From the Research
To calculate the number of platelets from a blood smear, count the average number of platelets in 10 oil immersion fields (1000x magnification) and multiply by 15,000 to estimate the platelet count per microliter. This method, known as the platelet estimate, is most useful when automated counts are unavailable or unreliable. For accurate results, examine well-stained areas of the smear where red blood cells are just touching but not overlapping. Be careful to distinguish platelets (small anucleate fragments that stain blue-purple) from artifacts, bacteria, or cellular debris. This estimation technique works because each oil immersion field contains approximately 1/15,000th of a microliter of blood under standard conditions.
Key Considerations
- The technique has limitations in accuracy compared to automated analyzers, with potential variability of ±20-40% 1.
- For critical clinical decisions, confirmation with automated methods is recommended when available.
- The method is reliable and can be used as a reference, as shown by the high correlation between manual and automated counts in a study published in the Turkish journal of haematology 2.
Steps to Estimate Platelet Count
- Count the average number of platelets in 10 oil immersion fields (1000x magnification).
- Multiply the average number of platelets by 15,000 to estimate the platelet count per microliter.
- For example, if you observe an average of 10 platelets per field, the estimated count would be 150,000 platelets/μL.
Important Notes
- Automated platelet counts are highly reliable and accurately predict clinical bleeding, as shown in a study published in the American journal of hematology 3.
- The Brecher & Cronkite reference method should always be indicated in thrombocytopenic patients for platelet counts below 30,000 plt/microliter obtained in one dimensional counters 4.