The objective of screening is to detect evidence of increased risk of disease development or to pick up disease at an early stage when the chance of successful treatment is optimised. Screening may include clinical examination by your doctor, blood tests, x-ray (e.g. mammography), ultrasound (e.g. ovaries, uterus, breasts), and cervical smears.
- A. The effectiveness of a screening test must be judged on several criteria:-
- B. The condition sought should be an important health risk.
- C. The disease should have a well understood natural history (course).
- D. The disease being screened should have an effective treatment when found early.
- E. The test should be inexpensive so that the majority of those at risk can have the test.
- F. The false negative rate should be low (high specificity) the disease should not be missed.
- G. The false positive rate should be low (high sensitivity) few patients should be given unnecessary anxiety.
- H. The test should be acceptable to the population.
- I. Screening should be repeated at intervals depending on the natural history of the disease.
- J. The risks of the test should be low.
An evaluation of screening tests for breast cancer has been applied to these criteria in Q32.35
The majority of cancers can be successfully treated if they are caught early. In an ideal world, screening tests would be 100% successful with no false positive results causing unnecessary anxiety. Sadly, no screening test is perfect. Screening has undoubtedly reduced the risks but there is still room for improvement.
The majority of blood screening tests have a cut-off limit as there is rarely a test where the chemical being measured is not present to some degree in healthy people. The ability of a blood screening test to diagnose a disease process (F - specificity) and its sensitivity (G) will often depend on the cut-off level (Figure 32.4). If the cut-off level is arbitrarily decreased specificity will be increased but sensitivity will fall leading to unnecessary anxiety for some. The reverse would apply if the cut-off level is increased the test would miss the disease more often but there would be less unnecessarily worried people.
