Platelet Estimate

Performing a Platelet Estimate on a Peripheral Blood Smear

Principle

• 8-10 microscopic fields are scanned using a 100x oil objective, in an area where the RBC’s are barely touching.
• Multiple the average number of counted by 10 to obtain an estimated total PLT count.

This is a good quality control measure that helps to validate the platelet count provided by the Hematology analyzer. Both the automated count and the estimate should correlate. The size variation of the platelets is measured via the platelet distribution width (MPV or PDW).

Note: We are using a patient estimation factor 10, based on the microscopes we are using, not 20,000 as indicated in the Rodak textbook. “A platelet estimation factor should be determined and validated for each microscope in use.” ^[1]

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

1. Using the 100x oil objective, scan 8-10 fields in the morphology zone, and count the number of platelets in each field. Refer to table 1-1 for platelet estimate values.
2. Calculate the average number of platelets counted in the scanned fields and multiply by 10. This will provide you with an estimated platelet count which should correlate with the automated count.
3. The results should agree within 10% of the machine count. If the count does not agree, repeat the estimate. If the results are still not in agreement, a manual platelet should be performed.
4. Document the platelet estimate on the hematology worksheet.

Procedural Notes

1. Stay within the morphology zone when performing the PLT estimate.
2. Use the table below to determine if the value is normal, low or high.
3. A new smear may have to be made if cellular distribution is poor.
4. Compare the estimate to the automated result.
5. “Because of the variation in the field diameter among different microscopes, an estimation factor should be determined for each microscope in use”^[2]

Quality

• PLT estimates aids in validating the counts from the hematology analyzer and the results should agree within 10% of the instrument count.
• Normal range: 150- 400 x 109/L

Interpretation

• Example: 10 fields were scanned and an average of 19 platelets were counted using the 100x oil objective
• 19 x 10 = 190 x 109/L

Sources of Error

Lack of correlation between the automated platelet counts and platelet estimates may be a result of:

1. Poor platelet distribution on the smear – make a new slide
2. PLT’s may be caught up in presence of fibrin (clotted sample) causing erroneous estimations – request a new sample.
3. Wrong patient – collect a new sample
4. Instrument problem – re-run the sample