1 Comunicación y Gerencia 18/4/2011Dr Salwa Tayel (Screening) بسم الله الرحمن الرحيم

2 Screening Associate Professor Family and Community Medicine Department King Saud University 18/4/2011 Dr. Salwa Tayel Dr Salwa Tayel (Screening)

Learning Objectives: 1.D efine screening and mention its purpose. 2.L ist WHO criteria for effective screening 3.C alculate and interpret measures of the validity of a screening test: ---Sensitivity ---Specificity 4.C alculate and interpret measures of the performance (yield) of a screening test: ---Predictive value positive (PV+) --- Predictive value negative (PV-) 318/4/2011Dr Salwa Tayel (Screening)

Screening l Screening: The application of a disease- detection test in asymptomatic apparently healthy individuals. l Purpose: To classify individuals with respect to their likelihood of having a particular disease. l Screening procedure itself does NOT formally diagnose illness. 418/4/2011Dr Salwa Tayel (Screening)

5 Screening Vs. diagnosis Tests  The purpose of screening is to segregate those who may have the condition. It is an initial step, as it needs further confirmation.  On the other hand, diagnosis is a procedure to confirm or disprove the existence of a disease or abnormality.  Both screening and diagnosis can be achieved by obtaining medical history, performing physical examination and the application of laboratory or non-laboratory test. 18/4/2011Dr Salwa Tayel (Screening)

Comparison between screening and diagnostic tests Diagnostic tests Screening tests Done to those with suggestive signs or symptoms Done to those who are apparently healthy or asymptomatic Applied to a single person Applied to a group of individuals Results are based on the evaluation of a number of symptoms, signs and investigations Results are based on one criterion Results are conclusive and finalResults are not conclusive More accurateLess accurate More expensiveLess expensive Basis for treatmentNot a basis for treatment 6 18/4/2011 Dr Salwa Tayel (Screening)

7 Types of screening programs  Mass screening Application of the screening program to the whole population or population subgroups as adults, school children, industrial workers.  High risk or selective screening The screening program will be applied to a selective group of population who are at a high risk e.g. breast cancer screening to females above 40 years. 18/4/2011Dr Salwa Tayel (Screening)

8 Early Intervention in the Natural History of Disease HEALTH OUTCOMES Cure Control Disability Death Disease Onset SymptomsDiagnosisTherapy Care Seeking Good Health Early detection through Screening 18/4/2011Dr Salwa Tayel (Screening)

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10 Flow diagram for a screening program Population Test -veTest +ve UnaffectedAffected Intervention Diagnostic procedures Screening test Re-screen 18/4/2011Dr Salwa Tayel (Screening)

Screening for Disease Control Screening Objective: To lower morbidity and mortality of the disease in a population (control, rather than elimination of disease). 1118/4/2011Dr Salwa Tayel (Screening)

WHO criteria for effective screening: 1)The disease should be important public health problem (relates to cost effectiveness, and prognosis). 2)There should be an effective and acceptable treatment for the condition if identified in an early stage. 3)Facilities for the confirmation of the diagnosis and treatment should be available. 1218/4/2011Dr Salwa Tayel (Screening)

WHO criteria for effective screening: 4)There should be a latent stage of the disease (long and detectable pre-symptomatic stage). 4) There should be a latent stage of the disease (long and detectable pre-symptomatic stage). 5) There should be a suitable screening test or examination that can detect the condition 6) The test should be acceptable to the population. 1318/4/2011Dr Salwa Tayel (Screening)

7) Natural history of disease should be adequately understood. 8) There should be an agreed upon policy on whom to treat. 1418/4/2011Dr Salwa Tayel (Screening)

9) The total cost of finding a case should be economically balanced in relation to medical expenditure as a whole. 10) Case finding should be a continuous process, not just a “once and for all” project. 1518/4/2011Dr Salwa Tayel (Screening)

Diseases for which screening has been recommended Cervical cancer l Breast cancer l Ovarian cancer l Colorectal cancer l Skin cancer l Diabetes l Hypertension 1618/4/2011Dr Salwa Tayel (Screening)

Characteristics of a screening test: Validity (Sensitivity, Specificity) Reliability (repeatability/precision) Yield (performance): Predictive values of the test. 1718/4/2011Dr Salwa Tayel (Screening)

Validity of Screening Tests How good is the screening test compared with the confirmatory diagnostic test (Gold Standard test)? l Gold Standard: A method, procedure, or measurement that is widely accepted as being the best available. Often used to compare with new methods of unknown effectiveness. (Dictionary of Epidemiology) 1818/4/2011Dr Salwa Tayel (Screening)

19 Results of screening test compared to gold standard Total Gold standard Screening test NegativePositive PS(FP)(TP)Positive NS(TN)(FN)Negative GTTHTDTotal 18/4/2011

Validity of Screening Tests a d c b True Disease Status + - Results of Screening Test Positive test + a = true positive (Agreement) b = false positive (Disagreement) c = false negative (Disagreement) d = true negative (Agreement) 2018/4/2011Dr Salwa Tayel (Screening) Negative test -

Validity of Screening Tests a d c b True Disease Status + - Results of Screening Test + - Sensitivity: The probability of testing positive if the disease is truly present Sensitivity = a / (a + c) 2118/4/2011Dr Salwa Tayel (Screening)

Validity of Screening Tests a d c b True Disease Status + - Results of Screening Test + - Specificity: The probability of screening negative if the disease is truly absent Specificity = d / (b + d) 2218/4/2011Dr Salwa Tayel (Screening)

Validity of Screening Tests True Disease Status + - Results of Screening Test Positive test + a = true positive (Agreement) b = false positive (Disagreement) c = false negative (Disagreement) d = true negative (Agreement) 2318/4/2011Dr Salwa Tayel (Screening) Negative test -

24 Results of screening 100 men for prostate cancer using (PSA) Total Gold standard (Prostatic biopsy) Screening test (PSA) No cancer Cancer 10 7 (FP) 3 (TP) Positive (TN) 2 (FN) Negative Total 18/4/2011

Sensitivity: a / (a + c) Sensitivity = 90% Specificity: d / (b + d) Specificity = 95% Prevalence of disease =(a+c)/(a+b+c+d) =100/200=50% 2518/4/2011Dr Salwa Tayel (Screening)

26 Number of false positives=bNumber of false positives=b False Positive Rate=b/b+dFalse Positive Rate=b/b+d Number of false negatives=cNumber of false negatives=c False Negative Rate=c/a+cFalse Negative Rate=c/a+c 18/4/2011Dr Salwa Tayel (Screening)

27 Sensitivity: a / (a + c) = 90% Specificity: d / (b + d) 95% False Positive Rate=b/b+d=5/100=5% False negative Rate=c/a+c=10/100=10% Prevalence of disease = (a+c)/ (a+b+c+d) 100/200= 50% 18/4/2011Dr Salwa Tayel (Screening)

28 Adverse effects of screening  Stress and anxiety caused by a false positive screening results.  Unnecessary investigation and treatment of false positive results  Prolonging knowledge of an illness if nothing can be done about it.  A false sense of security caused by false negatives, which may even delay final diagnosis.  Overuse/waste of medical resources. 18/4/2011Dr Salwa Tayel (Screening)

Reliability of Screening Tests Reproducibility RELIABILITY (Reproducibility) Precision: The extent to which the screening test will produce the same or very similar results each time it is administered (repeated). It can be assessed by repeating the test using the same or different observers. --- A test must be reliable before it can be valid. 2918/4/2011Dr Salwa Tayel (Screening)

Reliability of Screening Tests Sources of variability that can affect the reproducibility of results of a screening test: 1. Biological variation (e.g. blood pressure) 2. Reliability of the instrument itself 3. Intra-observer variability (differences in repeated measurement by the same screener) 4.Inter-observer variability (inconsistency in the way different screeners apply or interpret test results) 3018/4/2011Dr Salwa Tayel (Screening)

Yield (Performance) It is measured by: l Predictive Value Positive (PV+) l Predictive Value Negative (PV-) 3118/4/2011Dr Salwa Tayel (Screening)

Yield a d c b True Disease Status + - Results of Screening Test + - Predictive value positive (PV+): The probability that a person actually has the disease given that he or she tests positive. i.e. The ability to predict the presence of disease from test results.PV+ = a / (a + b) 3218/4/2011Dr Salwa Tayel (Screening)

Yield a d c b True Disease Status + - Results of Screening Test + - Predictive value negative (PV-): The probability that a person is truly disease free given that he or she tests negative. i.e. The ability to predict the absence of disease from test results. PV- = d / (c + d) 3318/4/2011Dr Salwa Tayel (Screening)

Calculate: PV+ =19/118=16% PV-= 1881/1882= /4/2011Dr Salwa Tayel (Screening)

Calculate: PV+=57/59=96.6% PV-=38/41=93% useful test 3518/4/2011Dr Salwa Tayel (Screening)

Sensitivity: a / (a + c)= 19/20 Sensitivity = 95% Specificity: d / (b + d)= 1881/1980 Specificity =95% Prevalence=20/2000*100 Prevalence=1% 3618/4/2011Dr Salwa Tayel (Screening)

Sensitivity: a / (a + c)= 57/60 Sensitivity = 95% Specificity: d / (b + d)= 38/40Specificity =95% Prevalence= 60/100*100Prevalence=60% 3718/4/2011Dr Salwa Tayel (Screening)

Prevalence (%) Sensitivity Specificity PV % 95% 1.8% 1.090% 95% 15.4% 5.090% 95% 48.6% % 95% 94.7% Factors affecting the yield of a screening test 3818/4/2011Dr Salwa Tayel (Screening)

Sensitivity:Specificity:Prevalence: The higher the prevalence of preclinical disease in the screened population, the higher the PV+. The higher the prevalence of preclinical disease in the screened population, the higher the PV+. PV+ is maximized when used in “high risk” populations since the prevalence of pre-clinical disease is higher than in the general population. PV+ is maximized when used in “high risk” populations since the prevalence of pre-clinical disease is higher than in the general population. Screening a total population for a relatively infrequent disease can be very wasteful of resources and may yield few previously undetected cases. Screening a total population for a relatively infrequent disease can be very wasteful of resources and may yield few previously undetected cases. Factors affecting the yield of a screening test 3918/4/2011Dr Salwa Tayel (Screening)

Characteristics of a suitable screening test: Validity – the extent to which the test distinguishes between persons with and without the disease: High validity requires: High Sensitivity High Specificity Reliability (High) Performance (Yield) Low cost, invasiveness, and discomfort Costs. 1. Costs of applying the test itself. 2. Costs of performing additional tests on people with false positives, in order to correct the test’s mistakes 4018/4/2011Dr Salwa Tayel (Screening)

41 Calculate: Sensitivity: Specificity: PV+: PV-: False Positive rate False Negative rate Disease prevalence 18/4/2011Dr Salwa Tayel (Screening)

Comparison of mammography results with findings from surgical excisional biopsies in women without palpable breast masses Total Gold standard (Surgical biopsy) Screening test (Mammography) No cancer Cancer Positive Negative Total 18/4/ Dr Salwa Tayel (Screening)

43 Thank you End of the lecture Bibliotheca Alexandrina 18/4/2011 Website