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Chamber counts and the NRBC correction

13 min

Skills you'll build4 skills
  • Calculate the volume counted in a hemacytometer
  • Calculate a cell concentration from a chamber count and dilution
  • Correct a WBC count that still includes nucleated red cells
  • Keep nucleated red cells out of the white-cell differential denominator

At the bench: Classify cells and describe morphology in a manual differential

Try first

Try first

Both sides of a hemacytometer are counted together: four 1-mm² corner squares on each side, eight squares in all, at a chamber depth of 0.1 mm. What total volume was counted?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Volume counted in a hemacytometer

Every manual chamber count depends on two values: the number of cells counted and the volume of specimen that contained them. An improved Neubauer hemacytometer has a chamber depth of 0.1 mm when the coverslip is seated correctly, and one large ruled square covers 1 mm². Because 1 mm³ equals 1 µL, one large square contains 0.1 µL.1

Volume counted (µL) = area counted (mm²) × chamber depth (mm)

When both chamber sides are counted, use either the sum of the cells from both sides with the total area, or the average count per side with one side's area. Mixing the two conventions changes the result by a factor of 2.1

Correcting for the dilution

A manual count is made from diluted specimen, so the count is multiplied back to the original concentration.

Cells/µL = cells counted × dilution factor ÷ volume counted (µL)

Leaving out the dilution factor reports the concentration of the diluted mixture. Multiplying by the dilution factor gives the specimen's true concentration.

Keep nucleated red cells out of the differential

A 100-cell white-cell differential counts 100 white cells only. A nucleated red blood cell (NRBC) seen during the count is tallied separately from the 100 and reported per 100 white cells.2 When NRBCs are frequent enough, conventionally more than 10 per 100 white cells, the counting method may include them in the WBC count, and a correction removes them:

Corrected WBC = uncorrected WBC × 100 ÷ (100 + NRBCs per 100 WBCs)

Apply the correction once. A platform that already reports an NRBC-corrected WBC count has made the adjustment, and applying the formula again would falsely lower the result.1

Follow the bench demonstration

Sources for this step
References
  1. Keohane EM, Preston MM, Mirza KM, Walenga JM, eds. Rodak's Hematology: Clinical Principles and Applications. 7th ed. Elsevier; 2025. Accessed September 26, 2026. https://www.us.elsevierhealth.com/rodaks-hematology-9780323936507.html
  2. American Society for Clinical Pathology Board of Certification. Medical Laboratory Scientist, MLS(ASCP) and MLS(ASCPi) Examination Content Guideline. Revised June 9, 2026. p 14. Accessed September 26, 2026. https://ascpcontentwebsite.blob.core.windows.net/boccontent/docs/default-source/explore-credentials/content-guidelines/ascp_ascpi_mls_content_guidelinef6ccee32-711a-4152-8b5b-65061928c57f.pdf

Watch one

Read each decision and its explanation.

A 1:20 dilution of blood is charged into a hemacytometer. The eight corner squares, four on each side and 8 mm² in all, contain 168 cells combined. What is the WBC count?

  1. Find the volume counted: 8 mm² × 0.1 mm = 0.8 µL.

    The area and depth give the volume counted.

  2. Set up the full relationship: cells/µL = cells counted × dilution factor ÷ volume counted.

    The dilution factor scales the counted concentration back up to the original specimen.

  3. Calculate: 168 × 20 ÷ 0.8 = 4,200/µL.

    Substituting the count, dilution factor, and volume gives the concentration.

  4. Convert to the conventional unit: 4,200/µL = 4.2 × 10³/µL.

    Reporting in the conventional WBC unit keeps the result comparable with the reference interval.

WBC = 4.2 × 10³/µL.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

An uncorrected white blood cell (WBC) count of 20.0 × 10³/µL includes nucleated red blood cells (NRBCs). The differential shows 25 NRBCs per 100 WBCs. What is the corrected WBC count?

Incorrect. Multiplying by 125 ÷ 100 inverts the ratio and raises the count. Removing NRBCs from a count that includes them lowers it.

Correct. Corrected WBC = 20.0 × 100 ÷ (100 + 25) = 16.0 × 10³/µL. Apply the correction once, and report an analyzer-corrected WBC without further correction.

Incorrect. Multiplying by 0.75 subtracts 25% of the count. The 25 NRBCs were counted alongside 100 WBCs, so they are 25 of every 125 nucleated cells and the divisor is 125.

Hint
  1. The NRBCs were counted in addition to the 100 WBCs.
  2. The correction divisor is 100 plus the NRBCs per 100 WBCs.
  3. Apply the correction once only.

Review Corrected WBC count for nucleated red cells

Problem 2 of 3 · Hints available

A cerebrospinal fluid (CSF) specimen is diluted 1:2 for a nucleated cell count. Four large squares, 4 mm² in all, on one chamber side contain 18 nucleated cells. What is the nucleated cell count?

This leaves out the 1:2 dilution factor. Cells counted ÷ volume counted gives the concentration in the diluted mixture. Multiplying by the dilution factor converts it to the specimen concentration.

Left out the prepared dilution factor

Cells counted ÷ volume counted gives the concentration in the diluted suspension. Leaving out a dilution factor of 10 reports 240 cells/µL for a specimen that holds 2,400 cells/µL.

This leaves out the factor of 10 that comes from the 0.1-mm chamber depth.

Nucleated cells/µL = cells counted × 10 × dilution factor ÷ squares counted = 18 × 10 × 2 ÷ 4 = 90/µL.

Hint
  1. Multiplying by 10 corrects for the 0.1-mm chamber depth.
  2. Include the dilution factor in the same calculation.

Review Manual whole-blood counts

Problem 3 of 3 · On your own

A blood film shows nucleated red cells during a 100-cell white-cell differential. Put these steps in the order a laboratory professional performs them.

  1. Classify white cells until 100 are counted, tallying each nucleated red cell separately outside the 100.
  2. Correct the WBC count once, when the nucleated red cells exceed the laboratory's trigger and the platform has not already separated them.
  3. Report the nucleated red cells per 100 white cells.
  4. Find the monolayer at low power, where cells lie singly.
Show the answer

Find the monolayer at low power, where cells lie singly., Classify white cells until 100 are counted, tallying each nucleated red cell separately outside the 100., Report the nucleated red cells per 100 white cells., Correct the WBC count once, when the nucleated red cells exceed the laboratory's trigger and the platform has not already separated them.

Find the monolayer first. Then count 100 white cells with the nucleated red cells tallied outside that 100, report them per 100 white cells, and correct the WBC count once when the trigger is met.

Review White-cell parameters

Use it

Use the specimen and readings to make the next bench decision. The step is complete without it.

Try this at the MicroscopeWBC0 / 100MANUAL DIFF ORDEREDPeripheral blood differentialPerform manual diff

For reference

Sources checked