The potential value of haematology in equine practice is well documented by numerous case series reports and experimental
studies. The maximal use of haematology is to help with diagnosis and/or prognosis of the ill horse. In cases where the
diagnosis can be arrived at from the history, signalment, and clinical exam, and the prognosis can be accurately predicted,
haematology may not be the best use of the client's financial resources. The information on hematology in this handout refers
almost exclusively to adult horses. Responses in foals may differ and will be mentioned in the presentation.
Hematocrit
Abnormally high values (spurious polycythemia) are most commonly seen in horses with abdominal pain and/or colitis or rhabdomyolysis.
The high values are caused by intravascular volume depletion and splenic contraction. A retrospective study on equine abdominal
pain found horses with HCT > 50% are at increased risk of death (Orsini 1988). Monitoring of HCT% and plasma protein after
abdominal surgery is routine and will provide prognostic information (Proudman et al, 2005). These values should be used
along with clinical findings such as mucus membrane color, heart rate, and response to treatment and peritoneal fluid analysis
in predicting prognosis.
On rare occasion, horses have absolute polycythemia, most commonly associated with neuroendocrine neoplasia or hepatic disease.
Low HCT
A moderately low HCT (21-26%) is most commonly a result of a chronic inflammatory disease. In many cases, plasma protein
will be elevated indicating increased globulin production from chronic antigenic stimulation and/or elevated acute phase proteins.
If the inflammatory disease involves the bowel (parasites or inflammatory bowel disease), the total protein is often abnormally
low.
Lower hematocrits (<20%) may occur from hemolytic or hemorrhagic disorders. For hemolytic diseases, the HCT%, along with
heart rate, clinical signs, PVO2, blood lactate, and persistence of the hemolytic process can be used to determine need of transfusion. There is no single
HCT number that serves as a "transfusion trigger" with a range from 10-20%. Low HCT will not be seen with acute hemorrhage;
in fact, animals may die from acute hypoxia/hypotension caused by acute hemorrhage but normal HCT (Divers 2000).
Severe non-regenerative anemias are rarely seen, but may result from adverse reaction to erythropoietin injections (red cell
aplasia), fell pony syndrome, and rarely from cytotoxic drug reactions or neoplasia. MCV would be expected to be low in
these cases.
When blood is spun in a micro hematocrit tube, the plasma should always be examined for icterus, hemolysis and lipemia.
Platelet Count
Platelet counts can help determine the severity of an illness and as a laboratory clue for neoplasia. Horses with severe
systemic inflammation/coagulopathy may have thrombocytopenia (usually in the 40-70,000 range), increased D-dimers and low
anti-thrombin III levels; fibrinogen is often normal or high. Horses with marked thrombocytopenia (< 20,000) usually have
drug-induced or neoplasia related (immune) thrombocytopenia. In the horse, another cause of "reportedly" low platelet count
is pseudothrombocytopenia (Hinchcliff 1993).
