Blood platelets are cellular particles produced by megakaryocytes under the influence of a variety of growth regulatory factors. The main source is the bone marrow, although the lung and spleen are also sources of platelets. Megakaryocyte and platelet production are regulated primarily by thrombopoietin, a cytokine that acts as a growth factor for platelet production. The normal platelet seen in a Wright's stained blood film is 3-5 µm in diameter, and circulating platelets are disk shaped.

The role of platelets in hemostasis is of equal importance to that of the coagulation mechanism, as platelets are involved with the blood vessel wall and the contact-activated coagulation factors (XII and XI) in initiating the hemostatic process. Decrease in the number of circulating platelets (thrombocytopenia) or presence of abnormal, nonfunctional platelets (thrombasthenia, thrombopathia) will impair hemostasis.

Platelets are metabolically active in biochemical, physiological, and pathological processes (see Table). Platelets actively synthesize proteins, lipids, carbohydrates, and nucleotides, and generate metabolic energy.

Thrombocytopenic states and thrombocytoses are diagnosed by direct or indirect platelet counting.

Platelet Count. The normal platelet count determined manually or by electronic cell counter for most animal species, except rodents, is ~ 170,000-500,000/µl, and less than 100,000/µl can be considered significant. Rodents usually have platelet counts of at least 900,000/µl. A satisfactory platelet estimate is made by examining the freshly made blood smear. For example, the number of platelets per 100 white blood cells multiplied by the total white count is an estimate of the platelet count. Another method is to simply count the number of platelets per oil immersion field (100x objective) where each platelet/field is equivalent to 15,000/µl.

Platelet Clumping. This is a common in-vitro phenomenon and prevents accurate automated or manual platelet counting. Any reported platelet count indicates only the minimum platelet number. The platelet estimate is the best indicator of the adequacy of platelet numbers in such situations.

Clot Retraction. The phenomenon whereby a whole blood or PRP clot retracts from the sides of a glass container depends on normal platelet quantity and function. Clot retraction time may be reduced in anemia and accelerated in polycythemia because the hematocrit tends to influence the retraction of whole blood clots.

Bleeding Time. The bleeding time measures hemostasis from a standardized mucosal incision or by transection of a toe nail. The transection or toe nail bleeding time measures all components of hemostasis (vessel wall, platelets, coagulation), whereas the buccal mucosal time measures just the contribution of primary hemostasis (platelet adhesion and aggregation). The toe nail method offers a crude but practical way to assess the bleeding risk of a patient prior to surgery as well as the efficacy of treatment to control a bleeding diathesis.

Meaningful results are obtained when the test is performed under controlled conditions which regulate the size and depth of the incision or transection in awake, sedated, or anesthetized animals. Normal toe nail bleeding times vary from 2-5 mins in dogs, and 2-3.5 mins in cats, and times are prolonged in platelet defects, vascular lesions, and von Willebrand's disease. The normal buccal mucosal bleeding time is shorter (< 4 mins). In healthy horses and cattle, the lip bleeding time is 8-10 mins.

Platelet Aggregation and Release. The effect of various aggregating agents such as ADP, collagen, adrenalin, and thrombin on PRP or platelet suspensions is used to evaluate platelet function.

Platelet Retention (Adhesiveness). Retention of platelets in a glass bead column or filter of standard size measures their ability to adhere to foreign surfaces.

The above tests are usually only available from platelet research laboraries.

Clot Retraction and Bleeding Time. See above.