Unopette® system for platelet counting with Newbauer hemocytometer.

Platelet counts can be done manually or using automated cell counters. Since many laboratories use instruments that count platelets, red cells and leukocytes concurrently, a platelet count is a routinely reported result on many samples of dog and horse blood. However, platelet clumping will lower (and in some instances invalidate) the platelet count when determined by any method. This occurs especially in samples of cat and cow blood (see image below). In these instances, an accurate platelet count cannot be provided, but for most purposes, significant changes in platelet number can be detected during the microscopic examination of the stained blood smear (see section on platelet estimation). In such samples, close attention should be paid to the platelet smear estimate part of our hemogram.

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 are done manually using a Unopette® diluting system, hemocytometer, and a microscope acquipped with phase microscopy. The platelets can be visualized in the counting chamber and directly counted, however this method is relatively inaccurate compared to automated counting. Platelet clumps also invalidate the count as the platelets cannot be enumerated accurately if they are all piled up in clumps on the hemocytometer. Since we have the Advia, manual platelet counts are rarely performed.

Impedance-based platelet counts
Our old hematology analyzer, the S+IV, used to count platelets by impedance methods (see RBC count). This is less accurate than the Advia as debris and small red blood cells are also included in the platelet count.

Flow cytometry-based platelet counts
This measures platelets by flow cytometry based on principles of light scattering.

1. Platelets 2. Large platelets 3. Red blood cells 4. RBC fragments 5. Debris 6. Ghosts

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 is plotted against the X axis and high light scatter 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 species with very small red cells, e.g. goats, some red blood cells may be counted as platelets.

The analyzer also detects platelet clumps and flags their presence. When this occurs, a comment is appended to the platelet count or smear estimate (if a count is not provided) demonstrating the presence of clumps.

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Last Updated: June 2000