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Along with a urinalysis, a complete blood count and blood chemistry analysis are typically considered part of the minimum metabolic database allowing the practitioner to gain an overall assessment of a patient's ability to oxygenate the body, respond to infection or inflammation, and perform the most basic organ functions (Table 4.3). Complete blood counts should include both a quantitative analysis (i.e. a count of the number of cells of a particular type) as well as a qualitative analysis (i.e. microscopic evaluation of cellular morphology in a blood smear). Quantitative analysis is typically performed by automated hematology analyzers, though manual counts can be obtained through the use of a hemocytometer. If neither automated nor manual counts are feasible, assessment of packed cell volume (PCV) is a readily available means of assessing the degree of anemia and the oxygen‐carrying capacity of the blood and can be combined with a simple blood smear for a more thorough assessment. The PCV represents the percent of the patient's total blood volume comprised of red blood cells and is directly measured after centrifugation of a sample. Though a measure of the same parameter, a patient's hematocrit (Hct) is calculated based on the red blood cell count and cell volume. Agglutination of red blood cells or inclusion of platelets in the count will, therefore, impact the Hct but not the PCV. Normal PCV values are 37–55% for dogs and 26–45% cats. An additional benefit of obtaining a PCV is the ability to subjectively assess plasma and measure protein concentrations (see below). Qualitative analysis of red and white blood cells can be assessed by microscopic examination of the long or “feathered” edge of a blood smear. Examination after air‐drying and staining with a Romanowsky stain can enhance the diagnostic value. Cells should ultimately be examined under the 100× objective with oil immersion and should be assessed for color, size, shape, inclusions in red blood cells (e.g. Howell‐Jolly bodies, Heinz bodies, basophilic stippling) and signs of toxicity in white blood cells (e.g. basophilia, vacuolation, Döhle bodies). A variety of infectious agents may also be identified in both red blood cells (e.g. Mycoplasma spp., Babesia spp., and Cytauxzoon) and white blood cells (e.g. intracellular bacteria, Ehrlichia spp., Anaplasma spp., Hepatozoon spp.). Viral inclusion bodies of canine distemper virus can also occasionally be seen within red blood cells and within the cytoplasm of white blood cells (Rosenfeld and Dial 2010d; Webb and Latimer 2011). Finally, particularly when subjected to centrifugation methods, microfilaria of D. immitis may also be identified on blood smear analysis.

Blood chemistry analysis may include a variety of parameters to assess the function of major organ systems (e.g. renal, hepatic), total proteins, electrolytes, and metabolism of carbohydrates and lipids. These assessments are most thoroughly and accurately performed through the use of calibrated, automated biochemical analyzers either in‐house or at a diagnostic laboratory. If complete biochemical profiling is not available, crude assessments of total proteins, blood urea nitrogen (BUN), and blood glucose can be conducted in almost any setting.

As mentioned above, after obtaining a PCV, an analysis of the remaining plasma can easily be undertaken. Plasma color and transparency should first be visually assessed. For both dogs and cats, plasma is normally clear and colorless; yellow plasma indicates icterus, pink or red indicates hemoglobinemia, and white to pink opaque plasma indicates lipemia. Finally, the plasma sample can be used to assess plasma protein concentration with a refractometer. Normal protein concentrations range from 5.5–7.5 g/dl in dogs and 6.5–8.5 g/dl in cats. Refractometric protein readings can be falsely elevated in samples that are lipemic, hemolyzed, or when there are high concentrations of glucose, urea, sodium, or chloride in the sample (Evans 2011). Complimentary analysis of the PCV and plasma protein can provide useful information to narrow the list of differential diagnoses:

 Low PCV, Normal protein: anemia

 Low PCV, Low protein: chronic blood loss

 High PCV, Normal protein: increased red blood cell production, hemorrhagic gastroenteritis, endocrinopathy

 Normal PCV, Decreased protein: protein‐losing disease, acute blood loss, liver failure

 Normal PCV, High protein: dehydration, increased immune response.

Urea is a waste product of dietary protein digestion formed in the hepatocytes and excreted through the kidneys. As such, assessment of the BUN concentration can be a useful indicator of decreased glomerular filtration and, when interpreted in light of the animal's hydration status and urine specific gravity, a crude screening test of renal function. Outside of a concentration reported from a biochemical analyzer, BUN levels are commonly assessed through the application of a whole blood sample on semi‐quantitative colorimetric reagent test strips. Such strips have been correlated with serum urea concentrations as measured with an automated analyzer in both dogs and cats. Dogs with BUN concentration estimates >15 mg/dl and cats with estimates >50 mg/dl were accurately categorized as azotemic in 98% of samples evaluated (Berent et al. 2005).

A final point‐of‐care quick assessment test is the measurement of blood glucose concentration. Glucose is the primary cellular energy source and is obtained from dietary carbohydrates, the breakdown of glycogen in the liver (glycogenolysis) and the synthesis of glucose from amino acids and fats (gluconeogenesis). Alterations in blood glucose concentration can indicate dysfunction in a wide variety of organ systems including the renal, hepatic, and endocrine systems. In addition, sepsis and congenital metabolic diseases are important causes of hypoglycemia while analysis immediately or shortly after a meal or a stressful event (particularly in cats) are noteworthy explanations for hyperglycemia (Duncan 1998). Normal fasting blood glucose concentrations range from 60 to 125 mg/dl in dogs and 70 to 150 mg/dl in cats.

Care must be taken when obtaining a blood sample for the measurement of glucose concentration. Acute stress, such as that due to aggressive handling or restraint, and a recent (<12 hours) meal are likely to result in falsely elevated measurements. Additionally, blood glucose concentration decreases ~10% per hour at room temperature, so samples should be processed within 30 minutes or the serum should be separated and refrigerated until testing (Evans 2011).

Glucose concentrations in whole blood can be measured by using colorimetric reagent strips or portable blood glucose meters. Reagent strips are not reliable for the detection of hypoglycemia, and the presence of anemia may result in falsely elevated concentrations. Excessive washing and incomplete coverage of the reagent pad can also result in erroneous results. Elevated blood glucose concentrations above the renal threshold can also be detected in urine samples through the use of urine chemical reagent test strips (Evans 2011). Portable blood glucose meters can provide measurements that are comparable to those obtained with reagent strips and biochemical analyzers; however, there is substantial variation between models (Cohen et al. 2009, 2000). In general, such meters tend to underestimate blood glucose concentrations and have greater divergence from reference measurements when samples are hyperglycemic. Meters that are designed and calibrated specifically for use in companion animals are preferred and thought to be less likely to report falsely decreased blood glucose concentrations (Kang et al. 2016). As with reagent strips, anemia can result in an overestimation of the blood glucose concentration. Despite these pitfalls, imprecise results from portable blood glucose meters are not likely to lead to alterations in clinical decision‐making (Cohn et al. 2000; Wess and Reusch 2000a, b).