Screening and Diagnostic Tests PDF

BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban Introduction and Review In session 4, we discussed: Crude Mortality Rate Cause-Specific Mortality Rate Case Fatality Rate Proportionate Mortality Challenges in mortality measurement and how these measures...

BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban Introduction and Review In session 4, we discussed: Crude Mortality Rate Cause-Specific Mortality Rate Case Fatality Rate Proportionate Mortality Challenges in mortality measurement and how these measures are applied in public health. Learning objectives 1. To define the validity and reliability of screening and diagnostic tests. 2. To compare measures of validity, including sensitivity and specificity. 3. To introduce positive and negative predictive value. Let’s go through each of these points in detail! The Iceberg phenomenon of disease Symptomatic diseases What the physician sees Clinical cases Diseases in the community Submerged portion of iceberg (the hidden mass of diseases): Hypertension Diabetes Pre-symptomatic diseases Anaemia Malnutrition What the Undiagnosed reservoir Mental illness physician of infection or disease in does not see the community Carriers of disease Sub-clinical diseases  The Concept of Screening The active search for disease among apparently healthy people is a fundamental aspect of prevention. Screening: "the search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals."  Evolution of Screening Programs in Healthcare Annual health examinations evolved into cost-effective screenings to detect ‘hidden diseases’ in larger populations. Various methods have been developed to make health examinations more accessible, affordable, and efficient for large populations. Screening programs, initially focused on specific diseases, have expanded and become essential for preventive healthcare. Evolution of Screening Programs in Healthcare  Screening and diagnostic tests Screening Test: Diagnostic Test: Used to identify individuals who Used to confirm or rule out the are at higher risk of having a presence of a disease in disease in a population. symptomatic individuals. The screening test (initial examination) helps to identify those who may have a disease (for early detection of disease), but it is the diagnostic test that confirms the presence of the disease and leads to the appropriate treatment. Diagnostic Tests Screening test Screening and diagnostic tests Concept of "lead time“improve the outcome (i.e., survival after diagnosis). In the No Screening scenario, the disease is only detected after symptoms appear, and treatment is initiated later, potentially affecting survival. In the Screening scenario, the disease is detected earlier, during the asymptomatic phase. The earlier detection creates a lead time, which is shown as the period between when the disease is detected by screening and when symptoms would have typically appeared. Uses of screening Case detection "prescriptive screening". E.g., Neonatal screening Control of disease  Research purposes Educational opportunities Case detection "prescriptive screening" Presumptive identification of unrecognized disease which does not arise from a patient’s request. It includes screening for conditions such as: Bacteriuria During Pregnancy Diabetes Mellitus Iron Deficiency Anaemia Deafness in Children Diabetes Mellitus Pulmonary Tuberculosis Haemolytic disease of the newborn, etc. Control of disease "prospective screening" Screening of immigrants from infectious diseases such as tuberculosis and syphilis to protect the home population; Screening for streptococcal infection to prevent rheumatic fever. The screening programme may, by leading to early diagnosis permit more effective treatment and reduce the spread of infectious disease and/or mortality from the disease. Research purposes To obtain more basic knowledge about the natural history of such diseases, as for example, initial screening provides a prevalence estimate and subsequent screening, an incidence figure. The focus of the screening is solely for research, not for offering healthcare or treatment. Educational opportunities Screening programmes (as for example, screening for diabetes) provide opportunities for creating public awareness and for educating health professionals. Criteria for screening The criteria for screening are based on two considerations: 1. the DISEASE to be screened 2. the TEST to be applied Criteria for screening 1. DISEASE the disease condition an important health problem, high prevalence; latent or early asymptomatic stage; a test that can detect the disease prior to the onset of signs and symptoms; there is an effective treatment; Criteria for screening 1. Screening test Acceptability, Repeatability (consistency) Validity (accuracy)  refers to what extent the test accurately measures which it purports to measure. Criteria for screening 1. Screening test: Validity (accuracy) Validity expresses the ability of a test to separate or distinguish those who have the disease from those who do not. Accuracy refers to the closeness with which measured values agree with "true" values. Glycosuria is a useful screening test for diabetes, but a more valid or accurate test is the glucose tolerance test (to screen for type 2 diabetes or prediabetes before you have symptoms of either condition, Or it can help find out whether diabetes is causing existing symptoms). Measures of Validity of a test : Sensitivity Quality criteria for the test Specificity Sensitivity and specificity are usually determined by applying the test to one group of persons having the disease, and to a reference group not having the disease. Example: A population of 1000 people; Screening test result by diagnosis Screening test results Diseased Non-diseased Total POSITIVE (+) Group A. Group B A+B (160) (80) True-POSITIVE False-POSITIVE NEGATIVE (-) Group C. Group D C+D (40) (720) False-NEGATIVE True-NEGATIVE TOTAL 200 800 1000 Group A  those individuals found positive on the test who have the condition or disorder being studied. Group B  those who have a positive test result but who do not have the disease. Group C  those with negative test results but who have the disease. Group D  those who do not have the disease.  How good was the test? 1. How good was the test in correctly identifying those who had the disease? “Of the 100 people with the disease, 80 were correctly identified as “positive” by the test, and a positive identification was missed in 20’’ Sensitivity of the test (true-positive) the proportion of diseased people who were correctly identified as “positive” by the test Sensitivity % = A/ (A+C) *100  80/ (80+20) *100 = 80% A population of 1000 people;  How good was the test? 2. How good was the test in correctly identifying those who did NOT have the disease? “Of 900 people who did not have the disease, the test correctly identified 800 as “negative.” Specificity of the test (true-negative)= the proportion of NON-diseased people who were correctly identified as “Negative” by the test Specificity % = D/ (B+D) *100  800/ (100+800) *100 = 89% A population of 1000 people;  How good was the test? 2. How good was the test in correctly identifying those who did NOT have the disease? “Of 900 people who did not have the disease, the test correctly identified 800 as “negative.” False-negative = (20/100)*100 = 20% False-positive = (100/900)*100 = % A population of 1000 people;  Evaluation of a screening test (a) Sensitivity = a/ (a + c) x 100 (b) Specificity = d/(b + d) x 100  Sensitivity test The ability of a test to identify correctly all those who have the disease, that is "true-positive". A 90% sensitivity  90% of the diseased people screened by the test will give a "true-positive" result and the remaining 10% give a "false-negative" result. Sensitivity = a/ (a + c) x 100  Evaluation of a screening test (b) Specificity = d/(b + d) x 100 Evaluation of a screening test: (c) Predictive value of a positive test = a/(a + b) x 100 (d) Predictive value of a negative test = d/(c + d) x 100  Evaluation of a screening test (e) Percentage of false-negatives = c/(a + c) x 100 (f) Percentage of false-positive = b/(b + d) x 100  Recap and Q&A Today, we covered screening of disease measures with examples: Definition of screening tests Validity components including: Sensitivity test Specificity test Predictive accuracy of test Preparation for Next Session For next session on Disease Surveillance, please:  Read Chapter 3 (Pages 41-64) from "Gordis Epidemiology" by Celentano DD and Szklo M.  Review Chapter 3 (Pages 138-139) from "Parks Textbook of Preventive & Social Medicine" by K. Park.  Familiarize with the concepts of disease surveillance and monitoring using examples from the reading.