7 Classification (Stage 4)
Clear definitions are crucial and avoids creating rubbish
The content of this chapter is the personal opionon of the author. This is escpecially true for the discouragment of the usage of the word case definition__
Classification means categorizing the recorded events into two or more groups. Sometimes the word “collation” is used for this stage, for example in the WHO definition of surveillance. Classification means a person or software applies a set of uniform criteria to define a disease, health event or condition for infectious disease surveillance. The classification is based on the specific objective.
Usually the word “applying a case definition” is used to describe this process. But it is better to use more specific words and use the word “case definition” for the general concept. Classification is often a hidden part of the surveillance system or built into the system in a way that it is not recognized as such. For example, the application of reporting definition is built into the reporting software.
Definitions enable public health officials to classify and count cases consistently for example across reporting jurisdictions. This avoids comparing “apples to oranges”. Classification is important because recorded events can be erroneous and should not be counted. Or there may not be enough information to decide whether a real event has occurred. Without classification, the events form an unclear collection with questionable significance. Even in seemingly trivial classifications, important definitions must be agreed upon: In a mortality surveillance deaths are counted. But does the death of a tourist with a foreign passport count as a death in the surveillance system?
7.1 Typical classification definitions
| Definition | Description |
|---|---|
| Reference definition | A reference definition is used for official reporting (e.g. national yearly reporting) |
| Case-finding definition | A case-finding definition is applied for an the search of cases in an outbreak |
| Notification definition | A notification definition is applied by physisicans or labs whether they should notify a diseases person |
| Reporting definition | A reporting definition is applied by local public health agencies before reporting a case to a state or national level |
| Intervention definition | A intervention definition is applied when to act upon an information |
| ECDC case definition | The ECDC case definitions are used for the reports of ECDC |
| Outbreak-study definition | An outbreak study definition is used for a cohort or case-study |
| Suspected case definition | This definition is used in an outbreak response |
| Probable case definition | This definition is used in an outbreak response |
| Confimed case definition | This definition is used in an outbreak response |
| Contact definition | This definition is used in an outbreak response |
| Clinical definition | A clinical definition is applied when someone in the medical systems uses his or her knowledge whether this constitutes a disease |
| ICD-10 definition | A medical classification system that is especiall used for the reimbursment of physicians |
| Infected person | Somebody who inoculated a pathogen, that subsequently replicates within the body |
| Diseased person | Somebody with an infection with any kind of symptoms that are caused by the infection process. |
Example for the application of definitions along the surveillance stages of a case-based surveillance system.
7.1.1 Example of the different usages of definitions
A person inoculates hepatitis B virus and the pathogen subsequently replicates in the body of the person (Fullfilling the infection definition). He then develops symptoms (therby fullfilling the diseased person definition) and he seeks the help of a doctor who gives him treatment (apllying a clinical definition) and enters the data into his software for reimbursment (applying the ICD-10 defintion). The physician notifies the local public health agency because the law requires to notify all diseased patients with acute hepatitis B (the notification definition). The local public health agency then decides to act (implicitly applying an intervention definition). They ensue an outbreak investigation and look for cases and contacts (applying case finding definitions or contact definitions). The local public health agency enters the data into their notification software that applies an algorithm and identifies that this event should be reported to the state and national agencies (reporting definition). The national agency reports the case in their official report (applying the referen definition). The software scripts in the national agency find that the event should be reported to ECDC (applying the ECDC case definition)
7.2 Components of definitions
Typical definitions in the field of infectious diseases include some or all of the following components
| Component | Explanation |
|---|---|
| Time | The timeframe in which a disease is looked at (in surveillance systems this is often missing) |
| Place | Where a disease is looked at. E.g. a region |
| Person | Who is looked at. It can be everybody in a certain region or it can be only a specific group in an outbreak session |
| Clinical | Include common and relevant signs and symptoms of the disease under surveillance Form either individually or in combination a clear or indicative picture of the disease |
| laboratory | Includes a list with methods used to confirm the pathogen Usually: One of the laboratory methods on the list is sufficient for confirmation of a disease |
| Epidemiological criteria | Are met when an epidemiological link is established Depending on: Incubation period of the disease Transmission Routes (person-to-person, contaminated food, …) Endemicity of the disease in the country |
7.2.1 Example for a clinical part of a case defintion
7.2.2 Example for a laboratory part of a case defintion
7.2.3 Example for an epidemiological part of a case defintion
7.3 Specificity and sensitvity
- A definition can be sensitive or specific. Sensitive means that more events are captured but likely this will increase the number of wrongly identified events. Specific means that less events are captured but the number of wrongly identified events is lower.
7.4 Creating you own definitions
- Identify the objectives of the surveillance system
- Involve a multi-sectoral team (e.g. physicians, reference laboratories, epidemiologists,…)
- Balance between sensitivity and specificity when collecting / reporting
- Define important and frequently used terms (e.g. fever)
- Use a standardised format and structure for all case definitions
- Plan the implementation of the case definitions (e.g. communicate, train, evaluate)