EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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T h e B e a u f o r t c o t t a g e l a b o r a t o r i e s 32 Plasma Lactate A number of hand-held and bench top blood lactate analysers have been used in horses. Most have been designed to help assess the human response to exercise. Results have been used to describe and predict aerobic endurance capacity in horses and it has been suggested that the rate of decline in blood lactate concentration, which occurs after exercise, may be a useful index of fitness. Persistent high blood lactate concentrations may suggest metabolic fatigue. Anaerobic capacity is essential for sprinters and so increases in blood lactate concentrations for fast, short distance Thoroughbred flat racehorses may be an advantage. Successful long distance endurance racehorses need to be able to maintain mainly aerobic metabolism (low blood lactate levels) rather than switching to increased dependence on anaerobic metabolism (high blood lactate levels) for prolonged periods of time. In horses, high circulating erythrocyte counts, particularly in response to exercise, makes measurement of whole blood lactate levels less accurate, repeatable and meaningful than plasma lactate levels. This necessitates deproteinisation and centrifugation before analysis of plasma samples, which is much less convenient than whole blood analyses, which can be performed for human athletes with instantreading hand-held machines. For racehorses, results are uninterpretable without carefully controlled standard exercise testing conditions, which are often difficult to organise, especially under Thoroughbred racehorse training conditions. Plasma lactate is also used for monitoring cardiovascular status in horses receiving critical care. It has been shown to be a useful prognostic tool in surgical colic patients and is particularly helpful in monitoring circulatory support in compromised foals. If you wish to assay plasma lactate, please contact Beaufort Cottage Laboratories for sample handling guidance. Therapeutic drug monitoring Therapeutic drug monitoring (TDM) is increasingly used in equine medicine and cardiology and is an invaluable guide to treatment regimens. The aminoglycosides, gentimicin and amikacin, have the potential to cause nephrotoxicity in horses while under-dosing can lead to treatment failure. Serum samples are drawn 30 minutes and 8, 12 or 24 (amikacin only) hours later to identify the peak and trough levels. Peak levels should be at least 8 times the MIC of the organism in question (up to 4 for some gentimicin-sensitive equine pathogens) whereas a delay in excretion is indicative of renal tubular dysfunction and should prompt discontinuation of aminoglycoside therapy. Several of the drugs that are useful in equine cardiac patients have a low therapeutic index. Assays for phenytoin and digoxin are available for close monitoring for potential drug toxicity in patients receiving these drugs.
G u i d e t o e q u i n e c l i n i c a l p a t h o l o g y blood gas analysis Analyses are essential for monitoring respiratory function under anaesthesia in cases of neonatal septicaemia or maladjustment under critical care and for older horses with respiratory and intestinal abnormality, where acidosis or alkalosis are suspected, prior to therapeutic correction. Analyses are helpful, alongside fluid and electrolyte balance assessments, for enterotoxaemia and other criticallyill horses under intensive care and for endurance horses and others performing in hot, humid conditions, suffering exertional exhaustion and/or heat stress. Samples must be taken in heparinised, airtight syringes (preferably glass), and transported on ice to the laboratory within an hour or two of collection, so are practically limited to immediate in-house testing. Endocrinology Pregnancy Tests Serum gonadotrophins (eCG) may be detected in mares where functional endometrial cups are present. For accurate results, serum samples should be collected between 45 and 95 days since the last date of mating. False negative results are unusual inside this period, but can occur in rare cases where eCG levels are below test 'threshold'. False positive tests are more common and may occur when early foetal death has left residual functional cups. In such cases the mare’s serum may remain eCG positive for the functional life of the cups, sometimes up to 100 days. Oestrone sulphate may be detected in the serum/plasma of mares over 120 days pregnant. At that time, levels of >100 ng/ml are usually found (0-25 ng/ml in non-pregnant mares). Most of the oestrone sulphate peak originates from the foetal gonads so this may be a useful test of foetal viability as well as a pregnancy test. Levels fall during the last few weeks of pregnancy. Urinary oestrogens, of placental origin, may be detected in mares after 150 days of gestation. Urine samples are required. In some mares the required fluorescent response may be unreliable to detect and the serum oestrone sulphate test is now a much more reliable test. Progestagens The analysis of plasma progesterone levels is a useful guide to diagnosis and treatment in the acyclic or irregularly cyclic mare. In the non-pregnant mare, levels >2 ng/l (6.3 nmol/l) indicate functional luteal tissue 33
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EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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