Pathways of Disease: Bridging Pathology and Epidemiology"
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Course
Epidemiology and Diagnosis
Institution
Epidemiology And Diagnosis
These notes examine the crucial role of diagnostic testing at the intersection of pathology and epidemiology. They explore how advancements in molecular diagnostics, immunology, and epidemiological surveillance techniques enhance our ability to detect and characterize diseases at various stages. By...
DIAGNNOSTIC TESTING
PURPOSE OF THE DIAGNOSTIC TEST
1. Screen for disease
2. Confirm disease
3. Monitor disease
1- Screen for disease
General criteria for screening
➔ Effective therapy that is safe and inexpensive must be available
➔ Disease must have a high enough prevalence to justify the expense
➔ Disease should be detectable before symptoms arise
➔ Test must not have many false positives (people misclassified as having disease)
➔ Test must have extremely high sensitivity
Screening for disease is an example of secondary prevention (identify latent disease)
Primary prevention Secondary prevention Tertiary prevention
“Prophylaxis” “Catch it early” “The rehab”
Prevention of disease or Detection of a disease Reduction of disability and
injury process in its earliest stage, the promotion or
before symptoms appear, and rehabilitation from disease
initiation of interventions in
order to prevent progression
of a disease
Regular physical activity to Chest X ray to diagnose Cardiac or stroke
reduce the risk of developing pneumonia rehabilitation programs
cardiovascular disease and
stroke
Examples of screening tests
a- Newborn screening for inborn errors of metabolism: Example are:
➔ Phenylketonuria (PKU) (build up of phenylaniline in body)
➔ Galactosemia
➔ Congenital hypothyroidism
➔ Maple syrup urine disease (Branched-chain alpha-ketoacid dehydrogenase complex
(BCKD) deficiency or severe reduction which is the second enzyme needed for
breakdown of BCAAs like isoleucine, leucine and valine)
,b- Adult screening tests: Examples are:
➔ Mammography for breast cancer
➔ Cervical Papanicolaou (Pap) smear for cervical cancer
➔ Screen for human papillomavirus (HPV) DNA
➔ Colonoscopy to detect/remove precancerous polyps
➔ Fecal occult blood test (FOBT) to detect colon cancer
➔ Bone densitometry scans to detect osteoporosis in women
➔ Fasting lipid profiles (measuring high-density lipoprotein + cholesterol [HDL-CH], low-
density lipoprotein + cholesterol [LDL-CH]) to evaluate coronary artery disease (CAD)
risk
➔ Fasting blood glucose or 2-hour oral glucose tolerance (OGT) test to screen for
diabetes mellitus (DM)
c- Screening for individuals in high-risk populations: Examples are:
➔ Abdominal ultrasound (US) for identifying an abdominal aortic aneurysm (AAA) in
current or previous male smokers 65 to 75 years of age (5%–9% have an AAA)
➔ Gonorrhea screen for individuals with high-risk sexual behavior (e.g., HIV-positive
individuals)
➔ Syphilis screen (rapid plasma regain [RPR] or venereal disease research lab [VDRL])
for individuals with high-risk sexual behavior (e.g., HIV-positive individuals)
➔ Hepatitis C (HCV) test for individuals born between 1945 and 1965 or those with a
history of intravenous drug abuse (IVDA)
➔ Purified protein derivative (PPD) for specific immigrant groups coming from
countries with a high risk for developing tuberculosis; prisoners; HIV-positive
patients
➔ Low-dose computed tomography (CT) scan for lung cancer in smokers with at least a
30 pack-year smoking history; current smokers or those who quit smoking within the
past 15 years
➔ Screen for depression by asking the following questions:
(a) In the past 2 weeks, have you felt hopeless, depressed, or down?
(b) In the past 2 weeks, have you lost interest or pleasure in doing things that you
normally enjoyed?
d- Screening people with symptoms of a disease: Example includes:
➔ Serum antinuclear antibody (ANA) test to rule out autoimmune disease (e.g.,
systemic lupus erythematosus [SLE])
, 2- Confirm the disease: Examples are:
➔ Anti-Smith (Sm) and double-stranded (ds) DNA antibodies to confirm SLE
➔ Chest x-ray to confirm pneumonia
➔ Urine culture to confirm a bacterial urinary tract infection (UTI)
➔ Serum troponins I and T to confirm an acute myocardial infarction (AMI)
➔ Tissue biopsy (Bx) to confirm cancer
➔ Fluorescent treponemal antibody absorption (FTA-ABS) test to confirm syphilis
3- Monitor disease status: Examples are:
➔ HbA1c to evaluate long-term glycemic control in diabetics
➔ International normalized ratio (INR) to monitor warfarin therapy (anticoagulation)
➔ Therapeutic drug monitoring (TDM) to ensure drug levels are in the optimal range
➔ Pulse oximeter to monitor oxygen saturation during anesthesia or asthmatic attacks
OPERATING CHARACTERISTICS OF CLINICAL LAB
a- Terms for test results for people with a specific disease
➔ True positive (TP): When a diagnostic test correctly determines the positive state
(diseased state) of the tested individual
➔ False negative (FN): When a diagnostic test has incorrectly classified a positive state
(person with disease) as negative for disease
b- Terms for test results for people without disease
➔ True negative (TN): When a diagnostic test correctly determines the negative state
(non-diseased state) of the tested individual
➔ False positive (FP): When a diagnostic test has incorrectly classified a positive state
in a person without disease
c- Sensitivity of a test
Sensitivity is the likelihood that a person with a specific disease will correctly be identified with a
positive test result (“positivity in disease”)
➔ Determined by performing the test on people who are known to have the specific
disease for which the test is intended
For example: Serum antinuclear antibody [ANA] test in people with SLE)
Formula for calculating sensitivity: Is TP ÷ (TP + FN)
➔ False negative (FN) percentage determines the test’s sensitivity
- Usefulness of a test with 100% sensitivity (no false negatives)
➔ Normal test result excludes disease (must be a true negative)
➔ Positive test result includes all people with disease:
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