WBC Estimate
Performing A White Blood Cell Estimate On A Peripheral Blood Smear
Learning Objectives
- Provides proper instructions for performing a white blood cell estimate on a blood smear.
- Calculate the WBC estimate using the formula
- Identify sources or error
- Interpret results
CSMLS Competency Profile References
- Category 2: Data specimen collection and handling – 2.07-2.08; 2.11-2.12
- Category 3: Analytical processes – 3.01 – 3.02.01; 3.09.02; 3.13; 3.14 – 3.14.02
- Category 4: Interpretation and reporting results – 4.01-4.06
- Category 5: Quality management – 5.02; 5.05; 5.08; 5.10
- Category 6: Critical thinking – 6.04-6.06
Principle
8-10 microscopic fields are scanned using a 40x dry or 50x oil objective, in an area where the RBC’s are barely touching. Multiple the average number of WBC’s counted by 2.0 (if 40x is used), or 2.5(if 50x is used), to obtain an estimated total WBC count.
White blood cell estimates on the smear provides a good quality control measure that helps to validate the total white blood cell count provided by the Hematology analyzer. Both the automated count and the estimate should correlate.
Materials
- Light microscope
- Immersion oil
- Cell counter
- Kim wipes
Sample Type
- A well-made wright-stained peripheral blood smear.
Safety Considerations
- Refer to MSD sheets for safety considerations, located in the reagent prep room.
- All specimens received in the lab must be regarded as potentially infectious so follow safe practices.
- Refer to regulations and safety precautions in the student lab manual located in each student lab.
Procedure
- Using the 40x or 50x objective, scan 8-10 fields in the morphology zone, and count the number of white blood cells in each field.
- Calculate the average number of WBC’s counted in the fields and multiply by 2 if you used a 40x objective or multiply it by 2.5 if you used a 50x objective. This will provide you with an estimated WBC count which should correlate with the automated count.
- The results should agree within 10% of the instrument count. If the count does not agree, repeat the estimate.
- Document the WBC estimate on the hematology worksheet.
PROCEDURAL NOTES
- Stay within the morphology zone when performing the WBC estimate.
- Use the appropriate calculation factor for the objective used.
- A new smear may have to be made if cellular distribution is poor.
- Compare the estimate to the automated result.
- “Because of the variation in the field diameter among different microscopes, an estimation factor should be determined for each microscope in use”
Quality
The manual estimate must be within 10% of the instrument count to be considered valid.
- Expected values: 3.6-10.6 x109/L.
Interpretation
Example: An average of 5 WBC’s is counted in 10 high power fields using a 40x objective – 5 x 2 = 10.0 x 109/L.
- If you use a 50x oil objective – 5 x 2.5 = 12.5 x 109/L.
SOURCES OF ERROR
Lack of correlation between the automated WBC counts and WBC estimates may be a result of:
- WBC or Platelet clumps resulting in poor WBC distribution on the smear – make a new slide.
- WBC may be caught up in presence of fibrin (clotted sample) causing erroneous estimations – request a new sample.
- Severe RBC agglutination.
- Wrong patient or mislabeled smear– collect a new sample
- Instrument malfunction – re-run the sample.