3 Chapter 4 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.

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.

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. [3] 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.

Table 4.3: Four Types of Diagnostic Tests and Examples

Table 4.4:Recommended Screening Exams/Schedule

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 epidiemiology, 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.

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. [2] 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.

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]