“Semantic qualifiers” (e.g. acute vs. chronic or unremitting vs. relapsing) are paired, opposing descriptive adjectives that can be used to compare and contrast diagnostic considerations.² Clinicians distinguish between diseases using key signs and symptoms and use these descriptors to assist with this discrimination in hypothesis generation. An example for this patient would be: A 67-year-old sheepherder living in rural New Mexico presents with persistent fevers and malaise for one month, along with subacute development of nonproductive cough and dyspnea, sepsis, anemia, and thrombocytopenia.
Note how the incorporation of epidemiologic information (sheepherder living in an earthen structure in rural New Mexico) creates a context in which the additional problems can be framed (persistent malaise, subacute cough). In this case, the persistent fevers help the clinician to narrow possibilities in the differential diagnosis and create focused hypotheses.
Although the benefit of teaching accurate and thorough problem representation seems self-evident, studies have not demonstrated that improved problem representation enhances diagnostic accuracy; however, we believe that there is still value in adapting and teaching this skill.3
Hypothesis refinement and the differential diagnosis. Initial hypotheses occur early in data collection, as the patient’s history and physical examination findings trigger connections to clinicians’ bank of known diseases (e.g. orthopnea triggers congestive heart failure). As the clinician collects additional data, he or she refines these hypotheses, changing the likelihood based on “fit” of the problem representation with known diseases or illness scripts.
When employing analytic reasoning processes, clinicians may benefit from using organizational frameworks to assist with hypothesis generation (see Table 1). For this patient, possible hypotheses could include CAP, TB, lymphoma, lung neoplasm, or other indolent pulmonary infection.
(Click for larger image)Table 1.
Illness scripts. Once discrete hypotheses (e.g. CAP, pulmonary embolism) have been generated, clinicians need a method to accurately compare disease processes. This can be done through the use of an illness script. Illness scripts are mental representations of diseases and are likely to include epidemiology, typical and atypical patterns of presentation, and distinguishing features.
For example, a clinician’s illness script for a typical presentation of bacterial CAP likely includes fever, productive cough, pleuritic chest discomfort, and infiltrate on CXR. Clinician educators who teach illness scripts should ensure that students understand that diseases have atypical presentations, even though they may only teach them the prototypical one. Conceptualizing diseases in this fashion allows clinicians to seek the disease with the “script” that best matches the patient’s story (i.e., clinical presentation).
In this case, the clinician is now thinking of causes of persistent fever + nonproductive cough + dyspnea + anemia + thrombocytopenia; possibilities include lymphoma or unusual infection (e.g. tick-borne relapsing fever, or TBRF).
Case Resolution and Script Selection
As the clinician processes the case, a known illness script of TBRF matches the patient’s clinical presentation, and a peripheral smear is ordered. The smear reveals presence of spirochetal organisms, later confirmed by PCR to be Borrelia hermsii, confirming the diagnosis of TBRF.
Errors in Clinical Reasoning
Although most clinicians are quite accurate in typical presentations of common diseases, they are more likely to commit diagnostic errors when faced with uncommon diseases, atypical presentations, and/or challenging contexts. The following sections categorize a selection of some common errors and offer some expert opinion from the literature on avoiding them.
Common diagnostic errors. Clinicians use heuristics, or mental shortcuts, which can occasionally induce diagnostic errors. By definition, the fundamental problem in all diagnostic error is premature closure, or acceptance of a diagnosis before it is fully verified. In the case presented, the clinician may have accepted the diagnosis of CAP without recognizing other possible diagnoses.