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The Master Clinician’s Approach to Diagnostic Reasoning

“The value of experience is not seeing much, but in seeing wisely.”

William Osler, MD

“There are only three things that are important in medicine: diagnosis, diagnosis, diagnosis.”

Charles Bryan, MD

 

Just as age, of itself, does not make a person wise, experience in medicine without clinical insight does not improve diagnostic skills. Clinical insight refers to the ability of experienced clinicians to assign relative “diagnostic weights” to clinical and laboratory findings. Master Clinicians are able to rapidly and readily differentiate between characteristic and consistent findings in differential diagnosis.

Characteristic findings are cardinal hallmarks that are nearly always present, and when present, support the diagnosis. Conversely, the absence of characteristic findings argues strongly against the diagnosis of a disorder. All clinical findings, even characteristic findings, differ in their diagnostic significance or “diagnostic weight.” For example, febrile travelers returning from the tropics may have, among other things, malaria or typhoid fever. There are several findings consistent with the diagnosis of either malaria or typhoid fever. Both have normal white blood cell counts, but if otherwise unexplained atypical lymphocytes are present, a characteristic finding in malaria, but not typhoid fever, then malaria is the likely diagnosis. Continuing along the same line of reasoning, if chills are prominent, a characteristic feature of malaria, they are notably less prominent or absent in typhoid fever. The absence of characteristic findings also has important exclusionary diagnostic significance and may be considered as “diagnostic eliminators” (eg, thrombocytopenia is a characteristic finding in malaria, but not in typhoid fever). Therefore, typhoid fever is unlikely in a febrile traveler returning from the tropics with prominent chills and thrombocytopenia.

Such differential diagnostic skills are often demonstrated during a clinicopathologic conference. In a clinicopathologic conference, physicians marvel at the ability of Master Clinicians to quickly narrow many diagnostic possibilities to 2 or 3. How does the Master Clinician do it? One of the hallmarks of a Master Clinician is the ability to assign relative “diagnostic weights” to characteristic findings. As the result of years of insightful experience, Master Clinicians use relative “diagnostic weights” of key characteristics findings to rapidly narrow the differential diagnosis. The Master Clinician’s initial differential diagnosis is based on recognizing key clinical findings, which is the basis of the Master Clinician’s diagnostic approach.

Without specific test results, most physicians are unable to effectively or efficiently narrow the differential diagnosis. Without clinical insight, important nonspecific laboratory findings are often overlooked or their clinical significance not recognized. Characteristic findings may have little clinical significance individually, but when considered in concert, form a diagnostic pattern that often points to the correct diagnosis.

Another important consideration in differential diagnosis is the disease time course. Clinical findings must be interpreted in the context of the disease time line (eg, bilateral upper eyelid edema is an early characteristic finding of Epstein-Barr virus infectious mononucleosis [Hoagland's sign], whereas splenomegaly is a consistent and late sign of many disorders). Similarly, diagnostic significance over time also applies to test abnormalities. For example, pneumonia with otherwise unexplained thrombosisis is unusual, but if present, occurs late and only with Mycoplasma pneumoniae or Q fever.

The diagnostic specificity of nonspecific test abnormalities may also be enhanced by the degree of test abnormality. For example, erythrocyte sedimentation rate (ESR) is sensitive, but not specific. However, if the ESR is highly elevated (>100 mm/h), differential diagnosis is limited to relatively few possibilities. To be most useful, diagnostic significance of clinical findings must be interpreted in the clinical context. For example, an adult with pneumonia and an otherwise unexplained ESR >100 mm/h limits differential diagnostic possibilities to only 2 pathogens: Streptococcus pneumoniae or Legionnaire’s disease. If the same patient also had otherwise unexplained highly elevated serum ferritin levels (2 times the upper limit of normal), Legionnaire’s disease is likely.

After the differential diagnosis is narrowed, age and disease prevalence should also be considered (ie, probability of diseases in the differential diagnosis relative to the patient’s age). Excluding respiratory viruses, a young adult with a persistent dry cough with little/no fever is more likely to have M. pneumoniae than pertussis. However, in the elderly, pertussis would be more likely than M. pneumoniae, particularly if accompanied by a prominent lymphocytosis. Although some pneumonias may be accompanied by lymphocytosis, the only pneumonia with a relative lymphocytosis > 60% is pertussis. This again illustrates, in the proper clinical context, that degree of abnormality itself confers diagnostic specificity.

In clinical problem solving, an oft-cited diagnostic error is early “anchoring.” This refers to physicians latching onto an initial impression on the basis of consistent, but not characteristic, findings. Master Clinicians anchor early, but the presumptive diagnosis is well “anchored” on the basis of the relative clinical importance (ie, “diagnostic weighting”) of characteristic clinical findings (Table).

 

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-Burke A. Cunha, MD

This article originally appeared in the January 2017 issue of The American Journal of Medicine.

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