Platelet Parameters

The platelet count is technically the platelet concentration in whole blood. Units = [platelets] ? 10³∕μL; SI units = [platelets] ? 10⁹∕L. Automated analyzers are routinely used to generate a platelet count.

Mean Platelet Volume (MPV)

The mean platelet volume (MPV) is the mean or average platelet volume and is measured in femtoliters (fL). This value is an average and does not represent the range of platelet volumes. Increased MPV is consistent with increased release of large or giant platelets and usually indicates a platelet regenerative response. The measurement of reticulated platelets (analogous to reticulocytes for red blood cells) is used diagnostically in human medicine and is likely to become an accepted index of platelet regeneration in veterinary medicine as well.

Platelet Distribution Width (PDW)

The platelet distribution width (PDW) is a calculated range of platelet volumes (representing anisocytosis), analogous to the red cell distribution width. The PDW is expressed as a percentage. A higher percentage indicates a greater variation in platelet volume. The PDW in combination with the MPV value can indicate whether there are increased numbers of smaller and∕or larger platelets.

Thrombocrit

Also called the plateletcrit, the thrombocrit value is a function of the platelet count and the MPV.

Thrombocytopenia

A decrease in the circulating platelet count is termed throm­bocytopenia. Thrombocytopenia is caused by one of three basic mechanisms: platelet sequestration, decreased platelet produc­tion, and decreased platelet life span due to consumption or

■ TABLE 27.1

Normal Values for Hemostatic Data in Ruminants and Horses

^aInsufficient data available.

Modified from Duncan JR, Prasse KW, Mahaffey EA, eds: Veterinary laboratory medicine, ed 3, Ames, IA, 1994, Iowa State University Press; and Kaneko JJ, Harvey JW, Bruss ML, eds: Clinical biochemistry of domestic animals, ed 5, San Diego, 1997, Academic Press.

■ BOX 27.1

Causes of Thrombocytopenia in Horses

Common Causes

Disseminated intravascular coagulation (DIC)

Immune-mediated thrombocytopenia (IMT) (drugs: penicillin, trimethoprim-sulfadoxine)

Endotoxemia, septicemia (e.g., acute toxic colitis, intestinal strangulating obstruction, neonatal septicemia)

Equine infectious anemia

Equine anaplasmosis (Anaplasma phagocytophilum)

Lymphoma

Less Common Causes

Salmonellosis

Equine viral arteritis

Equine influenza

Myeloproliferative disease (myelogenous leukemia)

Plasma cell myeloma

Aplastic anemia

Stachybotryotoxicosis

Neonatal alloimmune thrombocytopenia

■ BOX 27.2

Causes of Thrombocytopenia in Ruminants

Common Causes

Disseminated intravascular coagulation (DIC)

Bracken fern (Pteridium aquilinum) toxicosis

Septic mastitis or metritis (endotoxemia)

Less Common Causes

Salmonellosis

Gram-negative sepsis Trichloroethylene-extracted soybean meal Lymphoma

Plasma cell myeloma

Immune-mediated thrombocytopenia (IMT) Stachybotryotoxicosis

Myelofibrosis (pygmy goats)

destruction (Boxes 27.1 and 27.2).

Splenic platelet sequestration can occur as a result of splenomegaly. Splenomegaly occurs due to various disorders, including infiltrative diseases such as lymphoma, leukemia, and multiple myeloma; infectious and other inflammatory diseases such as immune-mediated hemolytic anemia that result in splenitis; lymphoid hyperplasia; and/or splenic extramedullary hematopoiesis. In addition, congestive splenomegaly occurs when venous outflow is occluded by torsion, intestinal displace­ments, or congestive heart failure. Platelet sequestration is a relative thrombocytopenia due to redistribution (versus an absolute thrombocytopenia), is reversible, and does not result in increased bleeding tendency.

Decreased platelet production can occur with various bone marrow disorders, including myelophthisis (replacement of normal marrow). Myelophthisis is often due to marrow infiltration with neoplastic cells (e.g., plasma cells in multiple myeloma) and can result in pancytopenia. Familial myelofi­brosis was reported in pygmy goats and resulted in marked pancytopenia, including thrombocytopenia with myelophthisis.⁵ Toxic damage to the marrow (e.g., from bracken fern toxicosis or nonsteroidal antiinflammatory drug administration) can result in myelosuppression and potentially aplastic anemia.

Blood loss is a rare cause of thrombocytopenia; it is more likely that any concurrent bleeding is the result rather than the cause of severe thrombocytopenia. Studies in dogs found that acute severe blood loss induces a mild to moderate thrombocytopenia, with up to 50% reduction in platelet count.² Rapid dilution of the blood volume with aggressive fluid therapy can decrease the platelet count as well as other hemostatic elements. This decrease is generally mild to moderate.

Platelet life span can be reduced by destruction of platelets or by platelet consumption in widespread coagulation. Immune- mediated platelet destruction is a common cause of thrombo­cytopenia in horses. IMT can be either idiopathic or secondary to various etiologies, including viral and bacterial infections, drug administration, and hemic neoplasia.^7-10 Thrombocytopenia due to idiopathic IMT is often marked, in contrast to throm­bocytopenia due to other causes. The thrombocytopenia of equine anaplasmosis due to infection with Anaplasma phagocy­tophilum is apparently multifactorial; an element of IMT is possible but not supported by experimental studies, and splenic sequestration is also a potential contributor.^11,12 Neonatal alloimmune thrombocytopenia has been reported in horses, mule foals, and pigs and is caused by colostral antibodies against foreign (paternal) epitopes on the neonate's platelets.²

Excessive consumption of platelets occurs with disseminated intravascular coagulation (DIC).

Clinical Signs of Thrombocytopenia

Thrombocytopenia is characterized by petechial or ecchymotic hemorrhages on the oral, nasal, and/or vaginal mucous mem­branes and the nictitans, sclerae, pinnae, and ventral abdomen. Epistaxis, melena, hyphema, and/or hematuria may occur, although spontaneous hemorrhage is rare unless the platelet count drops below 10,000 to 20,000∕μL. Concurrent inflamma­tion can increase a thrombocytopenic patient's propensity for spontaneous hemorrhage.¹⁹ Prolonged bleeding from injections or wounds and a propensity to form hematomas with minor trauma are relatively common with a platelet count below 40,000∕μL.

Thrombocytosis

Thrombocytosis is an increase in platelet count over the refer­ence interval. A physiologic increase in platelet count can occur at high altitude.² Relative thrombocytosis due to redistribution from splenic and other tissues stores can occur with exercise or epinephrine release. Splenectomy can result in thrombo­cytosis for several weeks due to both increased production and decreased platelet sequestration and destruction in the absence of the spleen.

Absolute thrombocytosis can occur as a secondary reactive change in a number of conditions. Thrombocytosis due to any cause other than myeloproliferative disease does not gener­ally result in increased risk of thrombosis or other clinical abnormalities; it is a reflection of the underlying disease(s). Many infectious and other inflammatory disorders, including immune-mediated diseases, can cause “reactive thrombocytosis,” as can surgery, neoplasia, and other inflammatory disorders. A less common cause of thrombocytosis is iron deficiency, and this may be true in large animal species as well. Chronic blood loss can be associated with thrombocytosis, and iron deficiency may be one factor. Administration of vincristine or vinblastine can result in thrombocytosis. Thrombocytosis can also occur as a rebound effect following prior thrombocytopenia with an appropriate bone marrow response. A very rare cause of persistent severe thrombocytosis is primary thrombocythemia or other chronic myeloproliferative disease; bone marrow evaluation is indicated in this situation.