From the Guidelines
To calculate the number of platelets from a blood smear and red blood cell (RBC) count, use the platelet estimate method or the ratio method, as there is no direct formula provided in the given evidence that relates platelet count to RBC count and smear results directly for calculation purposes 1.
Platelet Estimate Method
The platelet estimate method involves counting the average number of platelets in 10 oil immersion fields (1000x magnification) and then multiplying this number by 15,000 to get the approximate platelet count per microliter of blood. This method provides a straightforward approach to estimating platelet counts when more precise measurements are not available.
Ratio Method
Alternatively, the ratio method can be used by counting platelets in relation to RBCs. This involves counting platelets and RBCs in several fields to determine the platelet-to-RBC ratio. Once this ratio is established, it can be multiplied by the known RBC count (in millions/μL) and by 1,000 to get the estimated platelet count. For instance, if the observation is 1 platelet for every 20 RBCs and the RBC count is 5 million/μL, the calculation would be (1/20) × 5 × 1,000 = 250,000 platelets/μL.
Considerations
These methods are particularly useful in settings where resources are limited or when automated platelet counts are not available. However, it's crucial to note that these manual methods are less accurate than automated analyzers. Therefore, proper smear preparation and a consistent counting technique are essential for obtaining reliable results. The given evidence does not directly address the calculation of platelet numbers from smear and RBC count but emphasizes the importance of maintaining adequate platelet levels, especially in patients with bleeding or at risk of bleeding 1.
From the Research
Calculating Platelet Count from Smear and RBC
To calculate the number of platelets from a smear and RBC, several methods can be employed, as outlined in various studies 2, 3, 4.
- The International Council for Standardization in Haematology (ICSH) and the International Society of Laboratory Hematology (ISLH) recommend counting specifically labeled platelets relative to RBCs with a fluorescence flow cytometer, along with an accurate RBC count determined with a semiautomated, single-channel aperture-impedance counter 2.
- Another method involves estimating the platelet count indirectly by using the automated red blood cell (RBC) count and calculating the platelet count based on the red cell:platelet ratio in a stained blood film 3. This method has been found to be reliable, with a high correlation between automated and manual counts.
- A simpler approach involves visually counting platelets in a peripheral blood smear and comparing with an automated machine platelet count 4. This can be done by averaging the number of platelets in 10 high-power fields and multiplying by 15,000 to arrive at a platelet count in thousands per microliter.
Key Considerations
When calculating platelet count, it's essential to consider the following:
- The ratio of platelets to RBCs can vary, and this should be taken into account when estimating platelet count 3.
- The use of a standardized method, such as the one recommended by ICSH and ISLH, can help ensure accurate results 2.
- Visual counting of platelets in a peripheral blood smear can be a useful alternative to automated counting, especially in cases where automated counts are not available or are suspected to be inaccurate 4.
Related Conditions
Thrombocytopenia, a condition characterized by a low platelet count, can be diagnosed and monitored using these methods 5, 6. Understanding the causes and consequences of thrombocytopenia is crucial for effective management and treatment.