The thrombogram includes all tests that evaluate platelets, including the following:
- Assessment of platelet numbers: Platelet count and estimate.
- Assessment of platelet size: Mean platelet volume.
- Platelet morphologic features: This does not have its own category in a hemogram, but a comment is added if abnormalities in platelet features are observed
Platelet counts can be done manually using a hemocytometer or with an automated analyzer. Counts can also be estimated during blood smear examination. Since many laboratories use instruments that count platelets, RBC and WBC concurrently, a platelet count is a routinely reported result on complete or automatd hemograms. At Cornell University, all platelet counts from our analyzer are verified by examining a blood smear for platelet clumps (which will falsely lower the count) and estimating a platelet count from the monolayer (as needed). Sometimes, the platelet count from our machine is deemed inaccurate and the count is deleted, with an estimate provided instead (if do-able).
In a well-prepared smear, platelets are estimated by counting the average number of platelets seen per 100x oil immersion field in the monolayer. In general, 10 oil immersion fields are counted and the results averaged (this accounts for uneven dispersal of platelets in the smear). Then the following formula is applied:
Estimated platelet count/µL = average count in 10 fields x 15,000.
At Cornell University, we provide a semi-quantitative estimate of platelet numbers which are based on reference intervals for the species in question (if available) as follows:
- Increased (Incr.): The count is above the reference interval for the species.
- Adequate (Adeq.): The count is within the reference interval for the species.
- Low?: The count is within the low end of the reference interval for the species or may be mildly decreased (i.e. the count is “equivocally low”).
- Low: The count is below the reference interval for the species.
- Very low: The platelet count is below a medical decision limit associated with spontaneous hemorrhage, i.e. <30,000/uL.
For example, if an average field contains 7 platelets, the estimate would be 105,000/µL. This value would then be compared to the reference interval for the species in question; 105,000 would be “low” (below the reference interval) for a dog, but “adequate” (or within the reference interval) for a horse.
Mean platelet volume
Just like RBC, automated analyzers can measure the average volume (or size) of platelets in femtoliters (FL) or mean platelet volume (MPV). Large platelets are often young platelets (not all young platelets are large) and we look for large platelets in thrombocytopenic animals as a “soft” guide as to an attempted bone marrow response to the thrombocytopenia. However, not all large platelets are young and some platelets can be large due to defective production, so platelet size should not be relied upon to measure a regenerative response in this hematopoietic cell line. Platelet clumps will falsely decrease a platelet count and falsely increase the MPV (a platelet clump is “seen” by the analyzer as a single, large platelet versus multiple smaller platelets). Also, individual platelets can vary markedly in size within a given sample, especially in blood from dogs and cats (even in health), so the presence of some large platelets may not be an abnormal finding.