SH3/Lectures/Who should be screened

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Contents

What is screening?

  • “Testing for disease in people without symptoms within a target population”
    • Eg. no need for Faecal-occult blood test for a patient presenting with rectal bleeding as they've already presented with the symptom
  • “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, 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”
  • Screening involves applying a test to a target population
  • Target population based on prevalence of disease in the community (sometimes we screen for rare diseases, particularly in the newborn e.g. PKU)
  • A positive screening test identifies those who are at increased risk of the disease and require further investigation
  • The aim is to reduce the morbidity and mortality of various diseases by intervening early – secondary prevention

Levels of Prevention

  • Primary - no disease, no illness
  • Secondary - disease, but no illness
  • Tertiary - disease and illness

Disease = diagnosis, pathological process (not always symptomatic) Illness = subjective experience felt by patient (ie symptomatic)

Australian programs

  • BreastScreen
  • National Bowel Cancer
  • NSW Cervical Screening Program

Criteria for effective screening

  • World Health Organisation (WHO) criteria 1968
  • Adapted by national bodies worldwide
  • Population Based Screening Framework developed by Australian government to guide decisions about adopting screening programs
  • RACGP have developed recommendations based on WHO and United Kingdom National Health Services’ guidelines

Screening criteria

  • The condition
    • should be an important health problem (either common or rare+serious)
    • should have 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
    • should be simple, safe, precise and validated
    • should be acceptable to the target population
    • the distribution of test values in the target population should be known and a suitable cut-off level defined and agreed
    • receiver operating curve
  • Treatment
    • there should be an effective treatment for patients identified, with evidence that early treatment leads to better outcomes
    • there should be an agreed policy on who should be treated and how
    • facilities for diagnosis and treatment should be available
  • Outcome
    • there should be evidence of improved mortality, morbidity or quality of life as a result of screening and that the benefits of screening outweigh any harm
    • 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
      • cost benefit analysis
  • Consumers
    • Should be informed of the evidence so that they can make an informed choice about participation

Screening tests

  • Ideal test: 100% sensitivity and specificity
  • Real world test: some false positives and false negatives


Condition
(as determined by "Gold standard")
Condition Positive Condition Negative
Test
Outcome
Test
Outcome
Positive
True Positive False Positive
(Type I error)
Positive predictive value =
Σ True Positive
Σ Test Outcome Positive
Test
Outcome
Negative
False Negative
(Type II error)
True Negative Negative predictive value =
Σ True Negative
Σ Test Outcome Negative
Sensitivity =
Σ True Positive
Σ Condition Positive
Specificity =
Σ True Negative
Σ Condition Negative
  • Sensitivity = TP/ (TP+FN)
    • True positive rate
  • Specificity = TN/(TN+FP)
    • True negative rate
  • Creates anxiety for patients if there's a false result

Quality of screening tests Screening tests ideally need to be good at

  • identifying those who have the condition of interest i.e. true positives (high sensitivity)
  • identifying those who do not have the condition of interest i.e. true negatives (high specificity)
  • Minimising the false-positive and false-negative results

No test is perfect therefore there is always a trade-off between sensitivity and specificity

Quiz

  • Case 1: Claire 46 years, married with 2 children. Software engineer IT firm. No significant medical history or family history.
    • What screening tests would you recommend for this woman?
      • Pap smear
      • Ca 125 blood test (for ovarian cancer)
      • Faecal occult blood
      • Mammogram
  • Case 2: Geoff 55 years, married with one son. School teacher. Ex-smoker. Well-controlled asthma. No significant family history.
    • What screening tests would you recommend for this man?
      • Faecal occult blood
      • Chest x-ray (for lung cancer)
      • PSA (prostate specific antigen)
      • Lipids
  • Upshot = memorise requirements for each screening programme

BreastScreen Program

  • National screening program for breast cancer which operates in each state
  • Established 1991
  • Targets women age 50-70 years for 2 yearly mammograms
  • Available to those 40-50 and over 70 years
  • 1.62 million women screened 2005-06
  • 57% of 50-70 year old women participated (Aim of program is to screen 70% of target age)

Breast cancer screening

  • Condition
    • Common malignancy in women (1 in 11 lifetime risk)
    • Early stage identifiable
  • Test
    • Validated test
    • Safe and acceptable to women despite minor discomfort
  • Treatment
    • Effective evidence-based treatment available for early breast cancer
  • Outcome
    • Increased survival rates for early intervention
    • Reduction in mortality benefit of 35%

Further investigations

  • A positive screening test (e.g. abnormal mammogram) will require further investigation
    • FNAB
    • Core biopsy if +ve
  • Patients experience significant anxiety with a positive test which doctors need to bear in mind
  • Appropriate and rapid access to further assessment can help alleviate some of this anxiety
  • In the minds of many people a positive test = cancer, often fuelled by media coverage and celebrity cases
    • Not true: sens and spec aren't perfect

Talking to patients

  • Screening involves targeting healthy people therefore patients need:
    • to be provided with appropriate information about the screening test and its implications for further diagnostic procedures and treatment, so they can make an informed choice about participation
    • to be made aware that screening reduces their risk of developing a disease, it is not a guarantee they will be protected (false negatives happen.)

PSA: To screen or not to screen: the ongoing issue of screening for prostate cancer

  • PSA (usually in combination with DRE) test
  • Currently the following bodies do not recommend routine population based screening for prostate cancer
    • RACGP
    • Urological Society of Australia & New Zealand – USPreventiveServicesTaskForce
    • UKNationalScreeningCommittee
  • Patients may frequently ask for or assume it is being done when they are having other blood tests
  • Some within the medical community and consumer groups advocate for a national screening program
  • Why don’t we routinely screen for prostate cancer? Because...

Prostate cancer screening

  • Condition
    • Common malignancy in men
    • Natural history not well understood - ranges from indolent to aggressive
  • Test
    • Sensitivity and specificity not high
  • Treatment
    • There are effective treatments for early prostate cancer but can have significant side-effects (sexual, urinary function).
    • Given uncertainty about disease course can make treatment choices difficult.
  • Outcome
    • Trials to date have not shown clearly significant
    • Natural history not well understood – ranges from indolent to aggressive improvement in mortality

Evidence from RCTs 2009

  • Note that RR(death) overlaps 1 on meta analysis

SH3Screening1.png

What does this mean for practice?

  • There is a modest but uncertain reduction in prostate cancer death
  • Cannot conclude for or against screening – need longer term follow-up and other outcomes such as treatment side-effects.
  • Still uncertainty about testing intervals, PSA cut-offs and target age range and concerns about over diagnosis
  • Therefore the pros and cons of PSA testing should be discussed with men before undertaking it
  • If PC runs in the family, or they're symptomatic, then you do PSA. But you don't do it across the whole population.

Implementing screening programs

  • Large-scale activity
  • Requires recruitment of patients, facilities and workforce to initially conduct screening and subsequently provide assessment and management of identified cases
  • Quality of the program must be monitored by some type of accreditation process
  • Need to collect data and evaluate program
  • Needs to be cost effective

Participation in screening

  • For screening programs to be successful and cost-effective there needs to be large scale participation of the population
  • Need to include all groups within the targeted population. Certain groups e.g. low socio-economic status, CALD, ATSI, rural communities are sometimes less likely or able to access programs.
  • Ensuring equity of access is an important feature of any program

*In reality: Probably don't have the same health outcomes than higher SES

How do we reach the target population?

  • Screening programs commonly involve people being invited to participate either by media campaigns or by directly sending letters of invitation e.g. people are being sent FOBT kits as part of the National Bowel Cancer Screening Program
    • Reminders (1)
  • The other method of invitation is by case finding – this occurs mainly in general practice
  • Case finding: when doctors opportunistically invite patients (2) to participate in screening when they have presented for other reasons

General practice and screening

  • General practice is the setting where:
    • issues about screening, including the pros and cons of a particular program, are often discussed with patients.
    • screening tests may be performed
    • both systematic invitation and case finding are utilised to implement screening programs
    • systems in the practice such as disease registers and reminder systems are important if screening is to be done effectively

Is there a role for screening?

  • A 36 year old woman presents to her GP to discuss her recent investigations for infertility. Laparoscopy showed scarring of the Fallopian tubes secondary to chronic PID.
  • Could this situation have been prevented?
    • Chlamydia screening (STI) could have helped

Chlamydia screening

  • Condition
    • Prevalence rates not clearly known (range 1-5%). Increase in numbers in last decade.
    • Natural history if untreated not clearly understood
  • Test
    • Urine PCR
    • Simple and acceptable to patients
    • Good quality
  • Treatment
    • Safe and effective treatment (single dose of azithromycin)
    • Reinfection can occur so how often do you screen?
  • Outcome
    • Uncertain how much morbidity will be prevented
    • Do you only screen women?

Current state

  • In Australia public health and sexual health physicians are increasingly calling for a national Chlamydia screening program
  • RACGP guidelines recommend opportunistic screening of sexually active women under 25 years.

Answer case 1

  • (YES)Pap smear
    • Recommended 2 yearly for women who have ever been
    • sexually active
  • (NO) Ca 125 (for ovarian cancer)
    • Evidence does not support screening for general population
  • (NO) Faecal occult blood
    • Recommended 2 yearly age 50 – 75 years
  • (?) Mammogram
    • Recommended 2 yearly 50-70 years. Evidence shows marginal benefit 40-50 years for general population.
    • Need to weigh up against fairly high risk of requiring investigation of abnormal mammogram which will not turn out to be breast cancer (approx 24% of women screened 40-50 years)

Answer case 2

  • Faecal occult blood
    • Recommended 2 yearly age 50 -75 years
  • (NO) Chest x-ray (for lung cancer)
    • Evidence does not support screening for lung cancer
  • (NO) PSA
    • Evidence does not support screening for prostate cancer in general population. May do a PSA test if patient requests it following discussion.
  • (YES) Lipids
    • Recommended for those 45 years and over – frequency depends on other risk factors. Opportunity to assess **absolute cardiovascular risk and screen for diabetes with AUSDRISK tool


Memorise requirements for each screening test