Screening

In screening it is also important to have a target population to avoid large numbers of fruitless tests. In screening for the common cancers, where the incidence is highly age-dependent, the age range should be that in which the disease is relatively common and in which the patients are likely to be fit enough for treatment.

There are other high-risk groups, however, and family history is becoming important in this respect, particularly as it is now possible to detect specific genetic mutations from blood samples and to use these to screen close relatives. Examples of this are mutations in the APC gene in familial adenomatous polyposis, in the DNA mismatch repair genes in hereditary non-polyposis colorectal cancer, and in the BRCA 1 and 2 genes in familial breast and ovarian cancer.

A screening test must be acceptable and safe, so that it will be adopted by the target population. It must also be sensitive and specific. Sensitivity is the proportion of individuals with the disease who have a positive test, and specificity is the proportion of individuals without the disease who have a negative test. Thus the ideal test would be acceptable to everyone, have no associated morbidity or mortality, and have 100% sensitivity and specificity—but this ideal has never been realised and any screening test has to be a compromise between these factors.

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