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Mass screening

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Mass screening, more simply called screening, is defined in health care as "organized periodic procedures performed on large groups of people for the purpose of detecting disease."[1] The goal of screening is the primary prevention, "the prevention of disease or mental disorders in susceptible individuals or populations"[2], of disease.

When is mass screening justified?

According to World Health Organization guidelines published in 1968:[3]

  1. The condition should be an important health problem.
  2. There should be a treatment for the condition.
  3. Facilities for diagnosis and treatment should be available.
  4. There should be a latent stage of the disease.
  5. There should be a test or examination for the condition.
  6. The test should be acceptable to the population.
  7. The natural history of the disease should be adequately understood.
  8. There should be an agreed policy on who to treat.
  9. The total cost of finding a case should be economically balanced in relation to medical expenditure as a whole.
  10. Case-finding should be a continuous process, not just a "once and for all" project.

Simpler criteria have been proposed:[4]

  1. How great is the burden of suffering caused by the condition?
  2. How good is the screening procedure in terms of accuracy, cost, acceptability?
  3. If the condition is found, how effective is the ensuing treatment in terms of efficacy and patient compliance compared to waiting for later treatment.

Ideally, the evidence supporting these criteria comes from randomized controlled trials that show an improvement in the population's health when the proposed screening program is compared to no screening.[5] If a randomized controlled trial has been completed, then the number needed to screen (NNS) can be calculated to help individuals decide whether to undergo screening. The NNS is the number of patients who must be screened to prevent one bad outcome from the disease.[6]

Although lesser study designs may provide preliminary support, a randomized controlled trial reduces the chance of lead time bias or length time bias affecting conclusions.

Study methods of mass screening


For more information, see: Overdiagnosis.

In cohort studies and randomized controlled trials of mass screening, overdiagnosis is the diagnosis of non-harmful disease. [7] Overdiagnosis inflates the importance of the screening problem.


  1. National Library of Medicine. Mass screening. Retrieved on 2007-12-06.
  2. National Library of Medicine. Primary prevention. Retrieved on 2007-12-06.
  3. Wilson JMG, Jungner G (1968). Principles and Practice of Screening for Disease (pdf). World Health Organization. Retrieved on 2007-12-06.
  4. Fletcher, Suzanne W., Fletcher, Robert (2005). Clinical epidemiology: the essentials. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-5215-9. 
  5. Barratt A, Irwig L, Glasziou P, et al (1999). "Users' guides to the medical literature: XVII. How to use guidelines and recommendations about screening. Evidence-Based Medicine Working Group". JAMA 281 (21): 2029–34. PMID 10359392[e]
  6. Rembold CM (1998). "Number needed to screen: development of a statistic for disease screening". BMJ 317 (7154): 307–12. PMID 9685274[e]
  7. Marcus PM, Bergstralh EJ, Zweig MH, Harris A, Offord KP, Fontana RS (June 2006). "Extended lung cancer incidence follow-up in the Mayo Lung Project and overdiagnosis". J. Natl. Cancer Inst. 98 (11): 748–56. DOI:10.1093/jnci/djj207. PMID 16757699. Research Blogging.