Chapter 2.2 screening test
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Screening tests aim to identify unrecognized disease in apparently healthy individuals. They differ from diagnostic tests in that they are applied to groups rather than individuals, use a single criterion, and are less accurate. Validity refers to a test's accuracy while reliability is its precision on repeat tests. Sensitivity measures a test's ability to identify true positives, and specificity measures its ability to identify true negatives. Screening programs must consider factors like disease burden, test characteristics, and whether early detection improves outcomes.
Chapter 2.2 screening test
- 1. Screening test and evaluation of screening test. By Dr Nilesh Kucha
- 2. DEFINITION OF SCREENING The search for unrecognized disease or defect applied tests, examinations or other procedures individuals.* by means of rapidly in apparently healthy 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.* Tests done in individuals with no symptom or sign of an illness are referred to as screening tests.* 21st(* K. Park Textbook of PSM edition ; 4th* J M Last Dictionary of Epidemiology Papers 1968; edition, WHO Public Health * J H Abramson and Z H Abramson Survey methods in Community 5thMedicine edition)
- 3. BASIS OF SCREENING Iceberg phenomenon disease tip of the iceberg CLINICAL DISEASE of submerged portion HIDDEN BURDEN OF DISEASE
- 4. BASIS OF SCREENING Screening is a form of secondary prevention. It detects disease in its early asymptomatic phase whereby early treatment can be given and disease can be cured or its progression can be delayed. It has both diagnostic (?) and therapeutic components.
- 5. SCREENING TEST VERSUS DIAGNOSTIC TEST Screening test Diagnostic test 1. Done on apparently 1. Done on sick or ill healthy individuals Applied to groups individuals Applied on single patient Diagnosis is not final 2. 3. 2. 3. 4. Results final Based are arbitrary and Based on evaluation of a &4. on one criteria no. of signs/symptoms and cut-off lab findings 5. 6. 7. Less Less Not accurate expensive 5. 6. 7. More More Used accurate expensive a basis for as a basis for treatment Initiative treatment Initiative patient 8. comes from 8. comes from a investigator
- 6. TYPES OF SCREENING 1. MASS SCREENING Application of screening test to large, unselected population. Everyone in the group is screened regardless of the probability of having the disease or condition. Example: A) mammography in women aged 40 years or less
- 7. 2. HIGH RISK / SELECTIVE / TARGETED SCREENING The screening of selected high-risk groups in the population. 3. OPPORTUNISTIC / CASE FINDING SCREENING There is no accurate or precise diagnostic test for the disease and where the frequency of its occurrence in the population is small. The main objective is to detect disease and bring patients to treatment.
- 8. 4. MULTIPURPOSE SCREENING The screening of a population by more than one test done simultaneously to detect more than one disease 5. MULTIPHASIC SCREENING The screening in which various diagnostic procedures are employed during the same screening program.
- 9. USES OF SCREENING 1. CASE DETECTION prescriptive screening, people are screened for their own benefit (cancer, diabetes, hypertension) 2. CONTROL OF DISEASE prospective screening, people are screened for the benefit of others (HIV, STI) 3. RESEARCH to know the natural history of a disease 4. EDUCATION public awareness
- 10. CRITERIA FOR CHOOSING A SCREENING TEST 1. DISEASE a) Significant burden of disease b) Detectable and long preclinical stage of disease c) Adequately understood natural history of disease d) Appropriate test available for early detection of disease e) Facilities for diagnosis of disease f) Early detection of disease has outcome benefit g) Effective treatment available for disease h) Policy of screening program for disease
- 12. GOLD STANDARD • GOLD STANDARD: an external source of truth regarding the disease status of each individual in the population. Gold standard is a benchmark definitive. Infections – CULTURE and its results are considered • • Cancers – BIOPSY • Drug testing – RANDOMIZED CONTROLLED TRIAL • Cause of death – AUTOPSY
- 13. WHAT IS VALID AND RELIABLE? VALIDITY IS THE ACCURACY OF A TEST. RELIABILITY IS THE PRECISION OF A TEST. ACCURACY: “how close is result of a test to its true value?” PRECISION: “how close are the results of a test on repetition?”
- 15. COMPONENTS OF VALIDITY Ds present Ds absent Test positive Test negative
- 16. SENSITIVITY The ability of a test to correctly identify disease. a/ (a + c) expressed as percentage. thosewhohavethe Ds present Ds absent Test positive Test negative
- 17. SPECIFICITY The ability of a test to correctly identify those the disease. d/ (b + d) expressed as percentage. whodonothave Ds present Ds absent Test positive Test negative
- 19. PROBLEM OF FN AND FP FN: false reassurance ignoring of disease signs and symptoms postponement of treatment detrimental to overall health FP: further testing discomfort, inconvenience, anxiety burden on health facilities emotional trauma difficulty in “de-labeling”
- 22. SENSITIVTY AND SPECIFICITY OF IMPORTANT TESTS 1. Mammography: Sensitivity 75 – 95% Specificity 83 – 98% 2. PAP test: Sensitivity 29 – 56% Specificity 94 – 100% 3. PSA test (4 ng/ml): Sensitivity 20 – 32% Specificity 94 – 97% 4. FBS (5mmol/L): Sensitivity 85 – 89% Specificity 70 – 77% 5. RAPID ELISA: Sensitivity 99.5% Specificity 98%
- 23. USE OF MULTIPLE TESTS •Sequential Testing (Two-Stage Screening) After the first (screening) test is conducted, thosewho tested positive are brought back for the second test to further reduce false positives. Consequently, the overall process will increase specificity but with reduced sensitivity.
- 31. LIKELIHOOD RATIO The percentage of sick people with a given test result divided by the percentage of well individuals with the same result. Positive LR: The relative probability of a positive test in a diseased individual in comparison to a disease- free individual. TP / FP = SENSITIVITY / 1 – SPECIFICITY
- 32. Negative LR: The relative probability of a negative test in a diseased individual in comparison to a disease- free individual. FN / TN = 1 – SENSITIVITY / SPECIFICITY
- 33. WHY DO WE NEED LIKELIHOOD RATIO? • Many tests in clinical medicine have continuous results or multiple ordinal levels. Putting multiple categories into either positive or negative test causes loss ratios enable clinicians to interpret diagnostic test results. of information. Likelihood and use the full range of • While predictive values relate test characteristics to populations, patient. likelihood ratios can be applied to a specific • Likelihood ratios refine clinical judgment. Application of a likelihood ratio to a working diagnosis generally changes the diagnostic probability—sometimes radically.
- 37. EXAMPLE OF INTER-OBSERVER AND INTRA-OBSERVER VARIATION • INTRA-OBSERVER VARIATION: two readings of blood pressure measurement • INTER-OBSERVER VARIATION: chest x-ray films by different radiologists
- 38. YIELD YIELD is the amount of unrecognized disease that is detected brought to treatment as a result of screening. and YIELD =TP+FP/TP+FP+TN+FN It depends on prevalence of the disease and screening test. sensitivity of the Hence, yield of a screening test is high in high – risk screening.
- 39. ISSUES WITH SCREENING 1. Lead time bias - the systematic error of apparent increased survival from detecting disease in an early stage.
- 47. 2. LENGTH TIME BIAS Diseases with a long pre-clinical phase are more likely to be detected during screening. Moreover, pre-clinical phase for the same disease may be variable in different individuals.
- 48. 3. SELECTION BIAS Not everyone will take part in a screening program. There are factors that differ between those willing to get tested and those who are not. Willingness of patient. Example: outcome if outcomes if depends on perceived risk of disease and intelligence a) breast cancer screening – more positive only intelligent people participate. More negative only high risk patients participate.
- 49. 4. OVERDIAGNOSIS Screening may identify abnormalities that would never cause a problem in a person's lifetime. Causes overestimate of disease well as survival. as Example : a) PAP testing and Cervical CIS b) PSA testing and low grade prostate cancer c) mammography and DCIS
- 50. EVALUATIONOF SCREENING TEST 1.METHODS a) Experimental: conduct an RCT of the screening test to compare the disease specific cumulative mortality rate between the intervention and control group. this also eliminates confounding. allows study of distribution of lead time, effects of early treatment and identification of prognostic factors.
- 51. b) Non – experimental: cohort study (comparison of advanced disease or death rates in those who choose to screen and those who do not) case - control study (comparison of screening history in those who have advanced disease healthy) and those who are ecological study (correlation of pattern and disease experience populations) screening of several
- 52. 2.MEASURESOFEFFECT a) Comparison of survival experience b) Comparison of disease specific mortality rate
- 53. BAYES THEOREM A prior (pre-test) probability is an initial probability value originally obtained before any additional information is obtained. A posterior (post-test) probability is a probability value that has been revisedby using additional information that is later obtained. Example : Road traffic accident patients in a hospital are listed and one of the patients is randomly selected. a) what is the probability that he will have cerebral hemorrhage? b) during the later analysis of this patient it was found that he has CT scan positive for cerebral hemorrhage. Now, what is the probability that he actually has cerebral hemorrhage?
- 54. For the first question, we know that Probability of cerebral hemorrhage in a RTA patient is 28%. For the second question, we know that CT scan has a sensitivity of 90% and specificity of 95% for detecting cerebral hemorrhage. Bayes theorem gives us a way of calculating probability of an event in the light of presence of a second event, which itself occurs in the presence of the first one.
- 55. Probability that our patient has cerebral hemorrhage, in an already positive CT scan for cerebral hemorrhage, is… view of Pre-test probability of event (P1) X sensitivity of test (Sn) --------------------------------------------------------------------------- X 100 (P1X Sn) + (1 – P1)X (1 – Sp) Here, P1 = 0.28 Sn = 0.90 Sp = 0.95 So, Post-test probability (P2)= 0.875 or 87.5%
- 56. • What is meant by: a) True positive and False positive b) Positive and Negative predictive value c) • Statistical tests used in the analysis of effectiveness of a screening test. • With regards to a diagnostic test, define sensitivity,specificity, positive predictive value and negative predictive value. • Enumerate the general principles of screening. • Describe the terms 'sensitivity, specificity, positive predictive value, negative· predictive value and odds ratio' as used in statistical analysis DNB QUESTIONS ASKED PREVIOUSLY
- 57. THANKYOU…