Prerequisites for a control population
Revised: 2001-10-28

Question: What are the prerequisites for a control population? Please use the sore throat as a model although it may be applied to other diagnoses as well.

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%).

To calculate Etiologic predictive value (EPV) we need to estimate the proportion of carriers in patients with a sore throat caused by something else than the marker (in our scenario a sore throat caused by something else than GABHS). This is done by using information from a healthy control population.

By healthy we mean that this population lacks both the disease we are trying to predict (for example a sore throat caused by GABHS) and the symptom of interest (a sore throat). The healthy population should also lack other diseases that might interfere. For example if we are investigating an infectious disease then the healthy control population cannot have increased prevalence of diabetes mellitus or any other disease that might interfere with the immune response. The healthy population used must also be comparative to our population of patients in important confounding factors. Some important confounding factors in our example with a sore throat are age and season of the year. The latter is important if the proportion of positive samples in a healthy control population varies in time and geographical area.

The consequence is that if the test we want to evaluate is a throat cultures ability to predict the disease a sore throat caused by GABHS then we can only use samples from a healthy control population if the samples are collected from approximately the same geographical area during the same time periods as the samples from patients with a sore throat. In other situations where the fluctuation in positive tests is smaller it might be possible to use a historical healthy comparison group collected in another time interval than the patients.

Other WebPages of interest

Other pages with subjects that might be of interest is:

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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).

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

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