As an example, assume we want to develop an expert system that generates the probability a patient presenting to an emergency department (ED) has severe acute respiratory syndrome (SARS) given their free-text medical records. According to the World Health Organization and Centers for Disease Control and Prevention case definitions of SARS, the required input variables for diagnosing SARS are whether the patient (1) has an acute respiratory finding (Respiratory Fx), (2) is febrile (Fever), (3) has an abnormal chest radiograph consistent with consolidation or pneumonia (CXR), and (4) has recently traveled to a country currently affected by
SARS (Travel). Values for the first three variables are often described in electronic textual patient records. Figure 17.1 shows excerpts from a patient's medical record generated during an ED visit, including the triage chief complaint, history and physical exam, and chest radiograph report.A human physician reading these textual records could easily determine the correct values for the first three variables. Information from the fourth variable (Travel) may not be accessible anywhere in the patient's medical record unless the dictating physician had been concerned about SARS and had dictated the travel history. Retrieving the value for the Travel variable may require a semi-automatic technique in which patients with a high probability of SARS based on the three clinical values could be interviewed regarding their travel history.
A simple expert system may only use the variable Respiratory Fx, monitoring the number of patients presenting to the ED with respiratory complaints. A physician may be able to determine the true value of the Respiratory Fx variable from information in the triage chief complaint. A more complex expert system may use all three variables. According to the medical record in Figure 17.1, a physician would assign values to the variables in a more complex expert system as follows. Respiratory Fx: yes, because the patient had a chief complaint of cough, he complained to the ED physician of productive cough and shortness of breath, and the shortness of breath was probably not cardiac in nature, given that the patient did not have a history of CHF and denied chest pain; Fever: yes, because the chief complaint and the ED report said the patient was febrile; and CXR: yes, because the radiograph report described an opacity consistent with pneumonia.
It would be impractical to hire physicians to read medical records and extract values for the variables required by our expert system. Therefore, if we want to know the variables' values, we must determine them automatically using natural language processing.
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