Point Prevalence

Note: The "point" in point prevalence may, for all practical purposes, consist of a day, several days, or even a few weeks - depending on the time it takes to examine the population sample.

Formula

• When the term "prevalence rate" is used without further qualification, it is taken to mean point prevalence.
• Point prevalence can be made specific for age, sex, and other relevant factors.

Types of Prevalence

Illustrative Diagram

• Point prevalence (Jan 1): Cases 1, 2, and 7 (cases that were present/active on Jan 1)
• Point prevalence (Dec 31): Cases 1, 3, 5, and 8 (cases active on Dec 31)
• Incidence (Jan-Dec): Cases 3, 4, 5, and 8 (new cases that started during the year)
• Period prevalence (Jan-Dec): Cases 1, 2, 3, 4, 5, 7, and 8 (all cases present at any time during the year)

Relationship Between Prevalence and Incidence

• P = Prevalence
• I = Incidence
• D = Mean duration of illness

Prevalence = 10 × 5 = 50 per 1,000 population

• Diseases with long duration (e.g., tuberculosis, diabetes) have high prevalence relative to incidence.
• Diseases that are acute and short-lived (e.g., food poisoning) or rapidly fatal (e.g., some cancers) have low prevalence relative to incidence.
• A treatment that prevents death but does not produce recovery can paradoxically increase prevalence by prolonging duration.

Period Prevalence

• Cases that arose before the period but extended into or through it
• Cases that arose during the period itself

Formula

Period prevalence is considered a less commonly used measure compared to point prevalence.

Incidence Rate

Formula

Important: Incidence rate must always include the unit of time in the expression (e.g., "per 1,000 per year"). Writing "16.7 per 1,000" alone is inadequate.

Key Characteristics of Incidence Rate

Special Incidence Rates

a. Attack Rate

b. Secondary Attack Rate

Uses of Incidence Rate

• Monitoring effectiveness of disease control programmes
• If incidence is rising - may indicate failure of control programmes or improved reporting
• Reveals changes in disease aetiology (agent, host, environment)
• Analyzes differences across socioeconomic groups and geographical areas
• Essential for aetiological research and evaluating preventive/therapeutic measures

Visualizing All Three Measures Together

Summary Comparison

Case-Control Study

Definition & Key Features

1. Both exposure and outcome (disease) have already occurred before the study begins
2. The study proceeds backwards - from effect to cause
3. It uses a control/comparison group to support or refute an inference

The unit is the individual (not the group). The focus is on a disease that has already developed.

Framework - The 2×2 Table

• Exposure rate in cases = a/(a+c)
• Exposure rate in controls = b/(b+d)
• If a/(a+c) > b/(b+d), an association exists between the exposure and disease
• Measure of association = Odds Ratio (OR) = (a × d) / (b × c)

  Note: The Relative Risk (RR) cannot be directly calculated from a case-control study. The OR approximates RR when the disease is rare.

Basic Steps

1. Selection of cases and controls
   • Cases: use incident (newly diagnosed) cases; define diagnostic criteria before study begins
   • Sources: hospitals OR general population (disease registry)
   • Controls: must be comparable to cases in age, sex, occupation, social status (matching)
2. Matching - to control for confounding variables
3. Measurement of exposure - via interviews, questionnaires, or past records (hospital/employment records)
4. Analysis - exposure rates + Odds Ratio

Real-World Example: Vaginal Adenocarcinoma & DES (1971)

Types of Bias in Case-Control Studies

Advantages & Disadvantages

Cohort Study

Definition & Key Features

1. Cohorts are identified prior to appearance of the disease
2. Study groups are observed over time to determine disease frequency
3. The study proceeds forward - from cause to effect

Concept of "Cohort"

• Birth cohort: all persons born in the same year (e.g., birth cohort of 2010)
• Exposure cohort: persons exposed to a common drug, vaccine, or infection
• Marriage cohort: all those married in the same time period

Framework - The 2×2 Table

• Incidence in exposed = a/(a+b)
• Incidence in unexposed = c/(c+d)
• If a/(a+b) > c/(c+d), an association exists
• Measure of association = Relative Risk (RR) = [a/(a+b)] ÷ [c/(c+d)]

  Relative risk can be exactly determined only from a cohort study.

Types of Cohort Studies

1. Prospective Cohort Study

• Disease has not yet occurred when study begins
• Starts in the present, follows into the future
• Examples: Framingham Heart Study, Doll & Hill's study on smoking and lung cancer, Royal College of GPs study on oral contraceptives

2. Retrospective (Historical) Cohort Study

• Outcomes have all already occurred before study begins
• Investigator goes back 10-30 years using existing records, then traces subjects forward to the present
• Also called: historical cohort, non-concurrent prospective study
• More economical and produces results more quickly than prospective studies
• Examples: arsenic and carcinogenesis, lung cancer in uranium miners, angiosarcoma of liver and polyvinyl chloride

3. Combined Retrospective + Prospective

Indications for Cohort Studies

Elements of a Cohort Study

1. Selection of study subjects - general population or special exposure groups
2. Obtaining data on exposure - interviews, questionnaires, medical records, medical examination, environmental surveys
3. Selection of comparison group - general population or a non-exposed cohort
4. Follow-up - both groups followed under identical conditions; may take many years (especially for chronic diseases)
5. Analysis - incidence rates and Relative Risk

Advantages & Disadvantages

"A well-designed cohort study is considered the most reliable means of showing an association between a suspected risk factor and subsequent disease."

Key Comparison: Case-Control vs Cohort

Ultrasound Scanning in Pregnancy by Trimester

First Trimester (< 14 Weeks)

Timing

• Viability scan: 6-10 weeks (transvaginal)
• Dating + Nuchal Translucency (NT) scan: 11-14 weeks (ideally 12 weeks)

What is Assessed (CPT 76801 - for <14 weeks)

Nuchal Translucency (NT) Scan - 11-14 Weeks

• Measures fluid accumulation at the back of the fetal neck
• Increased NT is associated with:
  • Down syndrome (Trisomy 21)
  • Trisomies 13 and 18
  • Turner syndrome (45,X)
  • Triploidy
  • Wide range of structural abnormalities
• Risk for Down syndrome correlates with absolute NT value + maternal age
• NT is reported as a percentile for gestational age (80% of Down syndrome fetuses have NT above 95th percentile)
• Combined with blood tests (PAPP-A, beta-hCG) = Combined First Trimester Screen

Structural Anomaly Detection in 1st Trimester

• ~40-50% of major structural anomalies can be detected at 11-14 weeks
• Always detectable before 14 weeks:
  • Acrania / Anencephaly
  • Encephalocele
  • Ectopia cordis
  • Exomphalos (omphalocele)
  • Gastroschisis
  • Body stalk anomaly
  • Alobar holoprosencephaly
  • Sirenomelia
  • Megacystis
• ~40-50% of major cardiac defects can also be identified at ≤14 weeks (higher rate with dedicated fetal echocardiography)

Second Trimester (14-28 Weeks)

The Anomaly Scan (Mid-Trimester) - 18-20 Weeks

What is Assessed (CPT 76805 - for ≥14 weeks)

Biometry Measurements at Mid-Trimester

• BPD (Biparietal Diameter) - transverse head measurement at thalami level; insonation angle 90° to falx
• HC (Head Circumference) - same plane as BPD
• AC (Abdominal Circumference)
• FL (Femur Length)

Soft Markers for Chromosomal Abnormality at ~18-20 Weeks

Detailed Anatomical Scan (High Risk - CPT 76811)

• 3rd and 4th cerebral ventricles
• Cerebellar lobes with measurements
• Cisterna magna measurement
• Nuchal thickness (15-20 weeks)
• Palate assessment
• Detailed thoracic and abdominal evaluation

Third Trimester (>28 Weeks)

Timing and Indication

• Fetal growth monitoring (suspected IUGR/macrosomia)
• Placental location (known or suspected low-lying)
• Fetal wellbeing assessment
• Amniotic fluid monitoring
• Fetal presentation

What is Assessed

In diabetic pregnancies: "The goals of third-trimester management are to prevent stillbirth and asphyxia while optimizing safe vaginal delivery - involving monitoring fetal growth and biophysical profile or non-stress testing at least weekly."

Summary Table: Scans by Trimester

Safety Notes

• Ultrasound in pregnancy uses no ionizing radiation - considered safe
• Follow ALARA principle; thermal index (TI) kept ≤0.7
• Spectral Doppler should be reserved for specific clinical indications (higher energy output)
• Color Doppler may be used; necessary for detailed first-trimester cardiac evaluation

Diet History - S PERNPARK Mnemonic

Key Methods Explained in Detail

1. Food Balance Sheet Method

• National / macro level
• Estimates per capita food availability from production + imports - exports - non-food use
• Does NOT measure actual individual intake
• Used by FAO for international comparisons

2. Inventory / Larder Method

• Household level
• Records food stocks at beginning and end of a period
• Includes all purchases and home production during that period
• Estimates food available for consumption (not actual intake)

3. Weighment Method (Prospective)

• Gold standard for individual intake
• All food is weighed before eating
• Highly accurate but expensive, time-consuming, and may alter eating behavior
• Used in metabolic ward studies

4. 24-Hour Recall Method

• Individual level, retrospective
• Subject recalls everything consumed in the past 24 hours via interview
• Quick, cheap, no literacy required
• Limitation: single day may not represent usual intake; subject to memory bias
• Best when repeated over multiple days

5. Dietary Score Method

• Simple screening; assigns scores for consumption of protective foods
• Useful for quick community surveys

6. Food Frequency Questionnaire (FFQ)

• Subject reports how often (daily/weekly/monthly) they eat specific foods
• Long reference period (past month/year)
• Indirect method - does not measure exact quantity
• Most widely used in epidemiological studies

7. Duplicate Sample / Chemical Analysis

• Most precise method
• Exact duplicate of all food consumed is chemically analysed in laboratory
• Used in research settings

8. Expenditure Pattern Method

• Based on household food expenditure records
• Estimates nutritional status indirectly via economic data

9. Diet History Method (Burke's Method)

• Detailed in-depth interview about usual food intake
• Combines: 3-day food diary + 24-hour recall + food checklist
• Assesses meal patterns, dietary habits, food preferences
• Requires ~90 minutes and trained interviewer

Classification: Prospective vs Retrospective

Important note: The exact mnemonic "S PERNPARK" is a teaching aid used in Indian PSM coaching/notes and is not formally printed in Park's textbook. The textbook lists these methods under "Assessment of Nutritional Status - Dietary Surveys" without using this specific acronym. If your notes or coaching material assigns slightly different full forms to any letter, those should be given priority in your exam preparation.

Assessment of Nutritional Status - Dietary Assessment as per Park's

1. Clinical examination
2. Anthropometry
3. Biochemical / Laboratory evaluation
4. Functional assessment
5. Assessment of dietary intake ← (Diet Survey)
6. Vital and health statistics
7. Ecological studies

5. Assessment of Dietary Intake (Diet Survey)

"The value of nutritional assessment is greatly enhanced when it is supplemented by an assessment of food consumption. Direct assessment of food consumption involves dietary surveys which may be household inquiries or individual food consumption surveys."

Methods of Diet Survey (as per Park's)

(i) Weighment of Raw Foods

• Most widely employed method in India
• Practicable and fairly accurate when properly carried out
• Survey team visits households and weighs all food going to be cooked and eaten, plus food wasted/discarded
• Duration: 1 to 21 days - commonly 7 days = "one dietary cycle"

(ii) Weighment of Cooked Foods

• Foods analyzed in the state they are normally consumed
• More accurate in terms of actual nutrient intake
• Not easily acceptable among people (culturally less acceptable)

(iii) Oral Questionnaire Method (24-Hour Recall)

• Useful for large-scale surveys in a short time
• Inquiries made retrospectively about nature and quantity of foods eaten in the previous 24 or 48 hours
• If properly carried out, gives reliable results
• Can also collect data on dietary habits and practices

What the Data is Translated Into

• Cereals
• Pulses
• Vegetables
• Fruits
• Milk
• Meat, Fish, and Eggs

Reference used: ICMR publication - "Nutritive Value of Indian Foods"

7. Ecological Studies (Indirect Dietary Assessment)

(a) Food Balance Sheet

• Indirect method of assessing food consumption
• Food supplies related to census population to derive per capita supply availability
• Refers to the country as a whole - conceals regional and group differences
• Advantage: Cheaper and simpler than direct assessment
• Gives indication of general pattern of national food consumption

(b) Socio-economic Factors

• Family size, occupation, income, education, customs, cultural patterns, feeding practices of children and mothers - all influence food consumption patterns

(c) Health and Educational Services

• Primary health care services, feeding programmes, immunization programmes

(d) Conditioning Influences

• Parasitic, bacterial, and viral infections that precipitate malnutrition
• Requires an "ecological diagnosis" before prevention measures can be implemented

What Diet Survey Provides

• Information about dietary intake patterns
• Specific foods consumed
• Estimated nutrient intakes
• Indicates relative dietary inadequacies judged by present standards
• Valuable for planning health education activities
• Helps assess changes needed in agriculture and food production

Summary Table

Note on "S PERNPARK": Park's textbook does NOT use this mnemonic. It simply lists the above methods under "Assessment of Dietary Intake" (Method 5 of the 7 assessment methods). The mnemonic is a coaching/notes-based teaching aid not present in the original textbook.