Page 531 - WSAVA2018
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When interpreting a blood result, care must be taken to distinguish between abnormal results due to disease, and abnormal results due to other factors. These other factors, referred to as artefacts, can occur for a variety of reasons, including:
· Physiological changes
Stress due to transport and handling of the patient can lead to a release of endogenous corticosteroids, resulting in changes in the haemogram and in blood glucose.
Lipaemia, while occasionally seen in diseases of the liver and reproductive system, can also occur naturally in the reproductively active female. Regardless of the cause, lipaemia can cause false elevations in bile acids, protein, calcium, phosphorus and uric acid.
It may also falsely decrease amylase.
Postprandial lipaemia is uncommon in pet birds and mammals, so fasting will not help; the clinician needs to check with the laboratory if the sample submitted was lipaemic before interpreting these biochemistries.
· Previous therapy
Before interpreting biochemistries, the clinician should consider if any treatment given prior to the sample collection could have had an effect on the results. Therapy given by another veterinarian or in an attempt to stabilize a crashing patient can have marked effects.
Parenteral fluids can dilute biochemistries; exogenous corticosteroids can markedly elevate aspartate aminotransferase (AST), creatine kinase (CK) and lactate dehydrogenase (LDH); intramuscular injections, particularly of irritant drugs, can do the same.
· The clinical condition of the patient
Glucose and albumin may be decreased. Calcium may be elevated or decreased, according to the methodology used.
· Storage and transport of the sample
Blood collected for biochemistry analysis should be placed immediately into a lithium heparin tube. Ideally, miniature tubes as used in medical paediatrics should
be used. The sample should be gently rolled or rocked; clotting must be avoided, but haemolysis must be as well. If the analysis is to be performed in-house, it should be processed immediately. If a delay is likely, or if the sample is to be shipped to an outside laboratory, the sample should be centrifuged and the plasma harvested. Sending whole blood to an outside laboratory can result in decreased glucose (as cell metabolism continues) and haemolysis.
EDTA tubes are unsuitable for biochemistry analysis in most species, but can be used for haematology, lead analysis and fibrinogen determination
The complete blood count (CBC) is an important test in determining many disease states. In most cases a CBC involves assessing:
• The erythrocytes, through the determination and assessment of:
• The haematocrit or packed cell volume (PCV).
• Erythrocyte morphology.
• Reticulocytes.
• The leucocytes, through the determination and assessment of:
• The total white cell count.
• The leucocyte differential count.
• The morphology of the leucocytes.
• Thrombocyte numbers.
· The PCV of most birds lies between 0.4 l/l and 0.55 l/l. Non-flighted birds, such as chickens, usually have a lower PCV as they do not have the same oxygen demand that flight requires.
· The normal PCV in most reptiles is lower than mammals and birds, and values of 20%- 35% are not unusual. Again, this may reflect their lower metabolic rate and subsequent lower oxygen requirement.
· Small mammals generally lie between these two extremes, with a PCV generally between 35 – 48%.
A low PCV can indicate blood loss, anaemia, shock or haemodilution following fluid therapy.
A high PCV indicates dehydration or polycythaemia (primary or secondary).
Trauma, starvation and dehydration can all have marked effects on biochemistries, and need to be considered when interpreting results. Trauma can cause elevations in AST and CK and possibly glucose; starvation can lower glucose and also elevate AST and CK if protein catabolism has begun; dehydration can elevate uric acid.
· The collection method
Ideally, sample collection should be performed in such
a manner that it has minimal impact on the patient while providing an artefact-free sample suitable for analysis. This usually requires venepuncture to be performed on a minimally stressed patient. Inexperienced clinicians may need to consider gaseous anaesthesia in order to collect a good sample without the bird struggling. Haemolysis can cause elevations in bile acids, LDH, CK, potassium and phosphorus.
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