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Diagnosis, screening and prevention

Diagnosis, screening and prevention. Puzzle. Virus present randomly in 1 in 1000 population Test for virus 99% reliable i.e. misses 1% of infected individuals and falsely labels 1% of non infected individuals Select 1 individual at random.

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Diagnosis, screening and prevention

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  1. Diagnosis, screening and prevention

  2. Puzzle • Virus present randomly in 1 in 1000 population • Test for virus 99% reliable • i.e. misses 1% of infected individuals and falsely labels 1% of non infected individuals • Select 1 individual at random. • If the test result is positive, what is the probability that that individual is infected?

  3. ~ 0 FN 1 TP ~ 1 1000 FP 10 999 TN 989 Probability that that individual is infected = 1 / 10+1 = 9% (= PPV)

  4. Diagnostic testsObjectives Student should be able to: • describe how tests are evaluated • define and calculate se\sp\pv and describe their inter-relationships • demonstrate the ability to correctly interpret test results • utilise prior probability in the judicious selection of diagnostic tests

  5. Objectivescontd. • describe how prior probablilities are derived • sources of health information in Ireland / other countries

  6. Key points 1. Purpose of testing • to move probability of disease towards 0% or 100% 2. Tests vary in performance (validity) • sensitivity and specificity 3. +ve test result  person has disease • PPV, NPV 4. Same test performs differently in different situation • Diagnostic vs screeningcontexts

  7. 1. Purpose of testing • To move probability of disease towards 0% or 100%

  8. 2. Test validity • Validity measured by se and sp. • Sensitivity is the probability of a positive test result in the presence of disease. • Specificity is the probability of a negative test result given the absence of disease. • Sensitivity and specificity are inversely related.

  9. Test validity

  10. Test validity • Sensitivity • probability of a positive test result in the presence of disease = a / a + c • Specificity • probability of a negative test result given the absence of disease = d / d + b

  11. Se = 99% Sp = 99% Puzzle: Pre-test probability (prevalence) = 1/1000Test for virus 99% reliable i.e. misses 1% of infected individuals,and falsely labels 1% of non infected individuals

  12. Comparison of fine needle aspiration test results with findings from surgical excisional biopsies in women without and with palpable breast masses (from C Smith et al. Surgery 1988 103:178))

  13. Comparison of fine needle aspiration test results with findings from surgical excisional biopsies in women without and with palpable breast masses (from C Smith et al. Surgery 1988 103:178))

  14. Se = 113/113 + 8 Sp = 181/181+15 = 93% = 92%

  15. 3. Positive test result person has disease • PPV: probability of disease, given positive test result = a /a + b • NPV: prob. of no disease, given negative test result = d / d + c • PPV rises: • as specificity increases • as prior probability/ prevalence increases

  16. PPV NPV Se = 99% Sp = 99% Puzzle: Virus present randomly in 1 in 1000 population. Test 99% reliable Select 1 individual at random: If the test result is positive, what is the probability that that individual is infected?

  17. PPV = a/a+b  1/11 = 9.1%NPV = d/d+c = 989/989  100.0% ( Se = 99% Sp = 99% Puzzle: Virus present randomly in 1 in 1000 population. Test 99% reliable Select 1 individual at random: If the test result is positive, what is the probability that that individual is infected?

  18. Comparison of fine needle aspiration test results with findings from surgical excisional biopsies in women without and with palpable breast masses (from C Smith et al. Surgery 1988 103:178))

  19. Comparison of fine needle aspiration test results with findings from surgical excisional biopsies in women without and with palpable breast masses (from C Smith et al. Surgery 1988 103:178))

  20. if prevalence is 20% if prevalence is 10% if prevalence is 1% if prevalence is 0.1% PPV is 83% PPV is 68% PPV is 16% PPV is 2% Positive predictive values (PPV) are highly dependent on PREVALENCEFor a screening test with:95% sensitivity and 95% specificity:

  21. if specificity is 60% if specificity is 80% if specificity is 90% if specificity is 95% if specificity is 99% PPV is 1.5% PPV is 2.9% PPV is 5.7% PPV is 10.8% PPV is 37.7% Positive predictive values (PPV) also vary greatly with SPECIFICITYFor a disorder with prevalence (pre-test probability) of 1%, & a test with 60% sensitivity:

  22. 4. Same test performs differently in different situations • Usefulness varies according to pretest probability (prevalence) of disease. • Implications of link between prior probability and PPV: • Clinical • if prior probability is very low - or very high - test contributes little information • Screening • PPV is usually very low, especially for disease of low frequency

  23. CASS: % with coronary artery disease according to symptom history(DA Weiner et al. NEJM 301:230-5,1979)

  24. CASS: % with coronary artery disease according to symptom history(DA Weiner et al. NEJM 301:230-5,1979)

  25. Prevention and screening Objectives Student should be able to: • describe the levels of prevention • list and apply the criteria for screening • describe the impact of prevalence on predictive values • explain why there are difficulties with screening

  26. Levels of prevention • PRIMARY PREVENTION • Prevention of disease by controlling risk factors, removing causes • e.g. non-smoking promotion • SECONDARY PREVENTION • Reduction in consequences of disease by early diagnosis and treatment • e.g. cervical cancer screening • TERTIARY PREVENTION • Reduction of complications of disease • e.g. MV crashes and ICU

  27. Screening:process of identifying unrecognised diseases/defects using tests that can be applied rapidly on a large scale • Tests sort apparently healthy from those with (subclinical) disease • Not diagnostic • investigative follow-up and treatment required • Safety paramount • initiative from health service, not client

  28. 1. Purpose of testing • To move probability of disease towards 0% or 100%

  29. NATURAL HISTORY OF DISEASE

  30. Screening Criteria • Important health problem • high prevalence \ radical consequences • Natural history known • long pre-clinical phase • Tests: valid, acceptable, ‘cheap’ • Effective treatment • adequate facilities for dx & Rx • Economically justifiable

  31. We screen for many different types of disease : • metabolic disorders • genetic disorders • congenital defects • developmental problems • infectious diseases • cancers and other chronic diseases

  32. Types of screening • Mass screening • Opportunistic screening (case finding) • Targeted screening • Multiphasic screening

  33. SCREENING BIASES • Lead time bias: • date of diagnosis automatically advanced for those cancers detected by screening • Length bias: • tendency of screening to detect cancers which spend longer in the asymptomatic state (slower growing) • Selection bias: • tendency for non-acceptors of screening to be at higher (or lower) risk of developing and/or dying from this disease than the general population

  34. NATURAL HISTORY OF DISEASE

  35. NATURAL HISTORY OF DISEASE

  36. SCREENING BIASES • Lead time bias: • date of diagnosis automatically advanced for those cancers detected by screening • Length bias: • tendency of screening to detect cancers which spend longer in the asymptomatic state (slower growing) • Selection bias: • tendency for non-acceptors of screening to be at higher (or lower) risk of developing and/or dying from this disease than the general population.

  37. Evaluation of screening programmes • RCT • Apparent ‘benefit’ of screeningenhanced if: • ‘poor’ quality RCT • Non RCT e.g. case control studies

  38. NEONATES "FOETUSES” PREGNANT WOMEN/PARENTS-TO-BE ADULTS ELDERLY PKU/CHT/GAL/HCU/MSUD, CDH, NTD, Down syndrome, thalassaemia UTI, STD, AIDS, rubella, blood sugar, Rhesus factor Rubella, AIDS, sickle cell anaemia, thalassaemia, Tay Sachs cancers (breast, cervix, rectal), hypertension, cholesterol glaucoma, cataract, hearing, mobility. Target groups for screening:

  39. Target group Neonates "Foetuses" Pregnant women Parents-to-be Children Adults Elderly Treatment Prevention of serious morbidity Termination Prevention of foetal damage (also maternal welfare) Prevention of conception Alleviation / prevention of morbidity Reduction in morbidity or mortality / prevention of disease onset Alleviation The treatment aims of screening differ

  40. Key points 1 - 2 • 1. Purpose of testing • to move probability of disease towards 0% or 100%. • 2. Tests vary in performance (validity) • Sensitivity • probability of pos. test result when disease present • Specificity • probability of neg. test result when disease absent • Trade off between sensitivity and specificity

  41. Key points 3 - 4 • 3. +ve test result  person has disease • PPV: probability of disease, given a pos. test result • NPV: prob. of no disease, given neg.test result • PPV rises: • as specificity increases • as prior probability/ prev. increases • 4. Implications of link between prior prob. and PPV: • Same test performs differently in different situations • Clinical context • if prior probability is very low - or very high - test contributes little information • Screening for disease of low frequency • PPV is very low

  42. Task force on sudden death in sport(Nov 2005) • Clinical screening for sudden death in all teenagers and adults who play sport • PPV = 1/1000 Better to use family history i.e.targeted screening

  43. *Screening test: blood spot trypsinogenNutritional benefits of neonatal screeningP. Farrell et al. NEJM 1997; 337:963-9

  44. Intervention Cost per QALY* in US dollars Opportunistic screening for type 2 diabetes $56,600 Intensive glycaemic control in type 2 diabetes $16,000 Plus tight blood pressure control additional $700 Detection of mild thyroid disease in women aged > 35 years, during a health check $9,000 Breast cancer screening with mammography in women aged 50 - 65 years $150,000 Colon cancer screening using FOBT in patients aged 50 – 75 years $16,000 Cervical cancer screening using pap smears every 4 years for women aged 20 – 75 years $16,000 * Quality adjusted life years

  45. Cochrane review of breast cancer screeningRelative risk of death from breast cancer in screened vs control groupsGotzsche & Olsen, Lancet 2000; 355: 129-34 • Total mortality - no difference • Breast cancer mortality - marginal difference

  46. Relative risk of death from breast cancer in screened vs control groupsGotzsche & Olsen, Lancet 2000; 355: 129-34

  47. Trials with adequate randomisation

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