Breast screening - definition of terms
This page presents information on the definition of terms used in breast screening, taken from the CancerStats report - Breast Screening - UK, published in 2003.
A screening test does not diagnose a particular condition, but sorts the population into test positive and test negative groups.
The sensitivity of a screening test is the proportion of the screened population that has the disease that tests positive. For instance, a sensitivity of 80% means that for every ten participants with the disease, eight will test positive and the other two will be false negatives. A test with poor sensitivity results in a high proportion of the population with the disease escaping detection. These people will be falsely reassured and could delay presenting important symptoms.
The specificity of a test is the proportion of the screened population that is disease free that tests negative. For instance, a specificity of 90% means that nine out of ten people who do not have the disease will have a negative result. One out of ten will have a false positive result and require further assessment before the possibility of disease can be eliminated. A test with poor specificity will have important consequences for the individual, including anxiety and unnecessary follow up.
An ideal screening test would have a high sensitivity (to reduce the number of false negatives) and a high specificity (to reduce the number of false positives). It is usually difficult to achieve this as there is a trade off between the two measures; tightening the criteria for one results in a decrease in the other.
Another feature of a screening test is the predictive value and there are two aspects to this. The positive predictive value of a test is the proportion of people who test positive who have the disease. The negative predictive value is the proportion of those who test negative who are disease free. The predictive value is influenced by both the sensitivity and specificity of the test, as well as the prevalence of the condition being screened for.
The relationships between specificity, sensitivity and predictive value are shown in Table 2.1.
Coverage of breast screening is the proportion of women resident in the screening target area (excluding those who are ineligible for screening, for example, those who have had a bilateral mastectomy) who have had a recorded test result at least once in the previous three years.
The uptake of a screening programme is the proportion of women invited for screening for whom a test result is recorded.
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