Screening 3 PDF

Screening Dr. SIJAL FADHIL F.I.C.M.S M.Sc. M.B.Ch.B.  Objectives 1-To introduce students to the main concept of screening. 2-To explain the differences between screening and diagnostic test. 3-To explain criteria of disease to be screened and criteria of screening test. 4-To ou...

Screening Dr. SIJAL FADHIL F.I.C.M.S M.Sc. M.B.Ch.B.  Objectives 1-To introduce students to the main concept of screening. 2-To explain the differences between screening and diagnostic test. 3-To explain criteria of disease to be screened and criteria of screening test. 4-To outline the sensitivity, specificity, predictive value positive and predictive value negative. 5-to outline types of screening Definition – Is the search for unrecognized disease by means of rapidly applied tests or examinations in apparently healthy individuals. – The intention of screening is to identify disease in a community early ,thus enabling earlier intervention and management in hope to reduce mortality and suffering from a disease. Screening test – in general: Is a test for a particular disease given to asymptomatic people. – What are the differences between screening test and diagnostic test?? Possible outcome of screening Uses of screening –1-Case detection: –2-Control of disease: –3-Research purposes: –4-Educational opportunities:  Types of screening A: Mass or population screening: involves the screening of a whole population with no reference to high-risk group: A-Single disease screening: cervical screening. B-Multiphasic screening: biochemical profiles on hospital patients. B-Selective (targeted) screening: involves identifying members of the population at risk, i.e. test for disease in high risk group: A-Single disease screening: CXR for pneumoconiosis. B-Multiphasic screening: antenatal examinations. Opportunistic screening: involves taking the opportunity to administer a screening test when the contact with the individual or group is not primarily for screening purposes, e.g. during a medical consultation when the individual was attending for an unrelated health problem. Examples on screening :programs #Childhood anemia screening programs are considered cost effective for targeted populations #Cervical cancer screening programs are considered cost effective for targeted populations Criteria for screening The criteria for screening are based on two considerations: 1-Disease to be screened: 2-Screeing test to be applied: Disease to be screened -1 It should fulfill the following criteria: 1-The condition sought should be an important health problem(prevalence should be high ). 2-There should be a recognizable latent or early asymptomatic stage. 3-The natural history of the condition including development from latent to declared disease should be adequately understood. 4-There is a test that can detect the disease prior to the onset of signs and symptoms. 5-Facilities should be available for confirmation of the diagnosis. 6-There is an effective treatment. 7-there should be an agreed-on policy concerning whom to treat as patients (e.g border line diabetes) 8-There is a good evidence that early detection and treatment reduces morbidity and mortality. 9-The expected benefits of early detection exceed the risks and costs. screening test to be-2 applied 1-Acceptability:The test should be acceptable to the people at whom it aimed 2-repeatability(reliability): It means the test must give consistent results when it repeated more than once on the same individual under same conditions It means the results of the test are precise(exact),so repeatability is some time called precision, reliability or reproducibility It depends on: 1- observer variation : A-intra-observer variation(within observer): same observer taking 2 or more readings give varied results. B-Inter-observer variation (between -observer) : variation between different observers on same subject/material 2-Biological (subject) variation: it occurs due to: A-changes in parameters observed. B-variation in perceptions and answers of patients. C-Regression to the mean. 3-Error relating to technical methods: A-defective instrument B- erroneous calibration C-Faulty reagents D-inappropriate/unreliable test 3-validity(accuracy):it refers to what extent the test accurately measures which is suppose to measures, that means a valid test distinguish the people who have the disease From those who do not. It has 2 components: Sensitivity and specificity 4-Others(simplicity,safety,rapidity,low cost,and ease of administration)  Diagnosis Disease Not diseased positive TP (true positive) FP (false positive) Screening test negative FN (false negative) TN (true negative) Measures of validity of screening test Sensitivity: The ability of the test to identify correctly all those who have the disease, that is (true positive). SN = [TP / (TP +FN)] x100% = TP% …….the denominator represents diseased people. SN(TP%) = a/a+c x100% Specificity: The ability of the test to identify correctly those who do not have the disease (true negative) SP = [TN / (TN +FP)] x100 %= TN% … the denominator represents non-diseased people. SP(TN%) =d/b+d x100% Predictive value In addition to SN and SP ,the performance of screening test is measured by the predictive value which reflect the diagnostic power of the test. The predictive value measures whether or not an individual actually has the disease given the results of the screening test Predictive value of the positive) :(test PVP It is the percentage of truly diseased people among those who show positive test results PVP = [TP / (TP + FP)] x100 % PVP= a/a+b x100% Predictive value of the negative) :(test PVN It is percentage of healthy people among those who show negative test results. PVN = [TN / (TN + FN)] x100% PVN= d/c+d x100% 1- TP is directly related to SN FN is inversely related to SN TN is directly related to SP FP is inversely related to SP 2-PVP depends on: SN , SP , prevalence of disease FP %= [FP / (FP + TN)]x100% FP%=b/b+d x100% FP%= 1-SP FN % = [FN / (TP + FN)] x100 % FN%=c/a+c x100% FN%=1– SN  Criterion Standard Test Screening Disease (+) Disease (-) test Test (+) True positive rate (TP%) or (SN) False positive rate (FP%) or (1-SP) Test (-) False negative rate (FN%) or (1-SN) True negative rate (TN%) or (SP)  MCQ 1-PPV(positive predictive value) depends on : A-SN B-SP C-Prevalence of disease D-All of the above – References: – #Park’s text book of preventive and social disease 2021 – #Textbook in research methodology 2023 Thank you