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Screening of diseases

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Screening of diseases

  1. 1. Screening of diseases F D E Mtango
  2. 2. Objectives-To know the following: <ul><li>Definition of screening and screening test </li></ul><ul><li>Definition of diagnosis and diagnostic test </li></ul><ul><li>Difference between screening and diagnosis tests </li></ul><ul><li>The high risk and population approach for screening of diseases </li></ul><ul><li>The indications for screening of a disease </li></ul><ul><li>The evaluation of a screening test </li></ul>
  3. 3. What do we usually mean by screening of a disease? <ul><li>Screening is to investigate a population of apparently healthy people, in order to detect those most likely to be affected, and those most likely to be unaffected by a given disease </li></ul><ul><li>This is otherwise to separate those people who need additional investigation to detect a disease from those not needing any additional investigation (those most likely to be free of the disease) </li></ul>
  4. 4. What do we usually mean by diagnosis (or diagnostic test)? <ul><li>Diagnostic test the best criteria for regarding a patient as having a certain disease or a confirmatory test for a given disease as compared to a screening test </li></ul><ul><li>A diagnostic test allows us to evaluate a screening test or allows us to find the validity of a screening test </li></ul><ul><li>Validity of a screening test refers to the extent in which a screening test is able to correctly diagnose the presence or absence of disease </li></ul><ul><li>Any test can be said to be an inclusion/exclusion criteria </li></ul>
  5. 5. Difference between diagnostic and screening tests <ul><li>Diagnostic: more reliable than screening but either more costly or more painful or unpleasant. Also often impossible in the field and needs hospital or highly trained people to do it </li></ul><ul><li>Screening: Usually simple, quick and cheap test. Easily acceptable to community. Usually needs confirmation by another test, which is a diagnostic test </li></ul>
  6. 6. High risk or population approaches in screening <ul><li>High risk are people who are more likely to get a disease due to their age, sex, place, life style or habits </li></ul><ul><li>High risk approach in screening means screening only those at high risk of having the disease, e.g. smokers for lung cancer or bar maids for HIV </li></ul><ul><li>Population approach in screening is screening the whole population. This is, usually, in case of a high prevalence of disease </li></ul>
  7. 7. The indications for screening <ul><li>When the natural history of the disease is known </li></ul><ul><li>When a known, simple, cheap and acceptable test is available, e.g. breast palpation for screening of breast cancer </li></ul><ul><li>When the disease is of public health importance </li></ul><ul><li>When the yield of screening is good. Yield is the number of people confirmed to have the disease after applying a diagnostic test </li></ul>
  8. 8. Indications for screening 2 <ul><li>5. When majority of affected people are sub-clinical </li></ul><ul><li>6. For public health interest; when asymptomatic people can transmit the disease e.g. HIV. </li></ul><ul><li>7. When people at high risk for the disease can be easily identified </li></ul><ul><li>8. When there is an effective treatment of the disease and therefore screening will improve the prognosis (or outcome) of the disease </li></ul>
  9. 9. Examples of screening <ul><li>Pap smear to detect people with carcinoma of cervix, in situ, prevents development of invasive cancer </li></ul><ul><li>Test to detect phenylketonuria, if done in neonatal period, the child can be given a diet low in phenylalanine </li></ul><ul><li>Screening for breast cancer by observing breast lumps. Further investigation is needed to confirm or rule out breast cancer </li></ul>
  10. 10. The evaluation of a screening test <ul><li>Sensitivity of a screening test is the ability of the test to give true positives </li></ul><ul><li>Specificity of a screening test is the ability of the test to give true negatives </li></ul><ul><li>Predictive value of a positive screening test is the ability of the screening test to give a high yield or small number of false positives </li></ul><ul><li>Predictive value of a negative screening test is the ability of the screening test to give a small number of missed cases (false negatives) </li></ul>
  11. 11. Two by two contingency table <ul><li>Test results Dis. present Disease neg. </li></ul><ul><li>Test positive a (true pos) b (false pos) </li></ul><ul><li>Test negative c (false neg) d (true neg) </li></ul><ul><li>Total a+c b+d </li></ul><ul><li>Sen. = a / a+c x 100 Pred. + = a / a+bx100 </li></ul><ul><li>Spe.= d / b +d x 100 Pred. - = d / c+dx100 </li></ul>
  12. 12. Two by two contingency table 2 <ul><li>Test results Dis. present Dis. neg. Tot </li></ul><ul><li>Test positive 380 20 400 </li></ul><ul><li>Test negative 80 1520 1600 </li></ul><ul><li>Total 460 1540 2000 </li></ul><ul><li>Sen. = 380/460x100 =82.6% </li></ul><ul><li>Spe.= 1520/1540x100=98.7% </li></ul><ul><li>Pred. + = 380/400x100=95% </li></ul><ul><li>Pred. - = 1520/1600x100=95% </li></ul>
  13. 13. Application of screening <ul><li>1. Cephalo-pelvic disproportion: pregnant women who are less than 140 cm height are referred to hospital for delivery for they are high risk for delivery by caesarean section </li></ul><ul><li>2. Pap smear to detect people with carcinoma of cervix, The smear discovers women who are subjected to a biopsy to diagnose the first stage of this cancer or carcinoma in -situ. If this is treated it prevents development of invasive cancer. </li></ul><ul><li>3. Screening for breast cancer by observing breast lumps. Further investigation is needed to confirm or rule out breast cancer. Not all breast lumps can be proved to be breast cancer. </li></ul><ul><li>4. Using fever as screening test for malaria. All positive cases are subjected to a blood slide which if positive is diagnostic of malaria </li></ul>
  14. 14. <ul><li>Screening is practiced in day to day clinical practice as follows: </li></ul><ul><li>History taking, age, sex, social class, residence, </li></ul><ul><li>Symptoms: fever, diarrhoea, vomiting, heart palpitations, </li></ul><ul><li>Physical examination: fever, pulse, crepitations, wheezing </li></ul>
  15. 15. assignment <ul><li>What is the difference between a screening and a diagnostic test? </li></ul><ul><li>What are the indications of a screening test? </li></ul><ul><li>Make a 2 by 2 contingency table and use it to describe how you would evaluate a screening test </li></ul>

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