Pathology is the branch of medicine that studies the causes, mechanisms, and effects of diseases in an organism. It is a broad definition but reflects the character of this medical specialty that has its roots firmly attached to basic sciences such as molecular and cell biology, biochemistry, physiology, and anatomy. The term pathology originates from the Greek root pathos (meaning suffering) and logia (meaning study). While pathology is a science and disease is a process, words can acquire new connotations according to their use among professionals or the general public, and the term pathology is currently used as a synonym for disease by many people.

          So what does a pathologist do? Diseases typically deviate the affected organisms from its natural homeostasis or equilibrium, usually but not necessarily to a degree where function becomes impaired and unsustainable. The results of such dysfunctional behavior may be reflected in gross or macroscopic (visible to the naked eye), microscopic (histological or cytological), or submicroscopic (subcellular or molecular) changes. These changes can subsequently cause clinical signs and even death if not treated adequately. Anatomic pathologists rely on these morphologic changes, together with the signalment, clinical history, and a wide range of ancillary diagnostic techniques to work towards a preliminary and subsequently a final diagnosis. A preliminary diagnosis is a diagnosis based on partial evidence and often needs confirmation from other additional tests. For example, multiple areas of necrosis in the liver are highly consistent with an infectious process, but confirmation should rely on the detection of viral inclusions or microorganisms associated with the lesions. That evidence could be gathered via histology, histochemistry, immunohistochemistry, microbiology, or molecular testing. After direct or indirect evidence is collected and a causal relationship can be established between a particular agent or process and the observed tissue changes, a final diagnosis can be reached. It is important to know that when the necessary evidence is not available, an inconclusive diagnosis will be reached and that is part of the diagnostic routine. Pathology is not always diagnostic.

          The morphologic changes caused by a particular disease are referred to as lesions or pathologic changes. Expressions such as pathologic lesions should be avoided since they are redundant (all lesions are pathologic) and fall into tautology. The terms pathologic change or lesion are more appropriate since they refer to tissue changes that were caused by a particular disease process. This definition of terms is crucial since not every tissue change is pathologic. Postmortem changes or artifacts are examples of tissue changes that result from postmortem decay (autolysis) or from improper handling of tissues, respectively. There are many examples of such changes in the diagnostic routine. In fact, some of which could be even misinterpreted as a true pathologic change or lesion by the inexperienced examiner. The term tissue change will be employed here to describe pathologic changes (true lesions) and non-pathologic changes (non-lesions or artifacts).

          The visualization of a tissue change is a temporary event. You see it, you sample it, and it will be gone shortly after. That is why examiners usually document their findings with images and a complete written description of their findings. A pathology report is typically used to communicate and permanently record the results of a postmortem evaluation (necropsy or autopsy) or tissue changes present in any sample submitted for diagnosis (a surgical biopsy or a whole organ specimen, such a spleen, an amputated leg, etc.). For postmortem examinations, different institutions typically have idiosyncratic techniques that tend to be equally effective, as they are all based on the premise that a pathologist should always follow a systematic approach when working on a case. Make sure you have a system and that you employ that system every time you conduct a postmortem examination. If for any reason you need to occasionally or permanently replace your technique, do not worry, you will adapt. Take some time to learn the new steps and you will be automatically performing an entirely different technique sooner than you think.

          A pathology report will communicate all the pathological changes that were observed and interpreted so a final diagnosis could be achieved in a particular case. It will list the diagnosis or diagnoses that were based on the observed tissue changes, as well as ancillary tests that may have been required to support specific interpretations. There is no one single way to describe a tissue change, but there are definitely better ways to do so than others. Since the description is a visual exercise, it should be kept simple, concise, and impartial. No interpretations or premature conclusions should be attempted when describing a tissue change since those can lead to confirmation bias. The interpretation of a tissue change typically occurs within a few seconds after a tissue change is identified and it is independent of any analytic process; it is an intuitive mechanism based on shortcuts the brain takes to quickly achieve the expected result it desires to achieve. As such, it can be faulty.

          A final diagnosis should be based on the critical evaluation of patterns and pattern recognition, and confirmed by information available in the medical literature, previous experiences with the same or similar circumstances, and ancillary testing, if applicable. A diagnosis depends heavily but not solely on pattern recognition and to achieve that you need a good idea of the tissue changes that were present at the time of the gross inspection. To report these tissue changes, it is essential to understand your audience (another pathologist? a veterinary specialist? a general practitioner? a student? the animal owner?) and use appropriate language to communicate your findings efficiently. Further, when communicating your findings, do not forget to think of the big picture. If a dog had a brain tumor and a cutaneous lipoma, talk about the brain tumor, which probably caused the clinical signs and death, rather than the lipoma, which was likely a bystander. Moreover, remember one essential thing that most people tend to forget: proofread your report before sending it off. Poor grammar is a turn-off!

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