A key technique in differential diagnosis is using pertinent negative findings. Often one has a few common findings that are present, resulting in a wide differential diagnosis. One then narrows the differential diagnosis based on findings that are absent.
To do this one must know the frequency of findings in a disease. If a finding is absent in the patient but is present in 99% of individuals with a disease, that absence argues strongly against that disease. If the finding is present in only 10% of individuals with the disease, its absence does not mean much.
Example to work through
A 50-year-old woman had onset over the period of a year of diarrhea, weight loss and anemia. A variety of inflammatory, infectious and neoplastic conditions are in the differential diagnosis and there is a variety of useful findings.
Taking a travel history and a history of drinking water from streams (or untreated wells) is very important because such a history is very common in relevant infections and very uncommon in inflammatory and neoplastic conditions.
By clicking to select the Useful finding of Exposure and then clicking on the green “V” on the top black navigation bar, you can view the selected finding in the differential diagnosis and toggle the finding between present and absent to see the impact on the differential diagnosis.
Specifying absence of such a history focuses the differential diagnosis on the inflammatory and neoplastic conditions.
Specifying presence of such a history focuses the differential diagnosis on the infections.
