7.1.1 - Verifying the Diagnosis

It is also important early in an investigation to identify as accurately as possible the specific nature of the disease. You want to verify that both clinical and laboratory diagnoses are correct. Review the clinical findings and lab processes and results for the cases. If there is need for specialized laboratory analysis, collect the required specimens and materials. It may be necessary to sequence the genome of the bacteria. (for example, in a hospital-based outbreak of antibiotic resistant disease: Kupferschmidt, Kai. "Genome Study Helps Contain MRSA Outbreak--And Breeds New Questions. Science 23 Nov 2012 )

It is also often a good idea for a qualified clinician to visit some of the cases to confirm the initial reports. Try to gain a better understanding of the disease and those affected by it. Ask : What were their exposures before becoming ill? What do they think caused their illness? Do they know anyone else with the disease? Do they have anything in common with others who have the disease? Conversations with patients can be quite helpful in generating hypotheses about the cause, source, and spread of disease.

Establishing the Case Definition Section

Specifying the definition of a case of an adverse health outcome is one of the most important steps for successful investigation of a potential outbreak. Since the case definition is the standard for determining which individuals are cases and which are not, a case definition should be established early in the investigation.

As you learned in Lesson 2, cases can be defined by laboratory tests, clinical signs and symptoms and/or a physician's diagnosis. They can also be defined as being epidemiologically-linked, or even just exposed. A case definition usually includes four components:

  1. clinical information about the disease,
  2. characteristics of the people who are affected,
  3. information about the location or place, and
  4. specification of the time during which the outbreak occurred.

Cases can be classified according to the level of confidence the investigator holds regarding the individual's case status, such as confirmed, probable or suspect. Under certain special circumstances, individuals who have only been exposed to a contagious or contaminated agent and who remain free of symptoms may be included as a case.

The initial case definition may be quite broad with a risk of mistakenly identifying as cases some individuals without the condition. Ideally, the case definition catches all cases without picking up 'false positives' (when the case definition is met, but the person actually does not have the disease). A broad case definition can be helpful early in the outbreak, when there is a goal of reducing the spread of the disease. Using a broad case definition may help prevent the investigator from having to go back to clinics for additional data, illustrating the field epidemiology axiom: "Get it while you can."

As the disease spreads, the case definition may be refined, dropping 'possible' cases. Did you notice the case definitions in the 2009 H1N1 pandemic changing as the numbers of affected communities increased and the goals of health agencies were modified?

Using case definitions already established by prominent scientific or medical organizations or agencies is recommended. If no case definitions are recommended by professional organizations, investigators may use definitions published in the scientific literature. Using an established case definition yields more legitimate comparisons of cases and outbreaks between regions or over different time periods. We have already explored some sources of standard case definitions in Lesson 2.

Measuring the Frequency of Adverse Health Outcomes Section

In all outbreak situations, we will count the number of cases of the outcome. In many situations, we desire more information. For example, knowing the number of persons in the at-risk population would allow calculation of the proportion of those at-risk who developed the condition or disease. We may also wish to incorporate a time period over which the at-risk developed disease to produce a rate or risk of disease.

Think about it!

Come up with an answer to this question and then click on the button below to reveal the answer.

What was the case definition used In the Salmonella example? How was the frequency measured?

"An outbreak case was defined as infection with a Salmonella strain with the specific outbreak PFGE pattern and illness onset during January 1–December 31, 2007. During this period, a total of 401 outbreak cases from 41 states were identified." (count)

When identifying cases, use as many sources as possible, starting with health care facilities where the diagnosis is likely to be made. If an outbreak affects a population in a restricted setting, such as a school or worksite, you may decide to survey and/or collect samples from the entire population, particularly if asymptomatic cases are expected. Ask cases if they know anyone else with the disease. (Does this remind you of the survey in San Pablo from Lesson 5?)

Collect the following types of information on each case:

  • Limited identifying information: to allow the investigators to contact patients to ask additional questions and to notify them of laboratory results and the outcome of the investigation. Addresses allow mapping the outbreak.
  • Demographic and risk factor information: used to characterize the population at risk.
  • Clinical information: allows verification of case status. Date of onset allows you to create a graph of the outbreak. Supplementary clinical information may include whether the person was hospitalized or died.