4 Screening and Diagnostic Tests
Screening and Diagnostic Tests
Christy N Bazan
Screening and Diagnostic Tests
Overview
Diagnostic tests and health screenings have a vital role to play in the surveillance of disease in patients, communities, and from a public health perspective. By helping to identify disease early, treatment and interventions can begin in the fight against chronic and infectious disease.
Learning Objectives
By the end of this chapter, you will be able to:
- Know the three primary disease prevention strategies
- Differentiate between screening and diagnostic testing used to determine disease risk
- Know the types and methods of screening programs used for common causes of mortality in communities
- Understand the role of screening programs in public health
- Understand the link between disease screening and types of validity testing
The Three Disease Prevention Strategies
Primary Prevention
Primary prevention consists of measures aimed at a susceptible population or individual. The purpose of primary prevention is to prevent a disease from ever occurring. Primary prevention targets healthy individuals. Activities that limit risk exposure or increase the immunity of individuals at risk help prevent a disease from progressing . Examples of primary prevention include immunizations, use of condoms, abstinence education, and emergency contraception.
Secondary Prevention
Secondary prevention emphasizes early disease detection, and it targets healthy-appearing individuals with subclinical forms of the disease. The subclinical disease consists of pathologic changes but no overt symptoms that are diagnosable in a doctor’s visit. Secondary prevention often occurs in the form of screenings. For example, a Pap smear is a form of secondary prevention aimed to diagnose cervical cancer before progression.
Tertiary Prevention
Tertiary prevention targets both the clinical symptoms or screening results and outcome stages of a disease. It is implemented in patients with symptoms and aims to reduce the severity of the disease as well as any associated disease rebound. While secondary prevention seeks to prevent the onset of illness, tertiary prevention aims to reduce the effects of the disease symptoms once established in an individual. Tertiary prevention examples include rehabilitation efforts for cardiac concerns and strokes.
Overview of Diagnosis
Diagnosis is the act or process of determining or identifying the nature and cause of disease or injury through the following: evaluation of patient history, examination of the patient and review of laboratory or screening results. [1]
Overview of Screenings
Screening of disease involves using tests or procedures to identify disease or health concerns. Screening is most often done on individuals who are often well and do not have any observable signs or symptoms of disease. Most individuals getting screenings done are not aware that an illness is not detected or risk factors for disease exist. Often these screenings are done in combination with disease surveillance, the collection of morbidity or mortality data in a population.
Screening is not the same as diagnosis, but some screening tests are done as diagnostic testing to identify disease or disease risk. Screenings are medical tests that doctors use to check for diseases and health conditions before there are any signs and symptoms. Screenings help find problems early, allowing opportunity for earlier and possibly easier treatment of diseases.
Getting recommended screenings is one of the most important things you can do for. Your health. Depending on your age, sex, and medical history, you may need to be screened for various health diseases or conditions. See Table 4.4 for recommended screening exams your doctor may encourage you to consider.
There are concerns with screening tests which include false-positive results (the wrong results) , over diagnosis of potentially benign conditions, and at times access to the tests may prove challenging for those needing the screening. The benefits of possible early diagnosis, early treatment, and preventive treatment outweigh the concerns some have with screenings.
| What Screening Tests and Results are Used for | What Diagnostic Tests and Results are Used for |
| To detect diseases in the early stages | To detect the presence or absence of disease |
| Used for asymptomatic individuals especially those with Higer risks | Used for individuals who have symptoms to help diagnose disease or with asymptomatic individuals with a positive screening test |
| Depending on the results the screening test would indicate a high or lower probability or risk of disease | The result of the diagnostic test is more likely able to provide a definite disease diagnosis |
Table 4.1: The Main Differences Between Screening Testa and results and Diagnostic Tests and Results
Two Types of Screenings
Disease screening programs have a long history in strategies to control infectious disease epidemics and target chronic disease treatments. There are two types of screenings that help with collecting information that can help address the major causes of morbidity and mortality. These two screening efforts are mass screening and selective screening.
Mass screenings are tests that are administered to total populations groups regardless of disease risk. Some examples of these tests include TB tests, urinalysis, drug screenings, measuring temperatures of individuals to determine possible infection risk, eye exams, pap smears and mammograms.
Selective screenings are tests administered to high-risk population groups. These screening tests are used to help diagnose and detect infectious diseases, chronic diseases, and other conditions in individuals who have specific risk factors. These may be individuals who smoke, are obese, use IV drugs, or engage in unprotected sex or are at increased risk for adverse health concerns or outcomes. Regular screening of these individuals will help better diagnose and treat for possible disease concerns.
Ensuring Screenings help with Disease Diagnosis
For screening tests to be considered useful as well as necessary or recommended, they need to demonstrate reliability and validity.
Reliability is the degree to which an instrument can produce consistent results and consistent results on different occasions when there is no evidence of change. This type of measurement is often about the precision of the testing. [2] Validity is the degree to which an instrument measures what it is supposed to measure, and that the measurement is accurate.
The Gold Standard with any type of screening or medical testing is the definitive diagnosis. This would happen when the diagnosis has been determined by surgery, biopsy, autopsy, or other testing methods. These tests are what are used by many health professionals for verifying, confirming and diagnosis.
Screening measures need to be valid, reliable, and meet the gold standard for testing. When they are patient and health professionals can strategize treatment and care helping to limit morbidity and mortality risks.
| Type of Diagnostic Test | Testing Examples |
| Laboratory Tests | Blood, urine sampling |
| Imaging Tests | CT scan, MRI, Mammography |
| Endoscopy | Colonoscopy, Bronchoscopy |
| Biopsy | Lump or node removal, tissue or cell testing |
Table 4.3: Four Types of Diagnostic Tests and Examples
Table 4.4:Preventive Health Screenings By Age
Screening Calculations for Validity
There are four measures of validity when looking at screening measures. Sensitivity, specificity, positive predictive value and negative predictive value. These four measures are described in the table below.
When using these measures to determine if screening tests are valid, you must consider false positive and false negative results. False positive results may warrant unnecessary medical tests to further investigate results. Patients may experience anxiety because of the false positive result, only to find out that they do not have the disease. With false negative results, the disease is not detected in a patient who has the disease and the disease progresses. Being aware of these issues with screenings is important for patients to understand.
| Type of Screening Test Validity Measures | Description | Calculation |
| Sensitivity | The ability of the test to identify the presence of a disease or illness correctly; True positive and true negative results | |
| Specificity | The ability of a test to identify the absence of a disease or illness correctly; True negative or false positive results | |
| Positive Predictive Values (PPV) | The likelihood a person who has a positive test result does have the disease or illness | |
| Negative Predictive Value (NPV) | The likelihood a person who has a negative test result indeed does not have the disease or illness |
Table 4.5 The Four Measures of Screening Test Validity
The presentation of diagnostic exam results is often in 2×2 tables, such as Table 4.6. The values within this table can help to determine sensitivity and specificity, as well as other epidemiologic value data.
Table 4.6 Diagnostic Testing 2×2 Accuracy Table
Sensitivity and Specificity Testing
In epidemiology, sensitivity and specificity are mathematical calculations used to describe the accuracy of a test that reports the presence or absence of a disease or medical condition.
If individuals who have a disease or medical condition are identified as “positive” and those who are not are identified as “negative” , then sensitivity is a measure of how well the test can identify the positives and specificity is a measure of how well a test can measure the negatives.
Examples of Test Sensitivity and Specificity
If 100 individuals who were screened have a disease and 53 test positive, then the test has a 53% sensitivity. If 100 individuals with no disease are screened and 92 return a negative result, then the test has a 92% specificity.
Example: Hepatitis A Blood Test for Sensitivity and Specificity in a Population
A healthcare provider uses a blood test to determine whether or not individuals have Hepatitis A:
-
-
A total of 1,000 individuals had their blood tested for Hepatitis A.
-
427 individuals had positive findings, and 573 individuals had negative findings.
- Out of the 427 individuals who had positive findings, 369 of them had the disease.
- Out of the 573 individuals who had negative findings, 558 did not have the disease.
- The healthcare provider found that a total of 384 individuals actually had the disease,
- So how accurate was the Hepatitis A blood test in the 1000 individuals?
-
- Using the 2X2 table approach to sort this data, the following chart:
| Blood Test Results | Hepatitis A Present | Hepatitis A Not Present | Total |
| Positive | 369(A) | 58(B) | 427 |
| Negative | 15(C) | 558(D) | 573 |
| Total | 384(A C) | 616(B D) | 1000 |
- Calculation Results for Sensitivity and Specificity:
Sensitivity
-
Sensitivity=(True Positives (A))/(True Positives (A) False Negatives (C))
-
Sensitivity=(369 (A))/(369(A) 15 (C))
-
Sensitivity=369/384
-
Sensitivity=0.961 or 96.1%
Specificity
-
Specificity=(True Negatives (D))/(True Negatives (D) False Positives (B))
-
Specificity=(558 (D))/(558(D) 58 (B))
-
Specificity=558/616
-
Specificity=0.906 or 90.6%
Community and Public Health Benefits of Screening Programs
Disease screening programs have a long history in strategies to control infectious disease epidemics and target chronic disease treatments. [4] Goals of public health screening programs need to be clearly specified and shown to impact case findings.
Understanding the natural history of the disease, knowing what interventions and treatments the scientific and medical community believe work for individuals determined to have disease and finding or having evidence that early intervention improves outcomes are all important considerations for a well-organized public health screening effort.
Screening programs should measure conditions that can control possible epidemics of infectious disease and target treatment for chronic diseases. Understanding the tangible public health benefits of screening efforts boost confidence that the health care system is working and that all patients have been treated fairly and if needed will benefit from treatment that should be made accessible to them.
Tangible Public Health Benefits of Screening Programs
|
Table 4.2: Tangible Public Health Benefits of Screening Programs
The WHO, World Health Organization, states that screening programs should be done only for diseases with serious consequences, screening programs should have clear benefits to people’s health, and screenings should be reliable and not harmful. It is also critical that effective treatments for disease identified in early stages during screening have clear evidence that the treatment is more effective when started before symptoms arise like with Breast Cancer. The public should always have information available to them on screening options and make informed decisions on whether they want to have the screening test. [3]
Key Takeaways
In this chapter you learned about disease prevention strategies, screenings for disease, and diagnostic tests to confirm chronic and infectious diseases. Screening calculations, validity and understanding how test and calculations factor in to accurately and reliably diagnosing disease are important considerations in this chapter. Being aware of public health screening efforts and better understanding disease risk and prevention efforts can help improve your life and the lives of those around us.
- “Diagnosis-Health, United States.” Centers for Disease Control and Prevention. Published June 26, 2023, Accessed July 9, 2024. https://www.cdc.gov/nchs/hus/sources-definitions/diagnosis.htm#:~:text=The act or process of,and review of laboratory data. ↵
- “Measurement Properties: Validity, Reliability, and Responsiveness.” National Center for Chronic Disease Prevention and Health Promotion, Division of Population Health. Published October 31, 2018, Accessed July 15, 2024. https://archive.cdc.gov/www_cdc_gov/hrqol/measurement.htm ↵
- https://www.uclahealth.org/sites/default/files/documents/NewPatientGuide-PrevHealth_FINAL_091721.pdf ↵
- Institute of Medicine (US) Committee on Perinatal Transmission of HIV; National Research Council (US) and Institute of Medicine (US) Board on Children, Youth, and Families; Stoto MA, Almario DA, McCormick MC, editors. Reducing the Odds: Preventing Perinatal Transmission of HIV In The United States. Washington (DC): National Academies Press (US); 1999. 2, Public Health Screening Programs. Available from: https://www.ncbi.nlm.nih.gov/books/NBK230552/ ↵