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Medical screening 

Medical screening

Chapter:
Medical screening
Author(s):

Nicholas Wald

and Malcolm Law

DOI:
10.1093/med/9780199204854.003.030302_update_002

Update:

Antenatal screening – discussion of increasing use of DNA analysis of maternal blood for routine screening for Down’s syndrome and other chromosomal disorders.

Cancer screening – (1) manual breast self-examination by women shown not to reduce breast cancer mortality; (2) once-only flexible sigmoidoscopy screening for colorectal cancer; (3) low dose spiral computed tomographic screening for lung cancer.

Updated on 30 Jul 2015. The previous version of this content can be found here.
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date: 28 March 2017

Medical screening is the systematic application of a test or inquiry to identify individuals at sufficient risk of a specific disorder to benefit from further investigation or direct preventive action (these individuals not having sought medical attention on account of symptoms of that disorder). Key to this definition is that the early detection of disease is not an end in itself; bringing forward a diagnosis without altering the prognosis is useless and may be harmful.

Criteria for screening

Before a potential screening test is introduced into practice it must be shown to prevent death or serious disability from the disease to an extent sufficient to justify the human and financial costs. To this end, three screening parameters need to be determined: (1) the detection rate (sensitivity); (2) the false-positive rate (equivalent to the specificity); and (3) the odds of being affected given a positive screening result (equivalent to the positive predictive value). Where a detection rate cannot be directly determined, e.g. in cancer screening, or if the efficacy of the intervention is uncertain, a randomized trial is needed to show that screening and subsequent treatment reduce disease specific mortality.

Circumstances where screening is not appropriate

Screening tests should not be practised simply because they seem intuitively useful: chest radiography to screen for lung cancer and manual breast self-examination to screen for breast cancer were assumed to be worthwhile, but randomized trials showed they did not significantly reduce mortality from the cancer. Screening for prostate cancer is widely practised, yet it does harm (from hazardous treatment) with evidence of a relatively modest reduction in mortality from the disease. Causal risk factors, even important ones like serum cholesterol and blood pressure for cardiovascular disease, usually discriminate poorly between individuals who will and will not develop the disease they cause, because most of the population is ‘exposed’.

Particular disorders where screening is justified

The number of disorders for which medical screening has been shown to be worthwhile is perhaps surprisingly small, but includes: (1) antenatal screening—e.g. various single gene disorders, Down’s syndrome, neural tube defects, and some infections such as hepatitis B and HIV that may be asymptomatic in the mother but cause disease when transmitted to the fetus; (2) neonatal screening—e.g. congenital hypothyroidism, certain inborn errors of metabolism such as phenylketonuria, and congenital deafness; (3) adult screening—this has been shown to reduce mortality from only three cancers—breast, cervical, and colorectal; screening individuals with diabetes mellitus prevents blindness from retinopathy; screening men around the age of 65 prevents death from ruptured abdominal aortic aneurysm; and screening young women for chlamydia infection prevents pelvic inflammatory disease and its complications (including infertility).

Future prospects

Tests that arise out of technological development in the absence of a clear case of medical need, e.g. whole body scanning using MRI or CT scanning, should not be ‘sold’ to the public in the belief that they are helpful. As with all screening methods, their value needs to be shown before they are introduced into practice. Determining when medical screening is an effective method of preventing serious disease and disability is one of the most challenging areas in medical research.

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