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Factsheet: What is Screening?


In the medical world, screening is used to help detect a disease in well individuals (i.e. people who have no signs or symptoms of that disease).  The purpose of screening is to identify disease in a community early, thus enabling earlier intervention and treatment. The hope is that a good screening test or programme will lead to earlier diagnosis, and therefore reduced suffering and mortality.

What are the benefits of screening?
1. Screening can detect conditions at an early stage - before there are any symptoms present (e.g. cervical screening programme);
2. Screening may detect a disease at a stage when treatment is more effective (e.g. the newborn metabolic screening programme);
3. In the best situation, screening can save lives (e.g. breast screening).

What are the problems with screening?
There can be disadvantages to screening, so it is important to understand these before undertaking a screening test:
1. No test is perfect and screening tests do not give you a diagnosis - they simply put you into a group of people with either an increased or decreased risk of having that disease or condition; people at increased risk are then offered further testing to reach a diagnosis;
2. Screening tests may cause anxiety and discomfort (e.g. a blood test, and then an anxious wait until you get the result);
3. Risk of a "false negative" result - a person who does actually have the condition is put into the group of people with a lower chance of having the condition; this provides false reassurance and may delay treatment;
4. Risk of a "false positive" result - a person who does not have the condition is put into the group of people with a higher chance of having the condition; this leads to unneeded anxiety and referral for further diagnostic testing (which can be more invasive).

Because of these potential disadvantages, screening principles need to be adhered to before a programme will be offered. Some of the World Health Organisations principles of screening are:
1. The condition needs to 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 (i.e. a stage where it progresses slowly or not at all, during which treatment could be effective);
5. Tests for the condition should be acceptable to the population;
6. The natural history of the disease needs to be understood;
7. The costs of finding a case should not be prohibitive in relation to medical expenditure as a whole.

In New Zealand, the National Screening Unit is responsible for health screening programmes - specifically for the safety, effectiveness and quality of these programmes. There are currently 5 national programmes that the NSU looks after:
1. BreastScreen Aotearoa - screens women for breast cancer
2. National Cervical Screening Programme - screens women for abnormal smear tests
3. Newborn Metabolic Screening Programme - screens newborn babies for certain metabolic disorders
4. Antenatal HIV screening programme - screens pregnant women for HIV
5. Universal Newborn Hearing Programme - screens newborn babies for hearing loss.
The NSU is also responsible for the recent improvements to the antenatal screening for Downs syndrome in pregnant women.

As well as these universal screening programmes, there are other types of screening that you may be offered:
1. Case finding - this involves screening a smaller group of people who are known to be at higher risk of a certain disease or condition. This would include screening close family members for certain cancers (e.g. breast, bowel, ovarian) or some potentially inherited conditions (e.g. cystic fibrosis).
2. Opportunistic - this involves screening individuals for risk factors when they present with possibly unrelated issues. Examples of this type of screening might include discussing prostate cancer screening, testing for cholesterol, or completing a questionnaire about depression during a routine health check. This is common practice in New Zealand.

For more information about screening, and more details about national screening programmes within New Zealand, visit www.nsu.govt.nz

(TX: 18 July 2011)


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