Discussion

Repeated major body system assessments are the mainstay for the assessment of sick horses and for monitoring effectiveness of therapy in these patients. However, selective diagnostic tests can be extremely valuable in order to assess the severity of disease, allow assessment of where the case can be managed (in the field, at the practice or require referral), can guide appropriateness of treatment, and in some instances provide information on the likely prognosis for that patient.

From a practical point of view, these tests don't have to be expensive, many can be done horse-side and thus can provide instant results, but do require a move away from thoughts that all answers will be gleaned from a full haematology and biochemistry. This includes urinalysis, PCV, total solids and blood smear, blood lactate concentrations, blood gas analysis and electrolyte concentrations, fibrinogen and serum amyloid A concentrations and targeted biochemical testing (for example creatinine, glucose, cardiac troponin I, ammonia concentrations and bile acids).

Urinalysis:
Although horses rarely suffer from intrinsic renal disease, measurement of urine specific gravity and performing urine dipstick analysis (glucose, protein, blood, Hb, Mb) can be a very useful and cheap method for assessing appropriate renal function and likely degree of hypovolaemia when compared with the USG of plasma (1.008-1.012) and PCV, TS and blood lactate concentrations. Microscopy can be used to identify renal casts, cellularity and presence of bacteria (if collected aseptically). In addition more sensitive tests can be performed to assess for renal tubular damage (e.g. measurement of urine activity of gamma glutamyl transferase).

Assessment of fluid deficit:
Packed cell volume and total solids have traditionally been used to assess hydration status in horses. However they can be misleading in many of the clinical scenarios with sick horses. Reference ranges for PCV and many other haematological and biochemical markers are set using groups of healthy Thoroughbreds. For many of our native and heavy breeds, a normal PCV will be much lower than the reference range, thus meaning that we miss the severity of hypovolaemia based on this parameter.

In addition, when faced with acute haemorrhage, splenic contraction can artefactually increase the PCV thus underestimating both blood loss and degree of hypovolaemia. Total solids can also cause us some issues when trying to use this parameter to quantify hydration status in animals with protein-losing disease. Although protein-losing nephropathy is uncommon, concurrent gastro-intestinal disease with hypovolaemia is extremely common especially in our sick adult equine patients. In hypoproteinaemic, hypovolaemic animals, the total solids may appear in the reference range and therefore underestimate the severity of hypovolaemia in these cases. Both of these parameters can be misleading when assessing severity of hypovolaemia.

Lactate concentrations:
Lactate is produced in anaerobically respiring tissues and is therefore a valuable marker of poor perfusion to tissues. This parameter not only provides information regarding severity of hypovolaemia, but in many studies has been shown to be a valuable prognostic indicator due to what it measures. It can now easily be analysed using hand-held portable machines so results can be quickly obtained horse-side. Many of them are more reliable using plasma or serum, but even on whole blood will provide broad trends. The only practical comment about this is that it is often measured prior to the administration of intravenous fluids and then after a period of administration. Despite clinical improvement, the lactate concentration may increase. When animals become moderately to severely hypovolaemic, they conserve blood supply to essential organs. This means that lactate becomes trapped within the capillary beds of many organs. Following fluid administration, these capillaries are re-perfused and lactate is washed back into the circulation.

Blood smears:
Although these should be performed in conjunction with all abnormal haematology, when money is short or you dont have access to a haematology machine in the practice, these can provide fast, pertinent information in the sick horse. These include the presence of bands and percentage of bands, percentage of neutrophils and whether any of these are toxic. Cell counters can be purchased in order that more accurate cell counts can be obtained using the microscope.

Blood gas analysis and electrolyte concentrations Assessment of acid-base in any depth is outside of the remit of this lecture. However, valuable information can now be obtained relatively cheaply using portable machines about the severity of acid-base disturbances and concurrent electrolyte abnormalities that are present. Majority of the adult horses that present with colic have a metabolic acidosis primarily due to a lactic acidosis and require intravenous fluids. Bicarbonate is contra-indicated in animals when the acidosis is due to increased lactate concentrations. These horses also are, or become, hypokalaemic and hypomagnesaemic, due to low concentrations in resuscitation fluids and/or these electrolytes not being absorbed from the gastro-intestinal tract or are not being ingested. Colics are variably and usually mildly hypocalcaemic.

Fibrinogen and serum amyloid A concentrations:
Both of these are markers of acute inflammation. Serum amyloid A (SAA) is the more acute, acute phase protein of the 2 listed here and is now more frequently available on biochemistry machines and at specialist laboratories. Fibrinogen can also be obtained on many commercial biochemistry machines, but can also be estimated easily and cheaply in any small practice laboratory. This can be done either by measuring the difference in total solids between spun EDTA and serum samples or by using a heat precipitation test. In this test, take plasma (EDTA or heparin). Have one microhaematocrit tube that remains unheated and one that is heated to 58C for 3 min. Measure TS on both tubes and subtract the heated from the unheated to provide an estimate of fibrinogen. These tests are not completely accurate in terms of values in g/l, but will provide an idea about whether fibrinogen is normal, increased or very increased.