Screening

• Screening is a process of identifying apparently healthy people who may be at increased risk of a disease or a condition

  Screening is a pathway not just a stand alone test

  The screening test is NOT a diagnostic test
• Screening programme First described by Wilson and Jungner in 1968

  Three elements, disease, test and programme

  Disease

  High incidence in a target population

  Serious effect on health

  Treatable or preventable

  Test

  Non-invasive and easily carried out

  Accurate and reliable (high sensitivity, high specificity)

  Programme

  Widespread and equitable availability

  Voluntary participation

  Acceptable to the target population

  Full information and counselling provided
• False positive – an individual referred for further assessment who DOES NOT have the target condition

  False negative  - an individual not referred for further assessment who does have the condition
• Prevalence – number of individuals in a population with the target conditionj
• Specificity – refers to the screens ability to not detect unaffected individuals

  100% specific – all individuals without the target condition do not require further assessment True negatives
• Sensitivity – refers to the screens ability to detect those individuals who do have the target condition

  Perfect screen would be 100% sensitive – all individuals affected detected True positives
• All screening tests behave differently in terms of sensitivity and specificity 
• Prevalence = number of people with the disease or condition at a specific time divided by number of people in the population at risk at a specific time
• Sensitivity and specificity indicate the predictive performance from a population perspective, focusing on what proportions of patients with and without an adverse event are correctly predicted. 
• Positive predictive value (PPV) is the probability that subjects with a positive screening test truly have the disease.

  Negative predictive value (NPV) is the probability that subjects with a negative screening test truly don't have the disease.
• The UK National Screening Committee (UK NSC) defines a personal informed choice as:

  A decision made to accept or decline a screening test based on access to accessible, accurate, evidence-based information covering:

  • the condition being screened for
  • the testing process
  • the risks, limitations, benefits, and uncertainties
  • the potential outcomes and ensuing decisions
• Why ‘Personal’ informed choice?

  • Patients to consider what is right for THEM
  • According to their own unique circumstances
  • Some screening programmes offer clear benefits of participation (for example early diagnosis, or prevention of disease).
  • Others involve much more complex decisions (for example antenatal screening tests which can lead to difficult personal decisions about whether to continue with or end a pregnancy).
  • Screening information should in all cases make it clear that accepting or declining screening are equally valid choices.
• NHS bowel cancer screening programme

  • Detects bowel cancer at an early stage when treatment is more likely to be effective. 
  • also detects polyps, which may develop into cancers
  • Polyps can be removed, reducing the risk of bowel cancer developing. 
  • A screening kit is sent to people aged 60 to 74 every 2 years, Expanding to 50-59
  • Home test kit = faecal immunochemical test (FIT)
  • Tests for the presence of blood
  • The test is completed at home and posted to a laboratory for analysis. 
  • sample of poo with a special sampling stick sent for testing in laboratory.
• NHS Cervical Screening Programme 

  • Prevents cancer by detecting abnormalities of the cervix and referring for potential treatment. 
  • The programme uses liquid based cytology – still sometimes called a smear – to collect samples of cells from the cervix. 
  • These samples are examined in a laboratory to look for the presence of certain types of human papilloma virus (HPV)
  • If positive for HPV - cells examined for changes
  • Colposcopy / additional screening test in 1 year
  • Screening is offered every 3 years to all women aged 25 to 49 and every 5 years to those aged 50 to 64.
• NHS foetal anomaly screening

  • ultrasound in pregnancy - assess chance of baby having Down’s, Edwards’ or Patau’s or abnormalities.
  • first scan 10-14 weeks after conception includes blood test for Down’s, Edwards’ or Patau’s syndrome.  
  • 14-20 week Quadruple test – Down syndrome only
  • A scan for fetal abnormalities takes place around 18 to 21 weeks. (Edward, Patau and 11 other physical conditions including
  • Spina bifida, Cleft lip, congenital heart disease, bilateral renal agenesis
  • allows for further screening (NIPD) and diagnostic tests (CVS and amniocentesis)
• Combined (Down, Edward, Patau)

  • Between 10-14 week
  • Pregnancy associated plasma protein-A (PAPP-A)
  • Free ß-human chorionic gonadotrophin (free ß-hCG) 
  • Nuchal translucency (NT) (scan)
  • 84% detection rate
  • 2.2% false positive rate
• Quadruple (Down)

  • 14-20 weeks
  • alpha-fetoprotein (AFP)
  • total human chorionic gonadotrophin (hCG)
  • unconjugated oestriol (uE3)
  • inhibin-A (inhibin)
  • 80%  detection rate
  • 3.5% false positive rate
• Risk calculated taking into account:

  • Level of markers
  • Maternal age
  • Maternal weight,
  • ethnic group 
  • In Vitro Fertilisation (IVF)
  • Insulin Dependent Diabetes Mellitus (IDDM) 
  • Smoking

  High risk = greater than or equal to 1:150 (combined),

  1 in 200 (quadruple)
• non invasive prenatal diagnosis screening

  • Can be performed from 10 weeks
  • An additional screen after serum screening test (combined or quadruple)– reflex test for pregnancies with between 1 in 2 and 1:150 risk
  • Examines cell free DNA in maternal blood
  • Tests for the presence of extra chromosome 13, 18 and 21

  99% detection rate

  0.1% False positive rate

  STILL A SCREENING TEST 
• Diagnostic test can be offered

  Amniocentesis

  ~15 week of pregnancy

  ~1 % risk of miscarriage

  Chorionic villus sampling (CVS)

  11-14 weeks of pregnancy

  ~1% risk of miscarriage

  Decision can be made whether to continue with pregnancy or not
• NHS newborn blood spot screening

  Screens newborns for 9 rare but serious conditions:

  • phenylketonuria (PKU), congenital hypothyroidism (CH)
  • sickle cell disease (SCD), cystic fibrosis (CF)
  •  medium-chain acyl-CoA dehydrogenase deficiency (MCADD)
  • maple syrup urine disease (MSUD), isovaleric acidaemia (IVA)
  • glutaric aciduria type 1 (GA1), homocystinuria (HCU)

  Uses heel prick test to collect spots of blood.

  Babies who test positive can be treated early, improving their health and, in some cases, preventing severe disability or even death.