Screening Basic Public Health Concepts
SCREENING: DEFINITION“The PRESUMPTIVE identification of UNRECOGNIZED disease or defect by the application of tests, exams or other procedures which can be applied RAPIDLY to sort out apparently well persons who PROBABLY have a disease from those who PROBABLY do not”* Key Elements: disease/disorder/defect screening test populationIssues in Screening Disease
-Disease/disorder should be an important public health problem High prevalence Serious outcome -Early Detection in asymptomatic (pre-clinical) individuals is possible -Early detection and treatment can affect the course of disease (or affect the public health problem?)Screening Test Concerned with a Functional Definition of Normality versus Abnormality
Screening TestNormal
Abnormal
Test (Screening test)
DiseaseNo disease
total
Positive
80
100
180
Negative
20
800
820
total
100
900
1000
Sensitivity = 80 /100 = 80%
Specificity = 800 / 900 = 89%
A population of 1000 people, of whom 100 have a disease and 900 do not have the disease. A screening test is used to identify the 100 people with disease.
Test result
With diseaseWithout disease
Positive
True positive (TP)
False positive (FP)
False negative (FN)
True negative (TN)
Positive predictive value (+ PV)
The fraction of people with positive tests who actually have the condition.Negative predictive value (-PV)
The fraction of people with negative tests who actually don't have the condition.
Criteria for Evaluating a Screening Test
Validity: provide a good indication of who does and does not have disease -Sensitivity of the test -Specificity of the test Reliability: (precision): gives consistent results when given to same person under the same conditionsYield: Amount of disease detected in the population, relative to the effort -Prevalence of disease/predictive value
Validity of Screening Test (Accuracy) - Sensitivity: Is the test detecting true cases of disease? (Ideal is 100%: 100% of cases are detected) -Specificity: Is the test excluding those without disease? (Ideal is 100%: 100% of non-cases are negative)
True Cases of Glaucoma
Yes No IOP > 22: Yes 50 100 No 50 1900 (total) 100 2000Sensitivity = 50% (50/100) False Negative=50% Specificity = 95% (1900/2000) False Positive=5%
Screening for Glaucoma using IOP
Consider: -The impact of high number of false positives: anxiety, cost of further testing -Importance of not missing a case: seriousness of disease, likelihood of re-screening
Where do we set the cut-off for a screening test?
Reliability (reproducibility)Agreement within and between examiners ________________________________________________
Inter-Observer Agreement in Grading Severity of Cataract
Examiner <1 1-<2 2-<3 3-<4 4 2 <1 10 2 1 0 0 1-<2 1 20 2 0 0 2-<3 0 1 20 1 0 3-<4 0 0 1 10 2 4 0 0 0 2 5 % Agreement = 81.3% Kappa = 0.76
Examiner 1: Grade
Validity versus Reliability of Screening Test
Examiner 1 Examiner 2 Examiner 3
True cases
Good Reliability
Low Validity
Yield from a Screening Test for Disease X Predictive Value
XX
Screening Test
Negatives
Positives
X
X
X
X
Yield from the Screening Test: Predictive Value
Relationship between Sensitivity, Specificity, and Prevalence of Disease Prevalence is low, even a highly specific test will give large numbers of False Positives Predictive Value of a Positive Test (PPV): Likelihood that a person with a positive test has the disease Predictive Value of a Negative Test (NPV): Likelihood that a person with a negative test does not have the diseaseTrue Cases of Glaucoma
Yes No IOP > 22: Yes 50 100 No 50 1900 (total) 100 2000
Specificity = 95% (1900/2000) False Positive=5% Positive Predictive Value =33%
Screening for Glaucoma using IOP
How Good does a Screening Test have to be?
IT DEPENDS-Seriousness of disease, consequences of high false positivity rate: -Rapid HIV test should have >90% sensitivity, 99.9% specificity -Screen for nearsighted children proposes 80% sensitivity, >95% specificity -Pre-natal genetic questionnaire could be 99% sensitive, 80% specific
Screening tests
A test for a particular disease given to patients who have no symptoms Should be an important, morbid health condition Generally cheap Highly sensitive Not too demanding or risky There should be a treatmentScreening tests
Universal screening Screening all individuals of a certain category (e.g. PKU screening in kids) Case finding Screening a small group of individuals based on the presence of risk factors (e.g cancer clusters, family members diagnosed with hereditary disease)Screening tests
Adverse effects Stress and anxiety caused by false positive results Unnecessary radiation/chemical exposure and test discomfort Prolonged knowledge of a disease with no treatment False sense of security over false negative results Overuse of medical resourcesCommonly screened diagnoses
Cancer (Breast, lung, colorectal, prostate, pancreatic, cervical, ovarian, skin, testicular, thyroid) Cardiovascular (AAA, Blood pressure, Lipid disorders, carotid artery stenosis, PAD) Infectious disease (HIV, Hep B/C, STDs, Tuberculosis) Injury and violence (domestic violence, Youth violence/gang activity, seatbelt use) Mental health/substance abuse (Etoh, illicit drugs, tobacco, depression, suicide risk)Endocrine/Metabolism (Diabetes, IDA, obesity, physical activity)MSK –osteoporosisOB/Gyn (Pre-eclampsia, Rh incompatibility, neural tube defects, asymptomatic bacteruria, Down’s syndrome)Pediatrics (PKU, sickle cell disease, visual impairment, lead intoxication, hearing loss, dental caries)