Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Screening for diseases (epidemiology)

37,564 views

Published on

Published in: Health & Medicine
  • How to start a wildly profitable 7 figure marketing business and get your first commission check tonight, click here  https://tinyurl.com/y3ylrovq
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
  • DOWNLOAD FULL BOOKS, INTO AVAILABLE FORMAT ......................................................................................................................... ......................................................................................................................... ,DOWNLOAD FULL. PDF EBOOK here { https://tinyurl.com/yyxo9sk7 } ......................................................................................................................... ,DOWNLOAD FULL. EPUB Ebook here { https://tinyurl.com/yyxo9sk7 } ......................................................................................................................... ,DOWNLOAD FULL. doc Ebook here { https://tinyurl.com/yyxo9sk7 } ......................................................................................................................... ,DOWNLOAD FULL. PDF EBOOK here { https://tinyurl.com/yyxo9sk7 } ......................................................................................................................... ,DOWNLOAD FULL. EPUB Ebook here { https://tinyurl.com/yyxo9sk7 } ......................................................................................................................... ,DOWNLOAD FULL. doc Ebook here { https://tinyurl.com/yyxo9sk7 } ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .............. Browse by Genre Available eBooks ......................................................................................................................... Art, Biography, Business, Chick Lit, Children's, Christian, Classics, Comics, Contemporary, Cookbooks, Crime, Ebooks, Fantasy, Fiction, Graphic Novels, Historical Fiction, History, Horror, Humor And Comedy, Manga, Memoir, Music, Mystery, Non Fiction, Paranormal, Philosophy, Poetry, Psychology, Religion, Romance, Science, Science Fiction, Self Help, Suspense, Spirituality, Sports, Thriller, Travel, Young Adult,
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
  • Protect your brain from memory loss with brain pill. find out more... ◆◆◆ https://tinyurl.com/brainpill101
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
  • Tired of being scammed? Take advantage of a program that, actually makes you money!  http://scamcb.com/ezpayjobs/pdf
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
  • I am 31 years old and have had severe Yeast Infection for over 8 years. I have been suffering (yes! really suffering for so long) and I have indeed tried every over-the-counter and prescription that you could think of to treat this horrible disease. My wife also suffered from moderate Yeast Infection for years and we both started your Yeast Infection No More system about 4 months ago and I am so excited to report that we are both free from Yeast Infection. Both of us! For the first time ever. The symptoms are gone...completely! And the overall feeling is unbelievable. Thank you for your help... ◆◆◆ http://ishbv.com/index7/pdf
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here

Screening for diseases (epidemiology)

  1. 1. SCREENING PRESENTER: DR. SHAILAJA DARAL MODERATOR: DR. ANITA VERMA ASSO. PROFESSOR DEPT. OF COMMUNITY MEDICINE VMMC AND SAFDARJUNG HOSPITAL
  2. 2. DEFINITION OF SCREENING The search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals.* 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.* (* K. Park Textbook of PSM 21st edition; * J M Last Dictionary of Epidemiology 4th edition, WHO Public Health Papers 1968; * J H Abramson and Z H Abramson Survey methods in Community Medicine 5th edition)
  3. 3. BASIS OF SCREENING Iceberg phenomenon of disease tip of the iceberg CLINICAL DISEASE submerged portion HIDDEN BURDEN OF DISEASE
  4. 4. BASIS OF SCREENING
  5. 5. LEAD TIME
  6. 6. 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.
  7. 7. SCREENING TEST VERSUS DIAGNOSTIC TEST Screening test 1. Done on apparently healthy individuals 2. Applied to groups 3. Results are arbitrary and final 4. Based on one criteria and cut-off 5. Less accurate 6. Less expensive 7. Not a basis for treatment 8. Initiative comes from investigator Diagnostic test 1. Done on sick or ill individuals 2. Applied on single patient 3. Diagnosis is not final 4. Based on evaluation of a no. of signs/symptoms & lab findings 5. More accurate 6. More expensive 7. Used as a basis for treatment 8. Initiative comes from a patient
  8. 8. COMMON SCREENING TESTS • • • • • • FASTING BLOOD GLUCOSE FOR DIABETES BLOOD PRESSURE FOR HYPERTENSION PSA TEST FOR PROSTATE CANCER PAP SMEAR FOR CERVICAL CANCER MAMMOGRAPHY FOR BREAST CANCER FECAL OCCULT BLOOD FOR COLON CANCER
  9. 9. 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) visual defects in school children b) mammography in women aged 40 years or less c) newborn screening program in Japan
  10. 10. 2. HIGH RISK / SELECTIVE / TARGETED SCREENING The screening of selected high-risk groups in the population. Example: a) screening fetus for Down’s syndrome in a mother who already has a baby with Down’s syndrome b) screening for familial cancers, HTN and DM c) screening for CA Cervix in low SES women d) screening for HIV in risk groups
  11. 11. 3. MULTIPURPOSE SCREENING The screening of a population by more than one test done simultaneously to detect more than one disease Example: a) screening of pregnant women for VDRL, HIV, HBV by serological tests 4. MULTIPHASIC SCREENING The screening in which various diagnostic procedures are employed during the same screening program. Example: a) DM – FBS, Glucose tolerance test b) Sickle cell anemia – CBC, Hb electrophoresis
  12. 12. 5. 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. Example: a) RHD in children
  13. 13. 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
  14. 14. 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
  15. 15. 2. SCREENING TEST a) Inexpensive b) Acceptable c) Valid d) Reliable e) Yielding
  16. 16. GOLD STANDARD • GOLD STANDARD: an external source of truth regarding the disease status of each individual in the population. Gold standard is a benchmark and its results are considered definitive. • Infections – CULTURE • Cancers – BIOPSY • Drug testing – RANDOMIZED CONTROLLED TRIAL • Cause of death – AUTOPSY
  17. 17. 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?”
  18. 18. GOLD STANDARD AND SCREENING TEST
  19. 19. VALIDITY An expression of the degree to which a test measures what it purports to measure. How to understand validity? ESTIMATE: a measurement or statement about the value of a quantity under study. An epidemiological estimate is the end product of the study design, the study conduct and the data analysis. Error is a false or mistaken result obtained in a study or experiment. Systematic error is one sided variation of measurements from the true value. It has a recognizable source. A study that has little systematic error is described as VALID.
  20. 20. COMPONENTS OF VALIDITY Ds present Test positive Test negative Ds absent
  21. 21. SENSITIVITY The ability of a test to correctly identify those who have the disease. TP a/ (a + c) expressed as percentage. Problem of FP. Ds present Test positive Test negative Ds absent
  22. 22. SPECIFICITY The ability of a test to correctly identify those who do not have the disease. TN d/ (b + d) expressed as percentage. Problem of FN. Ds present Test positive Test negative Ds absent
  23. 23. ACCURACY ACCURACY = TP + TN / TP + FP + TN + FN ACCURACY = (SENSITIVITY) X (PREVALENCE) + (SPECIFICITY) X (1 - PREVALENCE)
  24. 24. 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”
  25. 25. CHANGING CUT-OFF POINTS:
  26. 26. 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%
  27. 27. USE OF MULTIPLE TESTS •Sequential Testing (Two-Stage Screening) After the first (screening) test is conducted, those who 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.
  28. 28. 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 diseasefree individual. TP / FP = SENSITIVITY / 1 – SPECIFICITY
  29. 29. Negative LR: The relative probability of a negative test in a diseased individual in comparison to a diseasefree individual. FN / TN = 1 – SENSITIVITY / SPECIFICITY
  30. 30. 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 of information. Likelihood ratios enable clinicians to interpret and use the full range of diagnostic test results. • While predictive values relate test characteristics to populations, likelihood ratios can be applied to a specific patient. • Likelihood ratios refine clinical judgment. Application of a likelihood ratio to a working diagnosis generally changes the diagnostic probability—sometimes radically.
  31. 31. 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
  32. 32. KAPPA STATISTIC Type of inter-observer agreement. Takes into account agreement between two observers purely by chance. It is the inter-observer agreement over and above agreement that is purely by chance. It is standardized to lie between -1 to 1. -1 = no agreement 1 = perfect agreement 0 = agreement purely due to chance
  33. 33. EXAMPLE OF KAPPA STATISTIC Resident 2 Are afternoon lectures helpful? yes Resident 1 yes no total 15 a 5 b 20 no 10 c 70 d 80 total 25 75 100
  34. 34. PERCENTAGE AGREEMENT (15 + 70) / 100 = 0.85 or 85% KAPPA statistic = PO - PE / 1 – PE Where PE = (a + b) / (a + b + c + d) X (a + c) / (a + b + c + d) + (b + d) / (a + b + c + d) So here, KAPPA = 0.57 X (c + d) / (a + b + c + d)
  35. 35. YIELD YIELD is the amount of unrecognized disease that is detected and brought to treatment as a result of screening. YIELD = TP + FP / TP + FP + TN + FN It depends on prevalence of the disease and sensitivity of the screening test. Hence, yield of a screening test is high in high – risk screening.
  36. 36. RECEIVER OPERATING CHARACTERISTICS (ROC) ANALYSIS For a given test, the true positive rate (TPR) or SENSITIVITY against false positive rate (FPR) or 1 – SPECIFICITY can be measured. All possible combinations of TPR and FPR compose a ROC space. One TPR and one FPR together determine a single point in the ROC space, and the position of a point in the ROC space shows the trade – off between sensitivity and specificity, i.e. the increase in sensitivity is accompanied by a decrease in specificity.
  37. 37. Thus the location of the point in the ROC space depicts whether the screening test classification is good or not. Plotting SENSITIVITY and 1 – SPECIFICITY at different values of a screening test gives the ROC curve. The area under the curve (AUC) gives a way to measure the accuracy of the screening test.
  38. 38. In an ideal situation, a point determined by both TPR and FPF yields coordinates (0, 1). This ideal point indicates that the screening test has a sensitivity of 100% and specificity of 100%. Screening test with 50% sensitivity and 50% specificity lies on the diagonal determined by coordinate (0, 0) and coordinates (1, 0). Theoretically, random error would give a point along this diagonal. A point predicted by a screening test that falls into the area above the diagonal represents a good screening classification.
  39. 39. METHOD TO FIND THE OPTIMUM CUT – OFF POINT OF A SCREENING TEST YOUDEN INDEX AND COST CONSIDERATIONS
  40. 40. EVALUATION OF 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.
  41. 41. 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 and those who are healthy) ecological study (correlation of screening pattern and disease experience of several populations)
  42. 42. 2. MEASURES OF EFFECT a) Comparison of survival experience b) Comparison of disease specific mortality rate
  43. 43. ISSUES WITH SCREENING 1. Lead time bias - the systematic error of apparent increased survival from detecting disease in an early stage.
  44. 44. 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.
  45. 45. 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 depends on perceived risk of disease and intelligence of patient. Example: a) breast cancer screening – more positive outcome if only intelligent people participate. More negative outcomes if only high risk patients participate.
  46. 46. 4. OVERDIAGNOSIS Screening may identify abnormalities that would never cause a problem in a person's lifetime. Causes overestimate of disease as well as survival. Example: a) PAP testing and Cervical CIS b) PSA testing and low grade prostate cancer c) mammography and DCIS
  47. 47. 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 revised by 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?
  48. 48. 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.
  49. 49. Probability that our patient has cerebral hemorrhage, in view of an already positive CT scan for cerebral hemorrhage, is… Pre-test probability of event (P1) X sensitivity of test (Sn) --------------------------------------------------------------------------- X 100 (P1 X 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%
  50. 50. REFERENCES • • • • • • • • • • • • • • Gordis Leon. Epidemiology;4:85 – 108, 311 - 331. Bonita R, Beaglehole R, Kjellstrom T. Basic Epidemiology;2: 110 – 114. Daly LE, Bourke GF. Interpretation and uses of medical statistics;5:381 – 421. Sackett DL, Haynes RB, Guyatt GH, Tugwell P. Clinical epidemiology;2:51 – 186, 305 – 358. Abramson JH, Abramson ZH. Survey methods in community medicine;5:171 – 204. Last JM. Dictionary of epidemiology;4:165-166. Rothman KJ, Greenland S, Lash TL. Modern epidemiology;3:128 – 182. Park K. Textbook of preventive and social medicine;21:124 – 131. Suryakantha AH. Community medicine;2:288 – 291. Oxford textbook of public health;4:621 – 637. Matthews DE, Farewell VT. Using and understanding medical statistics;4:282 – 309. Lectures on epidemiology by John Hopkins Bloomberg School of Public Health. Reeves M. Fundamentals of medical epidemiology and biostatistics. WHO Public health papers, 1968.
  51. 51. THANK YOU…

×