What is the expected final concentration of White Blood Cells (WBCs) if 25 WBCs are counted in 1mm^2 of a hemacytometer with no dilution?

Calculating WBC Concentration from Hemacytometer Count

The correct answer is B: 250/μL. When 25 WBCs are counted in 1mm² of a hemacytometer with no dilution, the final concentration is 250 WBCs/μL.

Hemacytometer Calculation Principles

The calculation of cell concentration using a hemacytometer follows specific mathematical principles based on the chamber's dimensions:

1. Chamber Depth: A standard hemacytometer has a chamber depth of 0.1 mm
2. Counting Area: When counting in 1 mm² area
3. Volume Calculation: The volume of fluid counted = Area × Depth = 1 mm² × 0.1 mm = 0.1 mm³

Since 1 mm³ = 1 μL, the volume counted is 0.1 μL.

Calculation Steps

To determine the concentration of WBCs:

• Number of cells counted = 25 WBCs
• Volume examined = 0.1 μL
• No dilution factor (specified in the question)

Therefore:

• Concentration = (Number of cells counted) ÷ (Volume examined)
• Concentration = 25 WBCs ÷ 0.1 μL = 250 WBCs/μL

Clinical Context

In hematology laboratories, accurate cell counting is essential for diagnosing conditions like leukemia, monitoring treatment responses, and evaluating immune function 1. Proper technique in cell counting using hemacytometers is crucial for accurate results in both clinical and research settings 1.

Common Pitfalls to Avoid

• Forgetting the depth factor: Always account for the 0.1 mm depth of the hemacytometer chamber
• Incorrect dilution calculations: In this case, no dilution was used, but when dilutions are employed, they must be factored into the final calculation
• Confusing mm³ with μL: Remember that 1 mm³ equals 1 μL when converting volumes

Alternative Methods for Low Cell Counts

For very low WBC counts, specialized methods may be required:

• Nageotte hemacytometers with larger chamber volumes (50 μL) can detect concentrations as low as 0.01 WBC/μL 2
• Flow cytometric methods can count WBCs at concentrations of 5-10 WBCs/mL in platelet products 3

In conclusion, when 25 WBCs are counted in 1 mm² of a hemacytometer with no dilution, the correct calculation yields a concentration of 250 WBCs/μL.