Test interpretation

 
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Generally, at Cornell University, we interpret results in several ways:
  • Detection of abnormal results: This is usually accomplished by observing test results outside reference intervals. If reference intervals are not established for the species, we generally go to the literature to provide guidelines as to how to interpret changes. Some abnormal results are not diagnostically relevant, e.g. a low red blood cell distribution width (RDW) provides no diagnostic information. There are some limits with reference intervals. For instance, they are usually established for adult animals and mixed breed and sex, where there can be breed-, age- and sex-specific changes (and intervals). However, it is far too costly to provide intervals for all species, ages and sexes and, in general, the differences are small, with some notable exceptions (e.g. foals and calves have mean RBC volumes below the reference interval for up to 1 year of age and their RBC are not microcytic due to a disease). More specific information on interpretation of test results is given under the specific sections for hematology, hemostasis and chemistry.
  • Interpretation of results: Results of clinical pathologic tests should always be interpreted with respect to what is known about the patient (signalment, history, clinical signs, results of other diagnostic testing) and should not be interpreted in isolation. First and foremost, you should ask yourself if the observed change is due to non-disease variables or artifact. These factors should always be considered when interpreting test results. Secondly, with complex testing, we often look for changes in patterns to help interpretation (pattern recognition). Thirdly, when examining individual results, we think of mechanisms responsible for the change, which leads us into differential diagnoses of diseases causing the change (final step).
    • Non-disease variables: include preanalytical variables that are associated with the patient, sample collection and sample handling. These generally affect the composition of the body fluid before analysis and can have a major impact on result interpretation. Some of these patient- or sample-related variables, such as hemolysis, icterus and lipemia can be semi-quantified (mild, moderate, marked) by visually assessing the sample or quantified by an analyzer, to provide an interference index. Analytical variables are factors which influence the analytical procedure, such as precision and accuracy. These are method, reagent, and laboratory-dependent (they can be optimized by good laboratory procedures). Post-analytical variables involve the different ways data from the laboratory is presented, stored and transferred to the clinician. Accuracy can be optimized by oversight of laboratory testing results by clinical pathologists.
    • Pattern recognition: Although results should be looked at as a whole overall and all results should be interpreted together and not in isolation, a lot of information is provided with clinical pathologic testing. It can be helpful to group results of clinical pathologic tests together.
      • Hemogram: We group results into an erythrogram (RBC-related results), leukogram (leukocyte-related results) and thrombogram (platelet-related results).
      • Hemostasis: We interpret results of routine screening panels (prothrombin time, activated partial thromboplastin time, thrombin clot time) based on the underlying pathways they test, i.e. intrinsic, extrinsic and common pathways. For instance, a prolonged PT but normal PT
      • Clinical chemistry: We group together results as acid-base and electrolytes, minerals, proteins, energy metabolites, liver, renal, iron and muscle. Urinalysis results are interpreted together with these results.
    • Mechanistic changes: The following are general mechanisms that cause changes in clinical pathologic results. However, all of these mechanisms will not apply to each test and some may not apply at all to a test.
      • Input: This can relate to dietary intake, absorption in the gastrointestinal tract, or production in the body. For example, a dietary deficiency of iron can result in an iron deficiency anemia. Intestinal inflammation can result in malabsorptive syndromes and diarrhea, decreased production of platelets, causes thrombocytopenia. For some constituents, hormones affect input, for example vitamin D facilitates absorption of calcium and phosphate from the intestinal tract.
      • Movement between cells, tissues or compartments: Constituents can move in and out of cells or between intravascular and extravascular compartments (including the gastrointestinal tract or body cavities). Cell lysis or injury (causing permeability of cellular membranes without overt rupture) will also result in movement of constituents inside cells into blood. For example, potassium can move intracellularly in response to an alkalemia, resulting in a peripheral hypokalemia; water can be retained within an obstructed stomach or abomasum resulting in dehydration with subsequent electrolyte abnormalities consequent to stimulation of antidiuretic hormone secretion to maintain blood volume; injured hepatocytes release ALT (small animals) and GLDH (all species); thus these are markers of liver injury. Hormones can also affect movement, for example, epinephrine  and insulin both cause intracellular movement of potassium.
      • Metabolism: Some constituents are used or metabolized by the body. For example, non-esterified fatty acids are taken up by hepatocytes and converted into energy, very low density lipoproteins or trigylcerides (fat). Again, this can be influenced by hormones. In the case of NEFA, stimulation of hormone-sensitive lipase causes lipolysis and NEFA release from adipose stores.
      • Output: This can occur through multiple routes, including the urinary system, gastrointestinal system, respiratory system, and skin. Metabolism or use could also be considered an output. Hormones can also affect output, for example, aldosterone causes loss of potassium (due to exchange for sodium) in the distal renal tubules.
      • Iatrogenic: Of course, drugs we give animals can affect test results. This can actually cause a change in the patient, e.g. phenobarbital treatment for seizures in dogs increases hepatocellular leakage enzymes (such as ALT), reflecting liver injury. In other cases, the drug may cause an artifactual change, for example bromide treatment for seizures in dogs falsely increases chloride results when measured by ion-selective electrodes.
      • Physiologic: Some changes are physiologic, e.g. microcytosis in foals and calves, higher ALP in younger animals.
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