What is screening?

Most people take part in screening in order to be reassured they are healthy, rather than with the expectation they are not. However, screening does not give you a diagnosis.

It does not tell you whether you do have the condition. Instead, screening sorts people into two groups: people who have an increased chance of having the condition (positive screening result) and people who do not have an increased chance of having the condition (negative screening result).

People with a positive screening result will be offered a diagnostic test, which will give more information about whether they do have the condition. People with a negative screening result will not be offered the diagnostic test. Depending on the type of screening they may be invited to come back for screening at a later date (eg, for a repeat cervical smear test).

However, screening does not guarantee that you will not get a condition, that the condition will be found, or that a condition can be treated. It just increases your chances that a condition will be avoided, found early, or be able to be cured.

The benefits of screening

Screening programmes detect some conditions, reduce your chance of developing or dying from some conditions, and can improve your quality of life. In some cases (for example, breast screening), screening may detect cancer at an early stage. In others (such as newborn metabolic screening) screening may find conditions which can be treated before the baby develops a preventable illness or disability. For example:

breast screening reduces your chances of dying from breast cancer by about 30 percent if you are between 50 and 65, and by about 45 percent if you are between 65 to 69
cervical screening reduces your chances of developing cervical cancer by about 90 percent
newborn hearing screening picks up hearing loss in babies and young children at an early stage. This means help such as resources, parent support groups, hearing aids, cochlear implants and the introduction of sign language can be offered as soon as possible
screening pregnant women to see if they have HIV and offering treatment will reduce the number of babies born with HIV or getting HIV through breastfeeding. Screening could prevent several babies from getting HIV in New Zealand each year
newborn metabolic screening enables certain conditions that can be harmful to babies to be picked up and treated early, reducing illness and sometimes death.

Does screening have risks?

Screening makes it more likely that conditions will be identified so early action can be taken. These are the types of screening programmes offered by the National Screening Unit.

However, all screening has the potential to cause harm. Screening tests carry a risk of false results.

A false negative result is when a person who does have the condition is put into the group of people who have a lower chance of having the condition. False negatives can provide false reassurance.

A false positive is when a person who does not have the condition is put into the group of people who have an increased chance of having the condition. False positives can result in increased anxiety and referral to diagnostic tests.

There is ongoing debate, both in New Zealand and overseas, about whether screening should be offered for some cancers, including for prostate cancer. There is evidence that the risks of screening for some cancers may outweigh the benefits.

For example, through screening you may find out that you have a certain condition, and get treatment. However, the condition may not be harmful, and you will have gone through unpleasant treatment for no reason. In this case, it may have been better never to have known you had the condition. Or if the condition is harmful, there may be no effective treatment for it. In this case you will not live longer because the condition was found, you will just know for longer that you have the condition.