Screening Test for Disease & Defects Essay

Chapter 11 Screening for Disease in the Community Learning Objectives • Define and discuss reliability and validity, giving differentiating characteristics and interrelationships • Identify sources of unreliability and invalidity of measurement • Define the term screening and list desirable qualities of screening tests Learning Objectives (Cont’d) • Define and discuss sensitivity and specificity, giving appropriate formulas and calculations for a sample problem • Identify a classification system for a disease Screening for Disease • Screening–the presumptive identification of unrecognized disease or defects by the application of tests, examinations, or other procedures that can be applied rapidly. • Positive screening results are followed by diagnostic tests to confirm actual disease. – Example: phenylalanine loading test in children positive on PKU screening Multiphasic Screening • Defined as the use of two or more screening tests together among large groups of people. • Information obtained on risk factor status, history of illness, and physiologic and health measurements. • Commonly used by employers and health maintenance organizations. Mass Screening and Selective Screening • Mass screening–screening on a large scale of total population groups regardless of risk status. • Selective screening–screens subsets of the population at high risk for disease. More economical, and likely to yield more true cases. – Example: Screening high-risk persons for Tay-Sachs disease. – Mass Health Examinations • Population or epidemiologic surveys-purpose is to gain knowledge regarding the distribution and determinants of diseases in selected populations. • No benefit to the participant is implied. Mass Health Examinations (cont’d) • Epidemiologic surveillance–aims at the protection of community health through case detection and intervention (e.g., tuberculosis control). • Case finding (opportunistic screening)–the utilization of screening tests for detection of conditions unrelated to the patient’s chief complaint. Appropriate Situations for Screening Tests and Programs • Social • Scientific • Ethical Social • The health problem should be important for the individual and the community. • Diagnostic follow-up and intervention should be available to all who require them. • There should be a favorable cost-benefit ratio. • Public acceptance must be high. Scientific • Natural history of the condition should be adequately understood. – This knowledge permits identification of early stages of disease and appropriate biologic markers of progression. • A knowledge base exists for the efficacy of prevention and the occurrence of side effects. Screening Test for Disease & Defects Essay

• Prevalence of the disease or condition is high. Ethical • The program can alter the natural history of the condition in a significant proportion of those screened. • Suitable, acceptable tests for screening and diagnosis of the condition as well as acceptable, effective methods of prevention are available. Characteristics of a Good Screening Test • Simple–easy to learn and perform. • Rapid–quick to administer; results available rapidly. • Inexpensive–good cost-benefit ratio. • Safe–no harm to participants. • Acceptable–to target group. Evaluation of Screening Tests • Reliability types Repeated measurements – Internal consistency – Interjudge – • Validity types Content – Criterionreferenced – • • – Predictive Concurrent Construct Reliability (Precision) • The ability of a measuring instrument to give consistent results on repeated trials. • Repeated measurement reliability-the degree of consistency among repeated measurements of the same individual on more than one occasion. Reliability (cont’d) • Internal consistency reliability-evaluates the degree of agreement or homogeneity within a questionnaire measure of an attitude, personal characteristic, or psychological attribute. • Interjudge reliability–reliability assessments derived from agreement among trained experts. Validity (Accuracy) • The ability of a measuring instrument to give a true measure. • Can be evaluated only if an accepted and independent method for confirming the test measurement exists. Validity (cont’d) • Content validity–the degree to which the measurement incorporates the domain of the phenomenon under study. • Criterion-referenced validity–found by correlating a measure with an external criterion of the entity being assessed. Validity (cont’d) Screening Test for Disease & Defects Essay

• Two types of criterion-referenced validity: Predictive validity–denotes the ability of a measure to predict some attribute or characteristic in the future. – Concurrent validity–obtained by correlating a measure with an alternative measure of the same phenomenon taken at the same point in time. – Validity (cont’d) • Construct Validity–degree to which the measurement agrees with the theoretical concept being investigated. Interrelationships Between Reliability and Validity • It is possible for a measure to be highly reliable but invalid. • It is not possible for a measure to be valid but unreliable. Representation of Reliability and Validity Sources of Unreliability and Invalidity • Measurement bias–constant errors that are introduced by a faulty measuring device and tend to reduce the reliability of measurements. – Example: A miscalibrated blood pressure manometer. Sources of Unreliability and Invalidity (cont’d) • Halo effect—the influence upon an observation of the observer’s perception of the characteristics of the individual observed. The influence of the observer’s recollection or knowledge of findings on a previous occasion. – Example: a health care provider’s tendency to rate a patient’s sexual behavior use in a particular manner, based on a general opinion about a patient’s characteristics without obtaining specific information about past sexual behavior. Sources of Unreliability and Invalidity (cont’d) • Social desirability effects – – Respondent answers questions in a manner that agrees with desirable social norms. – Example: Teenage boys might respond to a screening interview about sexual behavior by exaggerating their frequency of sexual activities because these behaviors might be perceived as socially desirable among some male peer groups. Fourfold (2 by 2)Table Measures of the Validity of Screening Tests • Sensitivity–the ability of the test to identify correctly all screened individuals who actually have the disease (a/a+c). • Specificity–the ability of the test to identify only nondiseased individuals who actually do not have the disease (d/b+d). Measures of the Validity of Screening Tests (cont’d) • Predictive value (+)–the proportion of individuals screened positive by the test who actually have the disease (a/a+b). • Predictive value (-)–the proportion of individuals screened negative by the test who do not have the disease (d/c+d). Other Measures from the Fourfold (2 by 2) Table • Accuracy of a screening test – determined by the following formula: (a+d)/(a+b+c+d). • Prevalence – determined by the formula: (a+c)/(a+b+c+d) Sample Calculation Effects of Disease Prevalence on the Predictive Value of a Screening Test • When the prevalence of a disease falls, the predictive value (+) falls, and the predictive value (-) rises. Exhibit 11-4 • Illustrates the importance of positive predictive value in the prostate cancer screening controversy. • PSA routine screening was widespread in the U.S. by 1991. Screening Test for Disease & Defects Essay

• The U.S. Preventive Services Task Force calculated that the harms of PSA screening outweigh the benefits. Relationship Between Sensitivity and Specificity • To improve sensitivity, the cut point used to classify individuals as diseased should be moved farther in the range of the nondiseased (normals). • To improve specificity, the cut point should be moved farther in the range typically associated with the disease. Relationship Between Sensitivity and Specificity (cont’d) Procedures to Improve Sensitivity and Specificity • Retrain screeners–reduces the amount of misclassification in tests that require human assessment. • Recalibrate screening instrument–reduces the amount of imprecision. • Utilize a different test. • Utilize more than one test. Evaluation of Screening Programs • Randomized control trials – Subjects randomly receive either the new screening test or usual care. • Ecologic time trend studies – Compare geographic regions with screening programs to those without. • Case-control studies – – – Cases–fatal cases of the disease. Controls–nonfatal cases. Exposure–screening program. Sources of Bias in Screening • Lead time bias – The perception that the screen-detected case has longer survival because the disease was identified early. • Length bias – – Particularly relevant to cancer screening. Tumors identified by screening are slower growing and have a better prognosis. • Selection bias – Motivated participants have a different probability of disease than do those who refuse to participate. Natural History of Disease Issues in the Classification of Morbidity and Mortality • The nomenclature and classification of disease are central to the reliable measurement of the outcome variable in epidemiologic research. • Nomenclature–a highly specific set of terms for describing and recording clinical or pathologic diagnoses to classify ill persons into groups. Issues in the Classification of Morbidity and Mortality (cont’d) • Classification–the statistical compilation of groups of cases of disease by arranging disease entities into categories that share similar features. Screening Test for Disease & Defects Essay

• Two types of criteria used for the classification of ill persons: – – Causal (e.g., tuberculosis or syphilis) Manifestational (e.g., affected anatomic site: hepatitis or breast cancer) Chapter 12 Epidemiology of Infectious Diseases Learning Objectives • State modes of infectious disease transmission • Define three categories of infectious disease agents • Identify the characteristics of agents • Define quantitative terms used in infectious disease outbreaks • Describe the procedure for investigating a disease outbreak Infectious Diseases (Importance) • They are a significant cause of morbidity and mortality worldwide. • Infectious agents are associated with some types of cancer. • Due to increasing world travel, infected passengers can transmit the communicable disease from within the time span of a longdistance plane flight. • They cause disease outbreaks in institutions. Epidemiologic Triangle • A model used to explain the etiology of infectious diseases. • Recognizes three major factors in the pathogenesis of disease: agent, host, and environment. Diagram of Epidemiologic Triangle Microbial Agents of Infectious Disease • • • • • Protozoa Bacteria • Helminths Viruses • Arthropods Rickettsia Mycoses (fungal diseases) Bacteria • Once were the leading killers, but now are controlled by antibiotics. • Remain significant causes of human illness. • Tuberculosis and salmonellosis are common diseases caused by bacteria. • Emergence of antibiotic-resistant strains a growing concern. Viruses • A microorganism composed of a piece of genetic material (RNA or DNA) surrounded by a protein coat. To replicate, a virus must infect a living cell. • Viral hepatitis A, herpes, and influenza are caused by viruses. Rickettsia • A genus of bacteria that can grow within cells. • Ectoparasites (e.g., fleas, lice, and ticks) transmit the majority of rickettsial agents, which cause a variety of diseases. • Rickettsial agents produce typhus fever, Q fever and Rocky Mountain spotted fever. Mycoses (Fungal Diseases) • Mycoses cause diseases such as coccidioidomycosis, ringworm, and athlete’s foot. – Example of disease: A fall 2012 outbreak of fungal meningitis was associated with a contaminated steroid medication and associated with more than 400 cases and 30 deaths in at least 19 states. • Opportunistic mycoses infect immunocompromised patients. – Candidiasis, cryptococcosis, and aspergillosis. Protozoa • Microscopic single-cell organisms. • Responsible for diseases, such as malaria, amebiasis, babesiosis, cryptosporidiosis, and giardiasis. • Example: malaria is transmitted by mosquitos in endemic areas. Screening Test for Disease & Defects Essay

Helminths • Organisms found most frequently in moist, tropical areas. • Include intestinal parasites such as roundworms, pinworms, and tapeworms. • Are responsible for trichinellosis and schistosomiasis. Arthropods • Act as insect vectors that carry a disease agent from its reservoir to humans. • Examples: mosquitos, ticks, flies, mites, and other insects. • Transmit diseases such as Dengue fever, Lyme disease, viral encephalitis, Rocky Mountain spotted fever, trypanosomiasis, and leishmaniasis. Characteristics of Infectious Disease Agents • Infectivity The capacity of an agent to enter and multiply in a susceptible host and produce infection or disease. – Polio and measles are diseases of high infectivity. – Measured by the secondary attack rate. – Characteristics of Infectious Disease Agents • Pathogenicity The capacity of the agent to cause overt disease in the infected host. – Measles is a disease of high pathogenicity, whereas polio is a disease of low pathogenicity. – Measured by the ratio of the number of individuals with clinically apparent disease to the number exposed to an infection. – Characteristics of Infectious Disease Agents (cont’d) • Virulence – – – – Refers to an agent’s capacity to induce disease in the host. Sometimes used as a synonym for pathogenicity. Measured by the ratio formed by the number of total cases with overt infection divided by the total number of infected cases. If fatal, use case fatality rate (CFR). Characteristics of Infectious Disease Agents (cont’d) • Toxigenicity Refers to the capacity of the agent to produce a toxin or poison. – The pathologic effects of agents for diseases such as botulism and shellfish poisoning result from the toxin produced by the microorganism rather than from the microorganism itself. – Characteristics of Infectious Disease Agents (cont’d) • Resistance – The ability of the agent to survive adverse environmental conditions. • Antigenicity – The ability of the agent to induce antibody production in the host. Related to immunogenicity. Host: Definition (Refer to Glossary) • A person (or animal) who permits lodgment of an infectious disease agent under natural conditions. Host • Once an agent infects the host, the degree and severity of the infection will depend on the host’s ability to fight off the infectious agent. • Two types of defense mechanisms are present in the host: nonspecific and disease-specific. Nonspecific Defense Mechanisms • Examples include skin, mucosal surfaces, tears, saliva, gastric juices, and the immune system. • Host responses to infectious agents – immunity may decrease as we age. – nutritional status of the host – Genetic factors Disease-Specific Defense Mechanisms • Immunity (resistance) of the host to a disease agent. • Types of immunity: Active: A disease organism stimulates the potential host’s immune system to create antibodies against the disease. Long lasting, but requires time to develop. – Passive: short-term immunity provided by a preformed antibody. – Active Immunity • Natural, active or natural immunity – Results from an infection by the agent. – Example: A patient develops long-term immunity to measles because of a naturally acquired infection. • Artificial, active or vaccine-induced immunity – Results from an injection with a vaccine that stimulates antibody production in the host. Screening Test for Disease & Defects Essay

Passive Immunity • Natural, passive–preformed antibodies are passed to the fetus during pregnancy and provide short-term immunity in the newborn. • Artificial, passive – Preformed antibodies are given to exposed individuals to confer protection against a disease. – Example: Prophylaxis against hepatitis by administration of immune globulin to individuals who have been exposed. Environment • The domain external to the host in which the agent may exist, survive, or originate. • The environment consists of physical, climatologic, biologic, social, and economic components that affect the survival of the agents and serve to bring the agent and host into contact. Reservoirs of Infectious Diseases • The environment can act as a reservoir that fosters the survival of infectious agents. • Examples: contaminated water supplies or food; soils; vertebrate animals. Animal Reservoirs • Animals can be reservoirs of infectious agents. • Zoonoses–infectious diseases that are potentially transmittable to humans by vertebrate animals. Examples: rabies and the plague. Direct Transmission from Reservoir • Spread of infection through person-to-person contact. • Example –Direct contact with the blood or bodily fluids of an infected person as in the spread of sexually transmitted diseases. Indirect Transmission from Reservoir • Spread of infection through an intermediary source: vehicles, fomites, or vectors. – Examples of vehicles – Contaminated water, infected blood on used hypodermic needles, and food. – Examples of fomites – Inanimate objects, such as a doorknob or clothing – laden with disease-causing agents. – Examples of vectors – flies and mosquitos Portals of Exit and Entry • Portal of exit—sites where infectious agent may leave the body, e.g., respiratory passages, the alimentary canal, and the openings in the genitourinary system, and skin lesions. • Agent must exit in large enough quantities to survive in the environment and overcome the defenses at the portal of entry into the host. • Portal of entry–locus of access to the human body, e.g., mouth and digestive system and the mucous membranes or wounds in the skin. Inapparent Infection • A subclinical infection that has not yet penetrated the clinical horizon–No symptoms of infection present. • Important because disease can be transmitted to unsuspecting hosts. • In asymptomatic individuals, clinicians can look for serologic evidence of infection. – Example: Increase in antibodies and enzymes in patients with hepatitis A virus. Incubation Period • The time interval between exposure to an infectious agent and the appearance of the first signs and symptoms of disease. Screening Test for Disease & Defects Essay

• Applies only to clinically apparent cases of disease. • Provides a clue to the time and circumstance of exposure to the agent. • Useful for determining the etiologic agent. Herd Immunity • Immunity of a population, group, or community against an infectious disease when a large proportion of individuals are immune either through vaccinations or prior infection. Generation Time • Time interval between lodgment of an infectious agent in a host and the maximal communicability of the host. • Can precede the development of active symptoms. • Useful for describing the spread of infectious agents that have large proportions of subclinical cases. • Applies to both inapparent and apparent cases of disease. Colonization and Infestation • Colonization–agents multiply on the surface of the body without invoking tissue or immune response. • Infestation–the presence of a living infectious agent on the body’s exterior surface, upon which a local reaction may be invoked. Iceberg Concept of Infection • The tip of the iceberg, which corresponds to active clinical disease accounts for only a small proportion of host’s infections and exposures to disease agents. Ice … Screening Test for Disease & Defects Essay