Case Definitions and Disease Classification

16.1 Case definitions

Establishing a case definition

In this chapter, we consider criteria for defining cases and the standardized system of disease nomenclature known as the International Classification of Disease (ICD). We also address how changes in disease nomenclature and coding practices can create artifactual fluctuations in reported rates of disease.

A case definition is a set of objective, uniform, and consistent criteria by which to decide whether an individual should be classified as having the disease under investigation. Use of carefully researched and constructed standardized case definitions is important in all epidemiologic endeavors, including epidemiologic surveillance, vital statistics, health surveys, health-care research, descriptive epidemiology, and the various types of experimental and observations analytic studies.

Case definitions may consist of clinical criteria, pathophysiological criteria, and epidemiologic “person, place, and time” criteria. For example, a case definition for gastroenteritis associated with a particular food vendor may have criteria that include fever (body temperature greater than or equal to 38.6 °C), diarrhea (bowel movements that conform to the shape of a container), nausea or vomiting, and malaise (self-reported body discomfort and fatigue) in individuals who had eaten at a particular establishment between specific dates. Criteria must be applied consistently throughout the study.

Multiple-choice criteria

Carefully constructed case definition alternatives can be built on multiple criteria. The case definition for coronary heart disease (angina, myocardial infarction, and sudden death) in the Framingham Heart Study (Illustrative Example 7.1), for example, was based on the best clinical and electrocardiographic criteria available at the time as recommended by the New York Heart Association. In cases lacking a clinical history of myocardial infarction, evidence of silent heart attacks were accepted only if an unequivocal pattern of myocardial infarction had developed since the previous electrocardiographic tracing was obtained or there was evidence of prolonged acute coronary insufficiency with electrocardiographic abnormalities (Kannel et al., 1961).

A common approach for building a case definition is to combine diagnostic criteria in different combinations of choices in an “either/or” fashion. This is sometimes referred to as a “Chinese menu”^a case definition. An example of a Chinese menu case definition for myocardial infarction, used by Henning and Lundman (1975), was to meet two of the criteria listed in A, B, or C (below), or criterion D alone.

Criterion A: central chest pain, pulmonary edema, syncope, or shock.
Criterion B: pathologic changes detected in the electrocardiogram (e.g., pathological Q-wave).
Criterion C: two elevated ASAT-values with a maximum approximately 24 h after onset of symptoms in combination with an ALAT-maximum approximately 36 h after onset with ALAT maximum lower than ASAT maximum.
Criterion D: autopsy findings of myocardial necrosis of an age consistent with the onset of symptoms.

The “either/or” aspect of the Chinese menu allows for adjustment to the case definition by either broadening or restricting criteria. A broad case definition is useful during the early phases of an investigation when one needs to gather information on all possible cases. The case definition can later be tightened to allow a sharper focus for testing causal hypotheses, as long as the revised case definition is applied to all past and future cases uniformly.

Chronic fatigue syndrome, as an example

Chronic fatigue syndrome has no known cause and is characterized by chronic fatigue and an array of nonspecific signs and physical symptoms. No laboratory test to confirm the existence of the disease is available, and diagnostic criteria for the syndrome have not been uniformly applied. Researchers, therefore, have lacked adequate tools for assessing the severity and functional limitations of the illness and its response to therapy (Klonoff, 1992). Moreover, without a consistent method for identifying cases, progress in understanding the epidemiology and clinical correlates of this syndrome have been hampered.

Recognizing these limitations, a group of epidemiologists and academic researchers developed a working case definition based on major and minor clinical and physical criteria (Table 16.1). According to these criteria, a case of chronic fatigue syndrome must fulfill two major diagnostic criteria and eight minor criteria (six or more symptom criteria and two or more of the physical criteria). In 1991, a national workshop updated this case definition by excluding specific psychiatric diagnoses and postinfectious disease fatigue that could explain the patient’s symptoms (Schluederberg et al., 1992). Because chronic fatigue syndrome is not a homogeneous abnormality and no single pathogenic mechanism is known, this update further emphasized the need to delineate patient subgroups for separate data analyses. These standards serve as the basis for conducting clinical and epidemiologic studies of chronic fatigue syndrome, providing a rational basis for evaluating patients and discovering the syndrome’s cause. (The exact cause of chronic fatigue syndrome is still unknown.)

Table 16.1 Working case definition of chronic fatigue syndrome.

A case of chronic fatigue syndrome must first fulfill both of the major criteria. In addition, six or more of the symptom criteria plus two or more of the physical criteria must be fulfilled.

Major criteria

1 New onset of persistent or relapsing, debilitating fatigue or easy fatigability in a person with no previous history of similar symptoms. The fatigue must be severe enough to reduce or impair daily activity below 50% of the patient’s premorbid activity level for a period of at least 6 months.

2 Other clinical conditions that produce similar symptoms must be excluded thorough evaluation, based on history, physical examination, and appropriate laboratory findings. (Examples of conditions that must be excluded are malignancy, autoimmune disease, localized infection, chronic or subacute bacterial disease, fungal disease, parasitic disease, diseases related to HIV, chronic psychiatric disease, chronic inflammatory disease, neuromuscular disease, endocrine disease, drug dependency or abuse, side effects of chronic medication or other toxic agents, or other known or defined chronic pulmonary, cardiac, gastrointestinal, hepatic, renal, or hematolic diseases.)

Minor criteria

Symptom criteria

1 Chills or mild fever (oral temperature between 37.5 and 38.6 °C, if measured by the patient; oral temperatures greater than 38.6 °C are less compatible with chronic fatigue syndrome and should prompt studies for other causes of illness).

2 Sore throat.

3 Painful lymph nodes in the anterior neck or armpit regions.

4 Unexplained generalized muscle weakness.

5 Muscle pain or myalgia.

6 Prolonged (greater than 24 h in duration) generalized fatigue after modest levels of exercise that would have easily been tolerated in the patient’s premorbid state.

7 Generalized headaches (of a type, severity, or pattern different from headaches the patient experienced in the premorbid state).

8 Migratory joint pain without swelling or redness.

9 Neuropsychologic complaints (one or more of the following: photophobia, transient visual scotomata, forgetfulness, excessive irritability, confusion, difficulty thinking, inability to concentrate, or depression)

10 Sleep disturbance (hypersomnia or insomnia).

11 Description of the main symptom complex as initially developed over a few hours to a few days. (This is not a true symptom but may be considered equivalent to the above symptoms in meeting the requirements of the case definition.)

Physical examination criteria

1 Low-grade fever (oral temperature between 37.5 and 38.6 °C or rectal temperature between 37.8 and 38.8 °C).

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