Screening Flashcards
Screening what and why
Screening is defined by the World Health Organization as the presumptive identification of unrecognized disease in an apparently healthy, asymptomatic population by means of tests, examinations or other procedures that can be applied rapidly and easily to the target population (1)
The aim of screening is to identify a disease before symptoms develop, so that it can be treated early. This is in contrast to diagnostic testing where a test is done to a patient with symptoms to confirm or exclude a medical condition.
List types of screening
1. Population screening:
This is where a screening test is offered systematically to all individuals in a defined target group within a framework of agreed policy, protocols, quality management, monitoring, evaluation and review (2).
2. Opportunistic screening:
This is when a test is offered to an individual opportunistically, when they present with symptoms that are unrelated to the disease. For example, testing for cholesterol in a middle aged man who presents with a sprained back. Planned opportunistic screening is when tests are offered in well-established patterns, such as school-based screening.
3. Targeted screening:
This is when a test is offered to selected high-risk groups. For example, ordering a Chest X Ray for someone who has occupational exposure to asbestos.
How to choose a disease for screening
The condition:
- The condition should be an important health problem
- There should be a recognisable latent or early symptomatic stage
- The natural history of the condition, including development from latent to declared disease should be adequately understood.
The test:
- There should be a suitable test or examination
- The test should be acceptable to the population
Treatment:
- There should be an accepted treatment for patients with recognised disease
The program:
- There should be an agreed policy on whom to treat as patients
- Facilities for diagnosis and treatment should be available
- The cost of case finding should be economically balanced in relation to possible expenditure on medical care as a whole
- Case finding should be a continuing process and not a ‘once and for all’ project
Describe how screening works
Recruitment
The process of screening starts with defining and recruiting a target population. The target group is the group that is gains the most benefit from screening and are encouraged to participate in the screening program. For example, the National Breast Screening program actively targets women aged between 50-74 years of age.
Screening
A proportion of the identified target group will choose to participate in the screening program and have the test done. Some of the participants will have a negative test and some will have a positive test. Those that have a negative test are usually asked to participate in another round of screening after a defined time period.
Assessment
Those who have a positive test will need a further assessment to confirm or exclude the condition that is being screened for.
Diagnosis
Further assessment may lead to early diagnosis of the condition that has been screened for. Diagnosis will generally trigger a management pathway with the ultimate outcome of reducing the mortality and morbidity from that disease
List screening programs in Au
In this section, we will take a look at 4 screening programs:
- National Bowel Cancer Screening Program
- National Cervical Cancer Screening Program
- Breast Cancer screening program
- The Newborn screening program
Describe bowel screening, cervical and breast screenign
The National Bowel Cancer Screening Program (NBCSP) was established in 2006.
Target group/Who is eligible for the program:
As of 1st July 2024,the following are eligible to do the screening testevery two years if they are
- arebetween 45* and 74
- have a Medicare card and entitlement type of either Australian citizen, permanent migrant, or registeredas a Department of Veteran Affairs customer
- have an Australian mailing address
*Before 1st July 2024, the program targeted men and women aged 50 to 74 every two years.Since__1 July 2024, the NBCSP has lowered the participation age to45-49.They can also request to join the bowel cancer screening program early by requesting alternative access to their firstscreening kit by asking their health care provider_or_by filling out a web form ( clickhereto see form).
- Once a person in the 45-49 age band requests their FIRST screening test, they will be in the NBCS program and automatically get regular screening kits every two years.
- However, if people aged 45-49 don’t ask for a screening kit_before_they turn 50, the cancer screening program will automaticallysend them onewhen they do.
- The screening test is an immunochemical faecal occult blood test (iFOBT)
Cervical Cancer
The National Cervical Screening Program (NCSP) was established in 1991. Until December 2017, the program targetted women aged between 18-69 years of age for a 2 yearly Papanicolaou smear (Pap test). The Pap test looks for cell changes in the cervix. For every 1000 women screened, about 7 have a high-grade abnormality detected.
In December 2017, the more effective - Cervical screening test replaced the Pap test. The Cervical screening test detects Human Papilloma Virus (HPV).
Target group: Women aged 25 to 74 years of age, repeated every 5 years.
Breast Cancer
BreastScreen Australia, established in 1991, provides 2 yearly screening mammograms to women aged 40 and over.
Describe NBS
The newborn bloodspot screening program tests newborn babies for a number of treatable genetic conditions. This includes about 25 conditions such as Phenylketonuria, Hypothyroidism, Cystic Fibrosis and Galactosemia. Screening involves testing a few drops of blood taken from a heel-prick. This program, which has existed for about 50 years, is funded by State and Territory Governments and there are some variations in the conditions screened by each state and territory.
Describe how to assess a screening program
No screening test is perfect. A test can be positive even though a person does not have a disease or negative when a person has a disease.
If a test is done on a person who has a disease, in simple terms, there can be one of two outcomes – It can be positive or it can be negative. If it is positive, it means that it has correctly picked up the disease – A true positive (TP). If however, the test is negative, it means that it has not picked up the disease – A false negative (FN).
Similarly, if you do a test on a person without the disease and the test turns out to be negative (which is what it is supposed to be), it would be a true negative (TN). If on the other hand, it turns out to be positive, this would be a false positive (FP).
Sensitivity
Sensitivity is a proportion of people with the disease who test positive for it.
Sensitivity = TP / (TP+FN)
A high sensitivity means that the proportion of True Positives is high and the proportion of false negatives is low.
Specificity
Specificity is the proportion of people without the disease who test negative for it.
Specificity = TN / (FP + TN)
A high specificity means that the proportion of True Negatives is high and the proportion of false positives is low
A good screening test will have a high sensitivity and a high specificity.
Positive Predictive Value
The Positive Predictive Value is the proportion of people who have a positive test who actually have the disease.
Positive predictive Value = TP / (TP+FP)
If the positive predictive value is high, it means that if a test is positive, the proportions of true positives are high (or in other words, the proportion of false positives are low)
The Negative Predictive Value
The Negative Predictive Value is the proportion of people who have a negative test who do not have the disease.
Negative Predictive Value = TN / (TN + FN)
If the negative predictive value is high, then it means that if a test is negative, the proportions of true negatives are high (or in other words, the proportion of false negatives are low).
The Positive Predictive Value and Negative Predictive Value are influenced by the prevalence of disease in the population.
The perfect test will have a sensitivity of 100% and a specificity of 100%. However, in reality most screening tests do not. For example, the sensitivity of a Pap Smear is about 51% (Range 30-87%) and specificity 98% (range 86% - 100%)
Discuss sources of bias in screening
1. Healthy Volunteer Bias (Self Selection Bias)
This describes the bias in which people who participate in screening programs are different to those that don’t. For example, people who participate in screening tend to be healthier, have a higher socioeconomic status and are more receptive to health messages compared to those who don’t (7). Due to this reason alone, they may live longer or have a better outcome, therefore overestimating the benefits of screening
With regards to Breast screening in Australia, the overall participation rate is about 55% of the target population. Participation is lower in Aboriginal and Torres Strait Islander women (37% of the Indigenous population), 47% for women living in very remote areas and 48% for women from a culturally and linguistically diverse background (5).
2. Lead Time Bias
The aim of screening is to diagnose a disease before the onset of symptoms (A) and before a diagnosis would otherwise have been made (B). Lead-time is the time between the detection of disease by screening and the detection of the disease if screening had not occurred (Figure 1)
Figure 1 - Lead time bias
Even if screening has made no difference to mortality, it may appear that it was due to the lead-time. This is called lead-time bias and leads to an overestimate of the benefits of a screening program. Easy recall prompt: lead time is due to early detection of a disease which gives the sense that survival outcomes have shifted when in fact time to death hasn’t changed.
3. Length Time Bias
Diseases progress at different rates either due to the differences in disease or individuals. Length time is the amount of time that it takes for a disease to progress to a given end-point such as death. Because slow-growing (or less aggressive diseases) tend to have a long asymptomatic phase, they are more likely to be picked up by a screening program. For these diseases, screening may appear to increase the length of survival and therefore overestimate their benefit. Easy recall prompt: length time is due to slow the progression of diseases which gives us more opportunities to detect the disease in the slow progression phase. However,earlier detection of disease through screening does not necessarily increase the survival period.
Discuss harms of screening
Screening saves lives and through early detection, screening can reduce morbidity and mortality. For example, an evaluation of the National Bowel Cancer Screening Program (NBCSP) using modeling estimated that with current levels of participation, over the period 2015-2040, it is expected to prevent 93,200 cases of cancer and 59,000 deaths (7).
Although screening has many benefits, there can be a harms associated with screening. This can be due to:
False Positives
This is when the person being screened does not have the disease but the screening test is positive. False positive tests can lead to a number of negative outcomes (7):
- Negative psychological consequences – such as depression, anxiety and changes in the overall perception of a person’s health status
- More testing – A positive screening test can lead to a range of potentially invasive tests, each with risk and harms associated with them
- Economic consequences – Follow up testing can result in economic burdens not just for the individual but for the health system as well.
False negatives
People who have a false negative test may experience delays in diagnosis and treatment. They may develop a false sense of security, continue/increase risky behaviour or ignore warning symptoms. False negative results may also lead to legal action for missed diagnoses.
Over-diagnosis
Labelling a person with an illness can have a negative impact on their quality of life and wellbeing. In addition to this, radical treatment for an otherwise indolent disease may lead to complications and risks associated with the treatment itself.
