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Flow cytometry controls, subset counts and detection limits

15 min

Skills you'll build

  • Choose the flow-cytometry control for gate placement, spillover, or autofluorescence
  • Calculate an absolute subset count using the correct parent population
  • Report a low result within the method's detection and quantitation limits

Read the full reference

Try first

Try first

In an eight-color tube, a marker stains its cells dimly, and you need to place the positive gate for it. Which control shows where the negative cells end once spread from the other seven labels is included?

Keep going to build the reasoning.

Keep going to build the reasoning.

Right. Keep going to follow the reasoning.

Keep going to build the reasoning.

Keep going to build the reasoning.

Get the idea

Use the explanation to check your first decision.

Each control answers one question

Flow cytometry controls are chosen by the question each one answers.1,3

  • Instrument performance particles track alignment, detector response and day-to-day stability.
  • An unstained specimen shows autofluorescence and baseline signal.
  • Single-stained controls measure spillover between labels, so compensation or spectral unmixing can be set.
  • A fluorescence-minus-one (FMO) control holds every label except one. It shows where the negative cells end in that label's detector, which matters most next to a dim population.
  • An isotype control shows part of the nonspecific binding. It cannot set every positive gate by itself.3

Count against the right parent

A dual-platform count combines a percentage from the cytometer with an absolute count from the hematology analyzer:2

absolute subset count = absolute parent-population count × subset fraction

A CD3/CD4 percentage is a share of the lymphocyte gate, so its parent is the absolute lymphocyte count. With 2,100 lymphocytes/µL and 30% CD3/CD4 cells, the count is 2,100 cells/µL × 0.30 = 630 cells/µL. Multiplying 30% by the whole white blood cell (WBC) count would put monocytes and granulocytes in the parent and inflate the result.

Detected, then quantified

Three limits describe how low a method can go.4

  1. The limit of blank is the highest apparent value expected from a sample with no measurand.
  2. The limit of detection is the lowest value reliably told apart from the limit of blank.
  3. The lower limit of quantitation is the lowest value measured with the method's stated accuracy.
Where the signal fallsWhat the report can say
At or above the lower limit of quantitationThe measured value
Between the limit of detection and the lower limit of quantitationDetected below the quantitation limit, when the procedure permits it
Below the limit of detectionThe procedure's wording for values below its validated limits

A signal between the two limits shows the measurand is present. It cannot support an exact number.4 A rare-event flow cytometry report states the assay's sensitivity when it affects interpretation.5

Follow the bench demonstration

Sources for this step
References
  1. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
  2. Clinical and Laboratory Standards Institute. Enumeration of Immunologically Defined Cell Populations by Flow Cytometry. 2nd ed. CLSI guideline H42-A2. Clinical and Laboratory Standards Institute; 2007. Reaffirmed June 2017. Accessed September 26, 2026. https://clsi.org/shop/standards/h42/
  3. Maecker HT, Trotter J. Flow cytometry controls, instrument setup, and the determination of positivity. Cytometry A. 2006;69(9):1037-1042. doi:10.1002/cyto.a.20333
  4. Clinical and Laboratory Standards Institute. Evaluation of Detection Capability for Clinical Laboratory Measurement Procedures. 2nd ed. CLSI guideline EP17-A2. Clinical and Laboratory Standards Institute; 2012. Reaffirmed June 2017. Accessed September 26, 2026. https://clsi.org/shop/standards/ep17/
  5. Clinical and Laboratory Standards Institute. Validation of Assays Performed by Flow Cytometry. 1st ed. CLSI guideline H62. Clinical and Laboratory Standards Institute; 2021. Accessed September 26, 2026. https://clsi.org/shop/standards/h62/

Watch one

Follow each move and the reason beside it.

A CBC gives a WBC count of 6.4 × 10³/µL with 30% lymphocytes. On the cytometer, 22% of the events in the lymphocyte gate meet the CD3/CD4 definition. What is the absolute CD3/CD4 count?

  1. Write the WBC count in cells: 6.4 × 10³/µL = 6,400 cells/µL.

    Both counts must share one unit before they are multiplied.

  2. Find the parent: 6,400 cells/µL × 0.30 = 1,920 lymphocytes/µL.

    The 22% describes lymphocytes, so the lymphocyte count is the parent.

  3. Multiply by the subset fraction: 1,920 cells/µL × 0.22 = 422.4 cells/µL.

    The subset fraction applies to its own parent population.

  4. Check the size: 422 cells/µL is smaller than 1,920 lymphocytes/µL, as it must be.

    A subset larger than its parent would mean the wrong parent or a reversed calculation.

  5. Round at the end: 422 cells/µL.

    Intermediate values stay unrounded, and the answer is rounded once.

The absolute CD3/CD4 count is 422 cells/µL.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

The white blood cell (WBC) count is 8,000 cells/µL with 25% lymphocytes, and 40% of the lymphocytes meet the CD3/CD4 definition. What is the absolute CD3/CD4 count?

Incorrect. Multiplying the whole WBC count by 0.40 gives this value. The 40% is a fraction of the lymphocytes, so the parent count is 8,000 cells/µL × 0.25 = 2,000 lymphocytes/µL.

Correct. Absolute lymphocytes = 8,000 cells/µL × 0.25 = 2,000 cells/µL, and the CD3/CD4 count = 2,000 cells/µL × 0.40 = 800 cells/µL. Each percentage is applied to its own parent population.

Incorrect. Dividing 2,000 lymphocytes/µL by 0.40 reverses the calculation and gives a subset larger than its own parent population.

Hint
  1. Name the population the 40% describes.
  2. Find the absolute lymphocyte count from the WBC count and the lymphocyte percentage first.
  3. Apply the subset fraction to the lymphocyte count you found.

Review Enumeration and immunophenotyping

Problem 2 of 3 · Hints available

A new lot of a tandem-dye antibody is put into use, and the compensation for the panel has to be set again. Which control do you run?

A fluorescence-minus-one control helps place a gate after compensation is set. It does not measure each label's spillover.

Each single-stained control shows how much of one label reaches every other detector. Those measurements set the compensation for this instrument and this lot.

An unstained specimen shows autofluorescence. With no label in the tube, it has no spillover to measure.

An isotype control looks at nonspecific binding. It does not measure how much of one label reaches the other detectors.

Hint
  1. Compensation removes the signal one label sends into another label's detector.
  2. To measure that spillover, each label has to be seen on its own.

Review Plots, gates, and controls

Problem 3 of 3 · On your own

The WBC count is 4.5 × 10³/µL with 20% lymphocytes. On the cytometer, 35% of the lymphocyte gate meets the CD3/CD8 definition. What is the absolute CD3/CD8 count?

Show the answer

315 cells/µL

The lymphocyte count is 4,500 cells/µL × 0.20 = 900 cells/µL, and the CD3/CD8 count is 900 cells/µL × 0.35 = 315 cells/µL.

Review Enumeration and immunophenotyping

Use it

Read the case evidence, then make each decision.

  • A follow-up bone marrow aspirate from a 9-year-old boy treated for B-lymphoblastic leukemia (MRN 0049208) is tested for measurable residual disease.
  • His leukemic cells carry one dim marker read in the PE detector.
  • Spread from the brighter labels widens the negative cells in that detector.
  • The assay's validated limit of detection is 0.002% of leukocytes, and its lower limit of quantitation is 0.01%.
  • The unstained tube, the single-stained controls and the instrument performance check are all acceptable.
Decision 1 of 2

Which control do you use to place the positive gate in the PE detector?

The single-stained control set the compensation. It shows PE alone, without the spread the other labels add to this detector.

It holds every label except PE, so it shows how far the negative cells reach in the PE detector with all the spread present. The gate starts where they end.

An isotype control shows part of the nonspecific binding and none of the spread from the other labels. A gate set from it can cut into the dim leukemic cells or take in negative ones.

Set a positive gate from an isotype control

An isotype control shows only part of the nonspecific antibody binding. Setting a positive gate from it ignores spillover spread and autofluorescence, which fluorescence-minus-one and unstained controls show, and can misplace the gate around a dim population.

The unstained tube shows autofluorescence. It has none of the spread from the other labels, which is what widens the negative cells here.

Review Plots, gates, and controls

Decision 2 of 2

With the gate set, the leukemic population makes up 0.005% of leukocytes. How is it reported?

The value falls below the lower limit of quantitation of 0.01%, so the assay cannot state it with its required accuracy. A number would claim more than the method measured.

Reported an exact value below the quantitation limit

A signal between the limit of detection and the lower limit of quantitation shows the measurand is present at an amount the method cannot state with its required accuracy. Reporting it as an exact number states a value the method never measured reliably. The procedure's wording, such as detected below the quantitation limit, reports only what the result supports.

The population is above the limit of detection of 0.002%, so the assay has told it apart from background. Calling it not detected would leave out disease the assay found.

The specimen and controls are acceptable. The procedure has a wording for a result between the two limits, so this specimen can be reported.

The population lies between the limit of detection and the lower limit of quantitation. The report says it is present and states the sensitivity, and gives no percentage.

Review Calibration, measuring interval, and interference control

The clue that settles this case is where 0.005% falls. It sits above the limit of detection of 0.002% and below the lower limit of quantitation of 0.01%, so the population is reported as present without a number. The FMO control set the gate that found it, because only that control showed how far the negative cells spread in the PE detector.

For reference

Sources checked