Leonardo P. Brandão, DVM, MsC, PhD
Before everything, as veterinarians, we must ask ourselves why we need to be thoughtful in order to choose the best laboratory testing to our patients.
The first and quit obvious answer could came from the fact that many pet owners don't have good economic conditions, and asking for too many laboratory testing could represent expenses in excess, making difficult to approach further necessary procedures, like surgical, for example.
We must also to stick out that when we ask for unnecessary laboratory testing we could make a very common error "when you don't know what is looking for, won't be able to recognize it when you find it", otherwise, a bad choice--as laboratory testing--could be more confusing than helpful. In order to be able to "extract" the best that a laboratory testing could give us, we must have a clear question on mind: what does it change? If the laboratory testing is able to answer something that would be useful and it changes the diagnostic, prognostic or even the clinical procedure to be adopted by the clinician, so it is worth it.
To get a specific and meaningful answer, the clinician must ask a specific and meaningful question and know whether a particular test is likely to yield a useful answer. Just before to enumerate the many different laboratory testing, we must remember that a deep knowledge of physiology and the possible interactions among many different organs and systems under some specific diseases is critical.
A critical analysis of laboratory testing includes to know its capabilities and limitations, or on the other side, we shouldn't ask for a specific test if we don't get a clearly view of its usefulness or if we don't know precisely how to interpret it. It's much better to ask from a simple, and very well known, exam than a much more sophisticated testing that we don't fully comprehend it. Regarding the red blood cell analysis, we should remember that its function is not only to determine if is there or not an anemia--this information could be roughly obtained from a accurate physical exam including a mucous membrane inspection regarding mucous' paleness--but the type of anemia (regenerative or nonregenerative). Determining the erythroid regeneration should help us to find out its cause (pathogenesis). After documenting the presence of anemia and grading its severity, one should evaluate the bone marrow's erythroid production by determining reticulocyte response. Blood loss or hemolysis causes regenerative anemia and a reduced or slightly increased reticulocyte response at 3 to 5 days after onset of moderate-to-severe anemia indicates inadequate marrow production (i.e., nonregenerative anemia) and chronic renal disease and anemia of inflammation (chronic diseases) are the most common causes.
In the vast majority of times the leukogram won't give us consistent information, unless in cases of severe leucopenia or leukocytosis. The severe leukocyte's reduction (< 2000 cells/μL in dogs and 3000 cells/μL in cats) occur infrequently in dogs and cats and constitute a poor prognostic sign. The two most likely causes of neutropenia are 1) excessive tissue consumption or neutrophils during severe inflammation and 2) disrupted bone marrow production. Leukocytosis> 30.000 cells/μL could be indicative of leukemic or localized infectious process (e.g., pyometra, abscess). Do not waste your client's money asking for platelet counts if you have problems to send the blood sample to the lab within 2 to 4 hours. As platelets tend to clump inside the tube, especially for cat's platelets, it could result on spurious thrombocytopenia most of the times, manly if the blood is kept refrigerated.
Urinalysis is much more useful to determine infections in the urinary tract than nephropathy (caused by loss in the capacity of urine concentration) if the patient is adequately hydrated. So the evaluation of patient's hydration should precede this kind of test.
In the same way, patients in which there are strong suspicions of severe hepatic insufficiency, as for dogs with ascites or extreme cachexy, should have normal serum activity of hepatic transaminases (ALT and AST) simply because they don't have enough amounts of those enzymes to be delivered by the hepatic parenchyma. In these cases, determination of serum albumin concentration or even the ultrasonographic exam of the organ should be much more useful.
Regarding the ultrasonographic exam, it is much more useful to evaluate the pancreas (in cases of inflammation or neoplasias) than amylase and lipase serum activity. Those enzymes have poor sensitivity and specificity and could show increases by other nonrelated causes, like peritonitis and acute renal failure.
Talking about cats, icterus is considered a major concern and a very important sign, though differential diagnostic must be done among mycoplasmosis, cholangitis/cholangiohepatitis and feline idiopathic hepatic lipidosis. Among them, only mycoplasmosis should be expected to cause anemia, although leukocytosis could be seen on 60% of cats with mycoplasmosis and more than 75% of cats with cholangitis/cholangiohepatitis.
Feline idiopathic lipidosis usually doesn't cause erythroid abnormalities (like anemia), but could be expected to be observed along serum elevation of GGT activity without a significant increase on SAP activity (serum alkaline phosphatase), although its definitive diagnostic should be given based on cytological or ultrasonographic exam of the liver. In summary, we should say the necessity of a good and complete clinical exam; obtained from a meticulous anamnesis, as though from a very detailed physical exam. That information could not only reduce the number of laboratory testing needed, as also, to shorten the time to obtain the diagnostic, increasing our success on treating the patient.
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