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Some researchers have divided the one-step chief-complaint-to-syndrome assignment process into two steps. Rather than using NLP to assign a syndrome directly to a patient based on the chief complaint, they use NLP to find all of the symptoms embedded table 23.3 Syndrome Classification System Used by Maryland

Department of Hygiene and Mental Health






Respiratory Sepsis Unspecified Other

From Sniegoski, C. (2004). Automated syndromic classification of chief complaint records. Johns Hopkins University APL Technical Digest 25:68-75, with permission.

in the chief complaint. They define a syndrome as a Boolean or probabilistic combination of symptoms. The two-step process then is: (1) NLP extracts symptoms from the chief complaint, and (2) a non NLP process determines whether the symptoms satisfy a Boolean or probabilistic syndrome definition. As an example, consider the chief complaint "n/v/d.'' With a two-step process, an NLP system would extract three symptoms from the chief complaint: nausea, vomiting, and diarrhea. Any syndrome definition that required nausea, vomiting, or diarrhea would be satisfied by this chief complaint. Biosurveillance systems operated by Washington State, New York State, and those using the CoCo classifier translate free text directly to syndromes; ESSENCE, SyCo2, and MPLUS classify to symptoms first.

The two-step approach has several potential advantages. It is more natural for epidemiologists and physicians, who conceive of a syndrome as a combination of symptoms. In fact, case definitions (discussed in Chapter 3) are Boolean combinations of symptoms. Additionally, it is possible to create a new syndrome definition "on the fly'' without retraining a Bayesian classifier or restructuring lists of keywords. To do this, one simply defines a Boolean or probabilistic combination table 23.4 Syndrome Classification System Used by New York City Department of Health and Mental Hygiene




Common cold Sepsis


Diarrhea Fever Rash Asthma


Nasal drip, congestion, stuffiness

Sepsis, cardiac arrest, unresponsive, unconscious, dead on arrival Cough, shortness of breath, difficulty breathing, croup, dyspnea, bronchitis, pneumonia, hypoxia, upper respiratory illness, chest congestion Diarrhea, enteritis, gastroenteritis, stomach virus Fever, chills, flu, viral syndrome, body ache and pain, malaise Vesicles, chicken pox, folliculitis, herpes, shingles Asthma, wheezing, reactive airway, chronic obstructive airway disease Vomiting, food poisoning

Chest congestion, sore throat


Hay fever

Thrush, diaper and genital rash

Heffernan, R., Mostashari, F., Das, D., et al. (2004b). Syndromic surveillance in public health practice, New York City. Emerg Infect Dis 10:858-64. dopt=Citation&list_uids=15200820 with permission.

of symptoms. This advantage is important because new syndromes emerge with regularity, so it is important to be able to create a new syndrome definition quickly. Human biology changes very slowly, so new symptoms do not occur and the NLP conversion from free-text to symptom will be relatively stable, except as the language patients and triage nurses use to record chief complaints slowly evolves (e.g., the first time a patient uttered "I think I have SARS'').

A limitation of the two-step approach is that it has not been validated. A real concern is that users of such systems can define syndromes for which the system's sensitivity and specificity may be extremely poor. A user may create a syndromic definition that is rational from an epidemiological standpoint but is not well-suited to the input data being classified. Without deep knowledge of the underlying processing method and its assumptions, a user will be completely unaware of this phenomenon and may be falsely reassured by the absence of disease activity when the newly created syndrome is put into operational use. As an extreme example of this problem, consider that a user might define a syndrome as a Boolean conjunction (AND statement) of five symptoms. Since the average registration chief complaint comprises four words, it is almost inconceivable that any patient would match such a syndrome definition.4

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Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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