Clinical Biochemistry

Core Analytes and Clinical Significance

• Basic Test Panel (Small Animals): Typically includes total protein, albumin, globulin (calculated), urea, creatinine, ALT, and alkaline phosphatase (ALP).

    ◦ Bilirubin should be measured if plasma exhibits a yellow color.

• Species Modifications:

    ◦ Horses/Farm Animals: Glutamate dehydrogenase (GDH) and/or gamma-glutamyl transferase (GGT) are more appropriate liver enzymes.

    ◦ Athletic Animals: Concentrate primarily on muscle enzymes (CK and AST).

• Key Analytes and Indications:

    ◦ Total Protein: Increases due to dehydration, chronic inflammation. Decreases due to protein loss (nephropathy/enteropathy), hemorrhage, or liver failure.

    ◦ Urea/Creatinine: Primarily used to indicate renal disease. Urea also indicates liver dysfunction. Creatinine concentration varies with muscle mass.

    ◦ ALT (Alanine Aminotransferase): Increases due to hepatocellular damage. Half-life is 2–4 hours (rises higher and recovers quicker than AST).

    ◦ ALP (Alkaline Phosphatase): Increases due to circulating steroids, biliary disease (most common causes). In dogs, levels in the thousands usually imply increased steroid levels. Any increase in cats is considered significant (half-life only 6 hours in cats vs. 72 hours in dogs).

    ◦ GDH: Increases in hepatocellular damage, particularly hepatic necrosis in horses and ruminants.

    ◦ CK (Creatine Kinase): Increases markedly in rhabdomyolysis and aortic thromboembolism. High serum levels can result from minor muscle damage (e.g., IM injections).

    ◦ Calcium: Increases due to dehydration (with increased albumin), primary hyperparathyroidism, or pseudohyperparathyroidism (neoplasms producing PRP). Decreases due to hypoalbuminemia, chronic renal failure, or parturient paresis.

    ◦ Glucose: Increases due to stress/excitement, glucocorticoid therapy, or diabetes mellitus. Decreases due to insulin overdose, insulinoma, or acetonemia.

    ◦ Bile Acids: More sensitive than bilirubin for hepatic impairment (liver dysfunction). A marked increase after feeding is characteristic of a portosystemic shunt.

▪ Diagnostic cut-off for EPI: Dogs ≤2.5 mcg/L; Cats ≤8 mcg/L.

    ◦ Fecal Tests: Fecal examinations for fat or proteolytic activity are unreliable or no longer justified.

    ◦ PLI: Current PLI assays are optimized for higher concentrations and are not suitable for diagnosing

In-Practice Laboratory Analysis

• Point-of-Care (POC) Tests:

    ◦ Total Protein: Measured by refractometry (also measures urine specific gravity).

    ◦ Glucose: Whole-blood glucose meters yield acceptably accurate results; however, unexpected hypoglycemia should be confirmed by a professional lab.

    ◦ Ketone Levels: Estimated on urine (preferred) or plasma/serum using dipsticks.

    ◦ Emergency Electrolytes: Sodium and potassium are the most important emergency analytes measured using a dedicated ion-specific electrode meter.

    ◦ Visual Assessment: Lipemia (triglycerides) and bilirubin levels can often be appreciated by eye.

• Practice Laboratory Instrumentation:

    ◦ Wet Chemistry (Transmission/Absorbance Photometry): The reference method on which interpretive guidelines are based; preferred for wider applications like enzyme analysis.

    ◦ Dry Chemistry (Reflectance Photometry): Best confined to simpler tests (glucose, urea); methods do not always compare well with the reference method.

• Quality and Cost:

    ◦ In-clinic analysis is generally more expensive, restricted in the range of analytes, and likely has lower accuracy/reliability than professional laboratories.

    ◦ Best Practice: In-practice analysis should be considered an interim emergency investigation, with non-emergency evaluation referred to a professional lab for accuracy, range of analytes, and clinical pathologist assistance.

• Quality Assurance (If relying on in-clinic): Requires meticulous attention, including running samples of known composition daily in both normal and pathologic ranges, and participation in an external quality assessment program. The veterinarian is legally liable for the accuracy and reliability of all results issued.

References: Merck Veterinary Manual