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Principles, methods, applications and organisation of screening for early detection, prevention, treatment and control of disease

Diagnosis and Screening: Principles, methods, applications and organisation of screening for early detection, prevention, treatment and control of disease

Definitions:

Screening is a public health service in which members of a defined population, who do not necessarily perceive they are at risk of, or are already affected by a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications (UK National Screening Committee http://www.nsc.nhs.uk).

Or

Screening refers to the use of simple tests across an apparently healthy population in order to identify individuals who have risk factors or early stages of disease, but do not yet have symptoms (WHO).

Principles:

The underlying concept of screening is that early detection of risk factors or early disease is beneficial for the clinical or public health outcome. Box 3.1.1 below outlines the classic Wilson and Junger criteria formulated to assess whether a condition potentially warrants screening efforts.

Box 3.1.1 Wilson and Jungner classic screening criteria, WHO 1968

  1. The condition sought should be an important health problem

  2. There should be an accepted treatment for patients with recognized disease.

  3. Facilities for diagnosis and treatment should be available.

  4. There should be a recognisable latent or early symptomatic stage.

  5. There should be a suitable test or examination.

  6. The test should be acceptable to the population.

  7. The natural history of the condition, including development from latent to declared disease, should be adequately understood.

  8. There should be an agreed policy on whom to treat as patients.

  9. The cost of case-finding (including diagnosis and treatment of patients diagnosed) should be economically balanced in relation to possible expenditure on medical care as a whole.

  10. Case-finding should be a continuing process and not a 'once and for all' project.

Wilson . JMG, Jungner G. Principles and practice of screening for disease. Geneva: WHO; 1968. Available from: http://www.who.int/bulletin/volumes/86/4/07-050112BP.pdf

The key questions to ask when deciding on whether to screen for a health problem are listed in Box 3.1.2.

Box 3.1.2

The disease

Is it an important health problem?

Is the natural history well understood?

Is there a long time between the presence of risk factors/sub-clinical disease to overt disease?

Does early intervention improve clinical/public health outcome?

Screening test

Is the test valid (sensitivity and specificity)?

Is the test simple, reliable and affordable?

Is the test acceptable to patient and staff?

Diagnosis and treatment

Is access to diagnostic facilities available and rapid?

Is treatment effective and accessible?

Is it cost-effective?

Is it sustainable?

Does benefit outweigh the harm?

For the assessment and evaluation of screening programmes see the specific section in this chapter.

Limitations of Screening

Screening has important ethical differences from clinical practice as the health service is targeting apparently healthy people, offering to help individuals to make better informed choices about their health. However, there are risks involved and it is important that people have realistic expectations of what a screening programme can deliver. Whilst screening has the potential to save lives or improve quality of life through early diagnosis of serious conditions, it is not a fool-proof process.

Screening can reduce the risk of developing a condition or its complications but it cannot offer a guarantee of protection. In any screening programme, there is an irreducible minimum of false positive results (wrongly reported as having the condition) and false negative results (wrongly reported as not having the condition). The UK National Screening Committee is increasingly presenting screening as risk reduction to emphasise this point.
(UKNSC, www.nsc.nhs.uk)

Methods, applications and organisation

Screening activities can be organised in formal screening programmes (e.g. breast screening) or opportunistically (e.g. assessment of smoking status during primary care visits). Screening instruments can vary from technological procedures (e.g. radiography or laboratory tests), simple clinical examinations (e.g. blood pressure) to a set of standardised questions (e.g. depression screening in primary care).

Brief overview of current UK screening programmes

For detailed information and updates on current programmes please visit the NHS Library for Health http://www.library.nhs.uk/screening/

Tables A, B, C, D amended from 'Mastering Public Health: A postgraduate guide to examinations and revalidation. Lewis GH, Sheringham J, Kalim K, Crayford, TJB. London: Royal Society of Medicine Press Ltd; 2008'

Table A:  Antenatal screening programmes

Standard

Screening for the following conditions is offered to all women:
• Anaemia • Bacteriuria • Blood group and rhesus D status • HBV (Hepatitis B virus) and HIV • Syphilis • Rubella immunity • Pre-eclampsiaNeural tube defects (including spina bifida)Sickle cell disease and thalassaemia (in high prevalence areas)

Down's syndrome

The combined test is available to women who request screening in the first trimester and understand the implications of doing so. It is based on combining an ultrasound measurement of nuchal translucency (NT; thickness of fluid at the nape of the fetal neck as assessed on ultrasound), and serum levels of human chorionic gonadotrophin (hCG), pregnancy-associated plasma protein A (PAPP-A) and the woman's age

The integrated test provides better performance than the combined test if the woman is prepared to wait until the second trimester for the result. It integrates measurements performed at different times during pregnancy into a single test result. Typically it refers to the integration of NT and PAPP-A in the first trimester with the quadruple test in the second

The quadruple test is offered to women who attend for screening in the second trimester, and also forms part of the integrated test as described above. It is a second trimester blood test consisting of measurement of alpha-fetoprotein (AFP), unconjugated oestriol (uE3), hCG, inhibin-A and the woman's age (the triple test does not include inhibin-A).

If the pregnancy is deemed to be high risk (>1/250 risk of carrying an affected baby with the risk derived from laboratory markers, maternal age, gestational age, and previous affected pregnancy), then the woman is offered amniocentesis (or chorionic villous sampling [CVS] if <13 weeks' gestation). These are diagnostic tests.

Fetal anomaly ultrasound scan

This is performed at 18−20 weeks' gestation to identify structural anomalies in the fetus. It is not universally performed in Scotland.

Other

Those women who are at high risk may be offered screening for psychiatric illness, Tay−Sachs disease, diabetes.

Table B:  Newborn screening programme

Heel-prick test

Used to test for:

Phenylketonuria (PKU)

Congenital hypothyroidism (CHT)

Sickle cell disease and thalassaemia (2005)

Cystic fibrosis (2007)

Hearing screening programme

Oto-acoustic emissions test or attenuated brain-stem audiometry

Physical examination

Offered during first 72 hours of life to detect, among others:

• Congenital heart disease

• Congenital cataract

• Congenital malformation

Cryptorchidism (undescended testes)

• Developmental dysplasia of the hip

Table C:  Children's screening programmes

Growth

Height and weight should be measured around the time of school entry. The measurements can be used to identify growth disorders, and are an important public health population measurement.

Hearing

Screening for hearing impairment is offered to school-aged children.

Vision

Screening for visual impairment is offered at age 4−5 years.

Table D:  Adult screening programmes

Disease

Screening Test

Confirmatory Test

Age group

Frequency

Breast cancer

Mammography

Fine-needle aspiration

50−70 (first invitation always before age 53)

3-yearly

Cervical cancer

Smear test or liquid-based cytology

Colposcopy

In England:

• 25−49 years

• 50-64 years

In Scotland: 20−60 years

 

• 3-yearly

• 5-yearly

• 3-yearly

Bowel cancer

Faecal occult blood

Colonoscopy

60−69

2-yearly

Genital chlamydia trachomatis

Nucleic acid test (NAT)  (taken by genital swab/urine)

Genitourinary clinic or outreach clinic appointment

Young men and women

Opportunistic

© Dr Murad Ruf and Dr Oliver Morgan 2008