Screening

• Screening is a major public health activity geared towards detecting disease before symptoms appear
• Screening does NOT diagnose disease - persons tested positive from a screening test must be evaluated further for the disease
• Screening test


  Usually done for individuals who are asymptomatic, applied to a group of individuals, results based on one criterion, less accurate, less expensive, not a basis for treatment
• Diagnostic test


  Usually done for individuals with signs or symptoms, applied to individuals, results based on evaluation of signs, symptoms and investigations, more accurate, more expensive, basis for treatment
• Why screen?


  • To prevent or delay development of advanced disease, early detection, prompt treatment, assumption - early detection leads to a more favourable prognosis
• Why screen?


  • Objective is to apply relatively inexpensive test to a large number of persons in order to classify them as likely having or likely not having the particular disease, desired outcome is a reduction in morbidity and mortality for the particular disease at a reasonable cost
• Why screen?


  • Identify diseased vs non-diseased persons, estimate prevalence or incidence of a disease
• Types of screening (approaches)

  1. Case finding

  2. Mass screening

  3. Multiphasic screening
• Considerations for screening

  • Important health problem, who should be screened, when to screen, risk to patient, reliability, validity and cost effectiveness of test, will early detection and treatment alter the course of the disease, facilities for diagnosis and treatment available
• Reliability


  Test result remains consistent when repeated to same person and under similar conditions
• Validity


  Ability of screening test to accurately identify diseased and non-diseased individuals (measures what its intended to measure) exquisitely specific and sensitive
• Sensitivity


  Ability of test to identify correctly those who HAVE the disease (true positives)
• Specificity


  Ability of test to identify correctly those who DO NOT HAVE the disease (true negatives)
• Predictive value


  Helps determine meaning of a positive or negative test result for a patient
• No test is 100% sensitive or 100% specific
• If prevalence is low


  It increases the number of false positives even if specificity is high
• False Negatives


  Negative results that are actually positive
• True Negatives


  Negative results that are actually negative
• Results


  • Negative
  • False Negatives
  • True Negatives
  • Positive
• Prevalence Formula: [a+c] / [a+b+c+d] x 100
• How Prevalence Affects Error Rate

  • A low prevalence increases the number of false positives
  • A high prevalence decreases the number of false positives
• • Community A - population 1000
  • Prevalence of DM - 5%
  • Screening test for DM - 90% sensitivity and 80% specificity
• • Community B - population 1000
  • Prevalence of DM - 20%
  • Screening test for DM - 90% sensitivity and 80% specificity
• Positive Predictive Value (PPV)


  Measures probability that a person indeed has a disease when tested positive
• Negative Predictive Value (NPV)


  Measures probability that a person does not have a disease when tested negative
• Clinical breast examination (CBE) was used to screen for breast cancer in 2500 women with biopsy proven adenocarcinoma of the breast and in 5000 age and race matched control women
• The results from the CBE were positive (mass palpated) in 1800 cases and in 800 control women, all of whom showed no evidence of cancer on biopsy
• Mantoux test and sputum culture tests were done on 200 persons suspected of having tuberculosis. The sputum culture is the Gold Standard test.
• The Mantoux test was positive for 50 persons and negative for 80 persons.
• A total of 110 persons had positive sputum culture
• Characteristics of a Good Screening Test

  • Safe
  • Acceptable
  • Valid
  • Economical
• Issues Regarding Screening Tests

  • False sense of security (missing diseased person) – false negative results
  • Anxiety – false positive results
  • Re-screening based on significance of disease and not wanting to miss a case
  • Cost for further testing or treatment of non-existent disease
  • Risk associated with screening test
• Some common screening tests


  • Blood pressure check
  • Blood sugar test
  • Pap smear
  • Breast self examination vs mammogram
  • Prostate examination (DRE & PSA)
  • Colonoscopy vs stool for occult blood
• What level of prevention is screening?

  Secondary/ stage of subclinical disease
• Why screenning?

  Prevent or delay development of advanced disease
• Types of screning 


  Case finding, mass screening, multiphasic screening
• Case finding 


  clinician conduct routine checks for disease unrelated to persons's visit
• case finding example 


  mamogram requested for old woman being followed up for diabetes
• Mass screening 


  selective/unselected screening in a high risk population
• Mass screening example 


  health fairs, sex workers for HIV, disease outbreak