Definition of a marker
Revised: 2001-10-28

Question: I do not understand your definition of a "marker". How is it different from the test and how does it relate to your definition of disease? Please use the sore throat as a model although it may be applied to other diagnoses as well.

There is an important difference between the marker and the test. To understand this let us carefully examine 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%).

This situation may be described as:

Relation between test outcome (T+, T-) and presence (D+) or absence (D-) of specified disease  in patients having a sore throat
  A sore throat caused by....
  ....GABHS (D+) ....other than GABHS (D-)
  GABHS present
(
M+)
(GABHS not present)
(
M-)
GABHS present
(
M+)
GABHS not present
(
M-)

Positive test (T+)

True positive ----- False positive* False positive

Negative test (T-)

False negative ----- True negative** True negative
*The positive test does not represent true disease (D+) and is therefore considered to be false positive. However, it indicates that the patient is a carrier of GABHS.

**Although the patient carry GABHS a negative test will correctly identify the patient as not having the disease, thus being (D-).

The marker is the etiologic agent that might cause the disease. The marker is always present at some stage if the symptoms are caused by the marker.

The concept of sensitivity and specificity might relate to presence/absence of the marker or presence/absence of a specified disease. These concepts usually relate to presence/absence of the marker the test is designed to detect and this is also the definition that is used in EPV.

If the test is negative in a patient with a sore throat caused by GABHS it might be due to:

All these possible events are included in the concept of sensitivity.

If the test is positive in a patient not harboring GABHS it might be due to:

All these possible events are included in the concept of specificity.

As we can see we may have a positive test although GABHS is not present. The positive test is then a false positive outcome. If we relate the test outcome to disease then a positive test in symptomatic carriers is also a false positive test (column three).

By definition the marker is always present in patients having the specified disease. However, the test is not always positive in patients having the disease.

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).

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

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