Clinical Chemistry

In the presence of clinical signs of disease, biochemical examinations, performed on serum (clotted) blood samples, may reveal abnormalities suggesting bacterial or viral infections or parasitic conditions, myopathy, hepatopathy, nephropathy, intestinal pathology or other metabolic abnormality. These examinations may also be valuable as part of a preventive medicine program for groups of horses, when examined on a regular routine basis, and interpreted carefully, providing information which can be used as a basis for advice to trainers of race and performance horses.

Our Hitachi automated clinical chemistry analyser provides highly repeatable and accurate quality controlled biochemistry results.

Proteins
Total protein, albumin and globulin estimations are useful in the assessment of general bodily condition and nutritional status and the response to infectious or parasitic disease. Albumin levels are high in excited horses or those suffering from haemoconcentration/dehydration and low in those suffering from inanition, advanced hepatopathy, protein losing enteropathy or nephropathy.

Protein Electrophoresis
Electrophoretic separation identifies elevations in specific globulin fractions i.e. α2 (acute-phase inflammatory protein response to tissue damage), ß1 (antibody response to strongyle larval activity), ß2 (response to liver pathology) and γ-globulins (antibody response to bacterial or viral infection). ß2 or γ-globulin 'monoclonal' peaks are sometimes seen in serum samples from horses with lymphoma or plasma cell myeloma. Please do not send heparinised plasma samples for protein electrophoresis examinations, as raised fibrinogen results often produce confusing ß2 globulin peaks.

IgG
This assay is valuable as an assessment of colostral transfer of immunity to the new-born foal. We recommend that serum samples are taken from foals on the second day of life when IgG levels should be >6 g/l. Levels <3 g/l suggest complete failure and 3-6 g/l suggest partial failure of colostral immunity, indicating the need for prophylactic measures. In cases where you suspect levels will be low, we suggest that you also send serum samples from the dam and other potential plasma donors, so that we can run compatibility tests, if indicated, without further delay.

Plasma Fibrinogen
This acute-phase inflammatory protein is elevated in the presence of tissue damage. It may help with diagnosis and prognosis in cases of internal abscessation, chronic infectious or parasitic disease and in cases of exercise induced pulmonary haemorrhage (EIPH). We prefer additional blood samples collected into sodium citrate (blue-topped vacutainer or green Monovette) to measure values accurately by direct coagulometry but we can, if necessary, perform this test on fresh, paired, non-haemolysed sequestrene (EDTA) and serum samples, by comparison of total protein results.

Plasma Viscosity
This is another measure of acute-phase inflammatory proteins which can help with the diagnosis and prognosis of tissue damage responses. We can perform this test on fresh, non-haemolysed sequestrated (EDTA) or heparinised blood samples. Some trainers of performance horses like to monitor PV and SAA as an indicator for health and fitness.

Serum Amyloid A
This is another important acute phase inflammatory protein in the horse. The assay is highly sensitive and levels over 20 mg/l suggest infectious disease.

Aspartate Aminotransferase (AST, SGOT)
Elevations are seen in the presence of acute hepatopathy or myopathy. After myopathy, levels peak at 24-48 hours and return to baseline by 10-21 days, assuming that no further damage occurs. This test, taken with CK at first visit and then 14-21 days later, can provide a useful guide to recovery from acute myopathy.

Creatine Kinase (CK, CPK)
Elevations are seen in the presence of acute myopathy. Levels peak at 6-12 hours and return to baseline by 3-4 days, assuming that no further damage occurs.

Troponin
Troponin is a protein complex composed of three subunits: TnI (troponin inhibitory component), which prevents muscle contractions in the absence of calcium; TnT (tropomyosin-binding component), which connects the troponin complex with tropomyosin; and TnC (calcium binding component), which binds calcium. The cardiac muscle-specific isoform cTnI exhibits approximately 60% homology with the skeletal isoforms (sTnI) and has a unique 31 amino acid extension of the N-terminus. Experience in human medicine has shown that after acute myocardial infarction (AMI), elevated cTnI levels appear in the circulation within 3-6 hours. Serum levels peak within 14-20 hours and return to normal after 5-7 days. The measurement of cTnI can therefore be a useful diagnostic aid for AMI and aid for the monitoring of recovery. Our results suggest that clinically normal horses and dogs have serum cTnI levels of less than 0.2 ng/ml. We believe that greater than 0.3 ng/ml is abnormal, i.e. suggests myocardial pathology and 0.2-0.3 ng/ml is currently a 'grey' zone. We have seen horses with confirmed cardiac myopathy with levels of 0.9-5.4 ng/ml and dogs with levels of 0.3-4.8 ng/ml.

Lactate Dehydrogenase (LD)
A variety of disease conditions can cause elevations in total LD and more differentiation can be provided by isoenzyme analysis, i.e. LD1 (haemolysis), LD2 (cardiac myopathy), LD4 (intestinal pathology) and LD5 (skeletal myopathy or hepatopathy).

Glutamate Dehydrogenase (GLDH)
Elevations are seen in the presence of acute hepatocellular damage. This is a mitochondrial enzyme found mainly in liver, heart muscle and kidney. Levels also rise in horses with enteropathy.

L-Gamma Glutamyltransferase (GGT)
Elevations are seen in the presence of chronic liver cirrhosis, pancreatitis and renal pathology. Idiopathic GGT elevations are sometimes seen in otherwise apparently healthy horses in training, for as yet undefined reasons. Urine GGT:creatinine ratios are elevated (>4.0) in renal tubular pathology.

Alkaline Phosphatase (SAP)
Elevations are seen in the presence of chronic biliary obstructive liver pathology, abnormalities of bone metabolism and intestinal malfunction. Reference ranges vary with skeletal maturity and therefore age.

Intestinal Phosphatase (IAP)
Elevations relative to total SAP are seen in the presence of intestinal pathology. Interpretation is unclear in skeletally-immature young horses.

Bilirubin
The analysis of bilirubin levels may aid the classification of anaemia and jaundice. Owing to the horse's unusual biliary excretion system, indirect (unconjugated) bilirubin levels may be high without clinical disease, and the significance of elevations without other liver enzyme abnormalities may be difficult to determine. A period of anorexia or inanition typically increases indirect bilirubin levels.

Bile Acids
Increased serum bile acid levels suggests impaired biliary function, therefore providing a useful prognostic guide.

Bromsulphalein (BSP) Clearance
Abnormally long clearance half-times for this dye are seen in the presence of gross liver malfunction and may be a useful diagnostic and prognostic liver function guide. We will supply reagents and protocol on request. After a baseline (time zero) jugular venous serum sample is taken, two vials of pre-warmed reagent are injected intra-venously. Serum samples are then taken from the other jugular vein at 2, 4, 8 and 16 minutes, post injection.

Our experience suggest that while in most cases, bile acid and BSP clearance results correlate well, the tests evaluate different aspects of hepatic function and therefore both tests may be indicated in some difficult cases.

Amylase
Elevations are seen in the presence of pancreatitis, seen only rarely in horses.

Glucose
This test is used for oral glucose and xylose absorption tests in adult horses and for nutritional monitoring in neonatal foals. Horses suffering from pituitary adenoma and showing signs of equine 'Cushing's' syndrome are frequently hyperglycaemic. Samples must be taken into fluoride anticoagulant.

Cholesterol and Triglycerides
Elevations are seen in the presence of abnormal lipid metabolism and hyperlipaemia.

Urea
Elevations are seen in the presence of abnormal renal function and uraemia and in many cases of grass sickness (not a specific diagnostic test).

Creatinine
Elevations are seen in the presence of renal pathology.

Fractional Electrolyte and Mineral (creatinine) Clearance Ratios
Impaired renal tubular function results in high urine clearance ratios for sodium, potassium and chloride. In secondary nutritional hyperparathyroidism, phosphate clearance is high and oral calcium supplementation is indicated. Experience has shown that in the UK., many healthy fit stabled performance horses, receiving high cereal rations, have phosphate clearance ratios <9%, emphasising the need for calcium supplementation.

Samples of urine and serum should be collected simultaneously or within an hour or two of one another, without the use of diuretics.

Sodium, Potassium & Chloride
Electrolyte imbalance may occur with diarrhoea, intestinal crises and exertional exhaustion. Serial assay may be helpful with intensive care cases.

Calcium, Phosphate & Magnesium
Mineral analysis may be helpful in young horses with signs suggesting abnormalities of bone metabolism. Serum levels are often normal, however, in the face of whole body abnormality, and thus urine and phosphate creatinine (fractional) clearance ratios are of greater value.

Mammary Secretion Biochemistry
Experience has demonstrated a relationship between the levels of protein, calcium, magnesium, sodium and potassium in pregnant mare mammary secretions and the 'maturity' of the foeto-placental unit. This can provide valuable information when considering elective induction or caesarean section.

Blood Gas Analysis
Analyses are useful in cases of respiratory and intestinal abnormality, where acidosis or alkalosis are suspected, prior to therapeutic correction. Samples must be taken in heparinised, air-tight syringes (preferably glass), and transported on ice to the laboratory within an hour of collection.