Definition of carriers
 Revised: 2001-10-28

Question: If you are suppose to evaluate a test and at the same time take carriers into account then you must have a definition of carriers. How do you define carriers? Please use the sore throat as a model although it may be applied to other diagnoses as well.

It is advisable to first read the page about defining disease and then return to this page. To understand this let us use the following situation:

Consider the following example: A conventional throat culture has been obtained during a summer period from 36 children of age 3-15 years having a sore throat possibly caused by group A beta-hemolytic streptococci (GABHS)1. Among those 36 cultures a throat culture indicated presence of the bacterium GABHS in 11 (31%).

However, some of the children might be ill due to a virus as well as carrying GABHS. To investigate this phenomenon we may collect data from healthy children. During the same period of time throat cultures were obtained from 290 healthy children of age 3-15 living in the same geographical area, and showing no signs indicating possible GABHS-caused tonsillopharyngitis1. Among those 290 cultures a throat culture indicated presence of GABHS in 37 (13%).

Carriers may be defined in many different ways, for example:

Definitions of carriers:
  --- Example from the strep throat ---
Definition Definition Comments
  1. Presence of a specified etiologic agent in patients having a specified symptom (or combination of symptoms/clinical signs). However, the patients does not respond differently to adequate treatment compared to individuals obviously not ill from the specified etiologic agent.

A group of patients with a sore throat and presence of GABHS in the throat. However, they do not improve more rapidly than individuals with a typical virus infection if adequate treatmentt is initiated.

 These individuals could be named symptomatic carriers.
  1. Presence of a specified etiologic agent and absence of an defined physiological response.

Presence of GABHS in the throat and no rise in titers of antibodies directed towards antigens from GABHS.

 These individuals could be named symptomatic carriers if they have a sore throat. If they lack the symptom a sore throat they could be named asymptomatic carriers.
  1. Presence of a specified etiologic agent and absence of a specified symptom (or combination of symptoms/clinical signs).

Presence of GABHS in the throat but no sore throat.

 This definition only applies to individuals lacking the specified symptom (or combination of symptoms/clinical signs). These could be named asymptomatic carriers.
  1. Presence of a specified etiologic agent but it does not cause the specified symptom (or combination of symptoms/clinical signs) that the patient has.

Presence of GABHS in the throat but it does not cause the sore throat.

 This definition only applies to individuals having the specified symptom (or combination of symptoms/clinical signs). These could be named symptomatic carriers. This definition is purely theoretical. It cannot be established if a single individual is a carrier according to this definition.

The first definition is related to effect of treatment. The second definition has been commonly used as gold standard for evaluating rapid tests and throat cultures. However, some studies indicate that using antibody titers to differentiate between those truly ill from GABHS and carriers might be wrong2. The second and third definitions might seem to be similar. However, there is a difference. The second definition deals with bodily response to the etiologic agent but the third definition deals with bodily responses whether they are caused by the etiologic agent or not.

Definition of carriers in EPV

Etiologic predictive value (EPV) use the third and fourth definition mentioned above. It uses information from a healthy control population. Individuals in this control population harboring the etiologic agent are defined as asymptomatic carriers. When investigating the population of patients with a sore throat EPV uses the fourth definition

Other WebPages of interest

Other pages with subjects that might be of interest is:

(You can click on these links to quickly see the pages)

References

  1. Gunnarsson, R.K., Holm, S.E. and Söderström, M. `The prevalence of beta-haemolytic streptococci in throat specimens from healthy children and adults. Implications for the clinical value of throat cultures' , Scand J Prim Health Care, 15, 149-155 (1997).
     
  2. Gerber MA, Randolph MF, Mayo DR. The group A streptococcal carrier state. A reexamination. Am J Dis Child 1988;142:562-5.

Ronny Gunnarsson MD PhD
Department of Primary Health Care
Göteborg University
SWEDEN

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