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Blood Biochemistry

Total Protein

Hyperproteinaemia can occur in dehydration, extreme hyperfibrinogenaemia and the hypergammaglobulinaemia of chronic infection. Measurement of total protein is therefore too non-specific to be useful in the evaluation of the elite equine athlete. Hypoproteinaemia, when identified, similarly provides better information when broken down into its sub-components. Hypoproteinaemia is always disturbing.

Albumin

Hyperalbuminaemia (Alb >35.0 g/l) can be a reflection of dehydration. Clinicians have to recognise that this may be a transient and self-correcting entity in otherwise healthy and well-managed horses sampled at rest and may not merit recourse to fluid therapy. Consistent minor hyperalbuminaemia and electrolyte elevations may be used to remind management of the need for sufficient provision of water. Frank dehydration obviously necessitates correction. Hypoalbuminaemia often occurs as a result of preferential production of globulins in chronic infection, in severe gastrointestinal disease and in rarer conditions such as renal disease. Albumin may also be lost into body cavities, e.g. in pleuritis and similar entities.

Globulin

Globulins can be divided into their alpha, beta and gamma components using protein electrophoresis. Alpha-2 globulins contain the acute phase proteins. Beta-1 elevations can be seen in parasitism and gamma elevations are seen in chronic infections, usually those that are bacterial in origin.

Plasma Fibrinogen

Plasma fibrinogen can be measured in a variety of different ways. The modified Clauss method reference range is 0.9 to 1.7 g/l. Elevation above these ranges are very seldom seen in elite racehorses consistently performing at the highest levels. Heat precipitation method reference ranges are usually from 2 to 4 g/l. Hyperfibrinogenaemia reflects inflammation. The elevation response usually lags the causal insult by about 24 hours. Serial plasma fibrinogen evaluations are of great value in monitoring response to treatment and in decisions on when normal athletic activity can be resumed. It is important to remember that this is a non-specific indicator of inflammation and even minor entities such as sore heels or girth sores can result in misleading elevations, emphasising once again, the ever present need for clinical examination.

Creatine Kinase (CK)

Rapid elevations occur in myopathy and other causes of soft tissue damage, which peak within 6-12 hours of the insult. This primary muscle enzyme has a short half-life and even very high elevations can return to normal within 3-4 days. Over-interpretation of minor CK elevations should be avoided, except as a warning in cases of recurrent rhabdomyolysis.

Aspartame Aminotransferase (AST) and Lactate Dehydrogenase (LDH)

These are also muscle enzymes but the latter is less muscle specific than the former. AST has a longer half-life than CK. Because AST levels peak more slowly (24 to 48 hrs post insult) but do not revert to normal (in the absence of further insult) until 10-21 days they are more useful in monitoring severity and the progress of recovery than CK. LDH can be fractionated into its iso enymes. LD1 elevations can be seen in haemolysis and LD2 in cardiomyopathy.

Gamma-glutamyl Transferase (GGT)

Consistent elevations can be seen in consistently successful racehorses. Elevation can also be a non-specific stress indicator, in association with introduction to fast exercise. Hepatic cirrhosis, pancreatitis and renal pathology can also be reflected in GGT elevations. Reliance on GGT as a sole indicator or monitor of hepatic pathology should be avoided.

Bilirubin (Indirect)

Elevations can occur in inappetance and when samples are taken prior to feeding, especially morning feeding. Elevation can be a useful indicator of a need for further hepatic evaluation which may include measures of bile acids, glutamate dehydrogenase (GLDH), perhaps followed by bromosulphthalein (BSP) clearance and or hepatic biopsy.

Urea and Creatinine

Elevations should be interpreted with caution in the presence of dehydration and in these circumstances, re-evaluation after correction of hydration status is always advisable. Elevations occur in the presence of renal disease and here, creatinine may be a more valuable diagnostic and prognostic indicator than urea. Renal disease is rare in elite racehorses and the inclusion of these renal parameters in standard screening profiles is therefore questionable.

Electrolytes

Most electrolyte values seen in screening profiles lie within or very close to, reference ranges in healthy horses. They are elevated in tandem with the elevated albumin levels seen in dehydration. Significant deviations from these norms are usually seen in profound illness such as systemic illness, GIT or urinary tract disease.
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