Lecture 11: Screening Tests, Diagnostic Tests, and Surveillance Tests PDF

Screening tests,diagnostic tests and surveillance 23-12-2024 First semester 2024-2025 Dr.Lama Rafieh DDS,MPH Week 11 Screening Screening people for disease – or risk factors which predict disease – is motivated by the potential benefits...

Screening tests,diagnostic tests and surveillance 23-12-2024 First semester 2024-2025 Dr.Lama Rafieh DDS,MPH Week 11 Screening Screening people for disease – or risk factors which predict disease – is motivated by the potential benefits of secondary prevention through early detection and treatment. Screening is the process of using tests on a large scale to identify the presence of disease in apparently healthy people. Screening tests do not usually establish a diagnosis, but rather the presence or absence of an identified risk factor, and thus require individual follow-up and treatment. screening Search for unrecognized disease or defect by means of rapidly applied tests,examination 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.  As the recipients of screening are usually people who have no illness it is important that the screening test itself is very unlikely to cause harm. Screening can also be used to identify high exposure to risk factors. For instance, children’s blood samples can be screened for lead in areas of high use of lead in paint. Types of screening There are different types of screening, each with specific aims: mass screening aims to screen the whole population (or subset) multiple or multiphasic screening uses several screening tests at the same time. targeted screening of groups with specific exposures case-finding or opportunistic screening is aimed at patients who consult a health practitioner for some other purpose. 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. Examples : Visual defects in school children Mamography in women aged 40 years or less Newborn screening program in japan Mass screening is not useful for preventive measures but it is useful for a suitable treatment that will reduce the duration of illness or alter its final outcome. Multipurpose screening The screening of a population by more than one test done simultaneously to detect more than one disease Screening of pregnant women for VDRL,HIV,HBV by seriological tests Multiphasic screening The screening in which various diagnostic procedures are employed during the same screening program. Examples : DM-FBS,Glucose tolerance test Sickle cell anemia-CBC,HB electrophoresis More accurate- we have more than one test to diagnose Targeted screening The screening of selected high-risk groups in population. Examples : Screening fetus for DOWN’S syndrome in a mother who already has a baby with DOWN’S syndrome Screening for familial cancers,HTN and DM Screening for HIV in risk groups Case finding-opportunistic 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 Examples : RHD in children Uses of screening Case detection Prescriptive screening , people are screened for their own benefit ( cancer, DM, HTN ) control of disease Prospective screening , people are screened for the benfits of others ( HIV, STI) Research To know the natural history of a disease Education Public awareness Criteria for choosing a screening test ( program) Criteria for choosing a screening test ( program) Significant burden of disease Detectable and long preclinical stage of disease Adequatly understood natural history of disease Appropriate test available for early detection of disease Facilities for diagnosis of disease Early detection of disease has outcome benefit Effective treatment available for disease Policy of screening program for disease Screening test The screening test itself must be cheap, easy to apply, acceptable to the public, reliable and valid. Inexpensive Acceptable Valid : to measure what it is supposed to measure if it correctly categorizes people into groups with and without disease, as measured by its sensitivity and specificity. Reliable: A test is reliable if it provides consistent results yielding Indicators to evaluate screening test Three key measures of validity: Sensitivity Specificity Predictive value Screening test result True positives _ sick people correctly diagnosed as sick False positives _ healthy people incorrectly identified as sick True negatives _ healthy people correctly identified as healthy False negatives _ sick people incorrectly identified as healthy Sensitivity Ability of a test to identify correctly all those who have a disease ( i.e. true positive ). If the disease is highly sensitive and the test result is negative , you can certain that they do not have the disease. 90% sensitivity means 90% of diseased are screened as true positives and remaining 10% are false negatives Sensitivity= A/(A+C)*100% specificity Ability of the test to identify those who do not have the disease correctly as true negatives If the test is highly specific and test results is positive it means they actually have a disease 90% of specificity means 90% of people are true negatives and remaining 10% will be wrongly diagnosed as diseased Specificity= D/(B+D)*100% Predictive accuracy Determined by predictive value 1. Positive predictive value 2. negative predictive value Useful to know what proportion of patient with abnormal tests results are truly abnormal. They reflect diagnostic power of a test. Positive predictive value Positive predictive value : the predictive value of a positive test indicates the probability that a patient with a positive test result has the disease in question. PPV of 90% means 90% of people who are diagnosed to be positive by the test in fact have the disease in question PPV= A/(A+B)*100% Negative predictive value The predictive value of a negative test indicates the probability that a patient with a negative test result does not have the disease in question. NPV of 90% means 90% of patients who are diagnosed to be negative by the test in fact do not have the disease NPV= D/(C+D)*100% Diagontic tests The first objective in a clinical situation is to diagnose any treatable disease. The purpose of diagnostic testing is to help confirm possible diagnoses suggested by the patient’s signs and symptoms. While diagnostic tests usually involve laboratory investigations (genetic, microbiological, biochemical or physiological), Screening versus Diagnosis A screening test is first performed to see if your heath is on track, and diagnostic test is then performed to either confirm or eliminate potential Surveillance The ongoing , systematic collection , analysis , and interpretation of health data essential to the planning , implementation , and evaluation of public health practice , closely integrated with the timely dissemination to those who need to know. Uses of surveillance Monitoring disease trends Describing natural history of diseases Identifying epidemics or new syndroms Monitoring changes in infectious agents Identifiying areas for research Evaluating hypothesis Planning public health policy Evaluating public health policy/interventions The purpose of public health surveillance ““information for action “ Types of surveillance Passive surveillance Active surveillance Sentinel surveillance Scanning surveillance Passive surveillance Local and state health departments rely on health care providers or laboratories to report cases of disease Primary advantage is efficiency : simple and requires relatively few resourses Disadvantage is possibility of incomplete data due to underreporting Majority of public health surveillance systems are passive Active surveillance Health department contacts health care providers or laboratories requesting information about conditions or diseases to identify possible cases Require more resourses than passive surveillance Useful when it is important to identify all cases Sentinel surveillance Data specifically collected for analysis of disease pattern Focus on data of a certain subgroup rather than the whole population Monitoring of key health events through sentinel : Sites Events Providers Vectors/animals For example collecting data from hospitals Scanning surveillance Focus on data of clinical syndrome occurring before complete progression to disease status , or signs and symptoms occurring prior to diagnosis of disease Examples of syndrome put under surveillance are ( influenza-like illness ILI , acute flaccid paralysis AFP Trigger a more detailed investigation to explore the likelihood of new disease occurrence Also known as syndromic surveillance Inexpensive and faster than systems that requires laboratory confirmation Sources of data Thank you