As previously mentioned, there is little value in performing laboratory tests for healthy horses undergoing elective procedures.

       However, when indicated, the appropriate tests should, ideally, be performed before general anesthesia is induced. Nevertheless, this may not always be feasible in emergency situations.

       Evaluation of erythrocyte numbers can begin with a determination of the packed cell volume (PCV) or hematocrit (HCT), which measure the percentage of the volume of whole blood that the red blood cells (RBCs) occupy.

       Erythrocyte numbers are 6.9–10.7 x 1012/l, which represent a normal PCV of 35–45% and a normal hemoglobin of 12–15 g/dl.

       A stained blood smear can be used to evaluate RBC morphology and the presence of infectious organisms on the RBCs.

       Increased RBC numbers (erythrocytosis) is most commonly associated with hemoconcentration – this is called a relative erythrocytosis as there is no increase in RBC mass.Splenic contraction can cause a transient increase in circulating RBC mass.

       In rare cases, there is an increase in numbers due to an increase in RBC production, absolute erythrocytosis.Absolute erythrocytosis is further categorized as primary, as in polycythemia vera, and secondary, as results from chronic hypoxemia.

       Decreased RBC numbers (anemia) can be caused by increased removal from the circulation by blood loss or destruction (hemolysis); or by decreased production by the bone marrow.Intravascular hemolysis is accompanied by a decrease in hemoglobin concentration; however, extravascular hemolysis is harder to confirm.Decreased production with iron deficiency secondary to chronic inflammation is a common cause of nonregenerative anemia in the horse, and is characterized by microcytic hypochromic RBCs and decreased RBC indices.Red cell distribution width (RDW) – a measure of RBC variation in size and volume ‐ is used to detect RBC regeneration and is interpreted in conjunction with other values on the complete blood count (CBC).

       Granulocytes (neutrophils, eosinophils, basophils, and mast cells), monocytes, macrophages, and lymphocytes are important for immune function, and changes in their numbers reflect a response to a disease process.

       Total leukocyte count is 5.1–11.0 x 109/l.

       Neoplasia, bone marrow disease, or functional defects can cause a decrease or increase in leukocyte numbers.

       Changes in numbers and morphology are important in evaluating the response to systemic inflammation, infection, and stress.

       However, the neutrophil count can be normal in the presence of inflammation or infection.

       Reference values include 2.8–7.7 x 109/l segmented neutrophils and 0.0–0.2 × 109/l bands.

       Neutrophilia is often associated with chronic bacterial infections.

       Neutropenia in adult horses is most often associated with endotoxemia (and other bacterial by‐products).

       Neutropenia in foals is most commonly associated with sepsis and systemic inflammation.

       Reference values are 0.0–0.7 × 109/l.

       Eosinophilia is rare in horses. Conditions to rule out include:Parasitism with cyathastomes.Eosinophilic colitis, enteritis, or multisystemic disease.

       Reference values are 1.3–4.7 × 109/l.

       Lymphopenia: Glucocorticoid release or administration, viral infection, old age, and immunodeficiency in foals (e.g. combined immunodeficiency disease in Arabian or Arabian crossbred foals).

       Lymphocytosis: epinephrine release or administration, exercise, equine herpesvirus 2 (foals), leukemia.

       Reference values for monocytes are 0.1–0.8 × 109/l.

       Monocytosis: Chronic inflammation, use of corticosteroids.

       Reference values for basophils are 0.0–0.1 × 109/l.

       The normal platelet count for horses is 75 000–300 000/μl.

       Platelets function in hemostasis, inflammatory responses, immunity, tissue regeneration, and disease pathology.

       Age < 3 years, intact males, pyrexia, infectious, or inflammatory conditions.

       Platelet count: 400 000–850 000/μl.

       A rare chronic myeloproliferative disorder.

       Platelet count >1 000 000/μl.

       Immune mediated.

       Causes include:Equine infectious anemia, anaplasma phagocytophilum.Drugs, toxins, snake bites.

       Hemorrhage.

       Disseminated