Platelet Count

Platelet Count

Platelet counts can be done manuualy 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, red cells and leukocytes concurrently, a platelet count is a routinely reported result on complete or automatd hemograms. At Cornell University, we use the ADVIA hematology analyzer for obtaining platelet counts and verify the obtained count by estimating counts on a Wright's-stained blood smear.

Automated platelet counts with the ADVIA

1. Platelets

2. Large platelets

3. Red blood cells

4. RBC fragments

5. Debris

6. Ghosts

The ADVIA measures platelets by flow cytometry based on principles of light scattering. Platelets are identified by their size (< 30 fL) and refractive index (n = 1.35 to n = 1.40). The platelet cytogram on the left is a graphical representation of how the Advia counts platelets. Low light scatter (refractive index or internal complexity) is plotted against the X axis and high light scatter (cell size) is plotted against the Y axis (B). Platelets are detected in the region labeled 1. Large platelets (section 2) are identified on the basis of size (> 20 fL) and refractive index (which distinguishes them from red cells). In camelids with iron deficiency anemia or hemolyzed samples from any animal, small or lysed red blood cells may be erroneously counted as platelets, falsely increasing the platelet count.

All blood smears are also examined for the presence of platelet clumps (see image to the right below), which will affect the accuracy of the platelet count. Platelet clumps decrease the platelet count obtained by any method. Thus any provided count should be considered a minimum platelet count in the sample. In some samples with severe platelet clumps (many blood samples from cats), the platelet count is totally invalid and a count will not be provided. Under these circumstances (or any other situations resulting in erroneous counts, such as an iron deficient camelid), a platelet count will not be provided and the smear estimate of platelet numbers (increased, adequate, low? or low) given with our hemograms should be used as a guide as to the actual platelet count. platelet clumps

Cats are notorious for platelet clumping (their platelets are activated at the slightest provocation) and it is difficult to obtain accurate counts in this species. Platelet clumping is usually due to a sample collection problem and can be minimized by collecting blood from a large peripheral vein (cephalic or jugular), such that blood flows smoothly into the vacutainer or syringe, and using a 22 or 23 g needle (in a dog or cat). The blood should be mixed with the anticoagulant as soon as possible after collection, by gentle rotation or inversion. Platelet clumping increases with time, so platelet counts should be done as soon as possible after collection to maintain accuracy.

Manual platelet counts

Platelet counts can be done manually with a commercial diluting system, hemocytometer, and a microscope. These counts are less accurate than automated counts, because platelets can be difficult to distinguish from debris. Platelet clumping will also decrease the hemocytometer platelet count.

Platelet count estimates from a blood semar examination

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.

For example, if an average field contains 7 platelets, an estimate of 105,000/µL would be appropriate. This value would then be compared to the normal range for the species in question ... 105K would be "low" (below reference intervals) for a dog, but "adeqate" (or within reference intervals) for a horse.