
|---|---|
| Coulter principle | One electrical pulse per particle; does not rely on the sample’s optical properties (PMC3032098) |
| Light obscuration (USP <788>) | An optical method; the review recommends the Coulter principle as a complementary technique alongside it (PMC3032098) |
For manual counting, see the hemocytometer cell counting and viability guide. For optical single-cell analysis, see what a flow cytometer is and how flow cytometry works.
Where it matters for labs
The principle underpins automated hematology, and several ASTM standards are based on the method (PMC3032098). In protein-formulation work it is recommended as an orthogonal check next to light obscuration. When comparing counting approaches, the practical question is which property you need to measure and which validation your method requires.
Common misunderstandings
- “Coulter counter” is the instrument and “Coulter principle” is the method; they are not the same thing as the company Beckman Coulter.
- Pulse size reflects particle volume, not an optical property.
- A low particle count does not give a precise result: counts below about 10,000 are much more variable.
FAQ
What does a Coulter Counter do?
It counts and sizes particles in a conductive liquid by detecting one electrical pulse per particle passing through an aperture.
What is the best automated cell counter?
No source ranks one instrument as best in general. Electrical-impedance counters, optical methods and manual hemocytometer counts measure different things, so the choice depends on the measurement you need and the validation your method requires.
Who invented the Coulter Counter?
Wallace H. Coulter, who developed the principle in 1948; US patent 2,656,508 dates from 1953 and the Coulter Counter was launched in 1958.
What is the principle of a cell counter?
For a Coulter-type counter, it is the change in electrical current as each particle passes through a constricted aperture: pulse number gives count and pulse size gives volume.
Sources
- “The Coulter principle for analysis of subvisible particles in protein formulations”, AAPS J 2010, PMC3032098, read 2026-11-07.
- Beckman Coulter, Wallace H. Coulter, read 2026-11-07.
