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 of diseases


Published on

Published in: Health & Medicine
  • Be the first to comment

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>