PGIM MD Community Medicine cheat sheet

Incidence rate = new cases in a period ÷ population at risk at start of period; it measures risk and requires a defined at-risk denominator.

Prevalence = all existing cases at a point (point prevalence) or during a period (period prevalence) ÷ total population; prevalence ≈ incidence × mean duration of disease when disease is stable.

Attack rate = new cases among exposed ÷ total exposed during an outbreak; secondary attack rate uses susceptible contacts of primary cases as the denominator.

Sensitivity = TP ÷ (TP + FN); specificity = TN ÷ (TN + FP); positive predictive value depends on prevalence, so PPV = sensitivity × prevalence ÷ (sensitivity × prevalence + (1 − specificity) × (1 − prevalence)).

Relative risk = incidence in exposed ÷ incidence in unexposed (cohort studies); odds ratio approximates RR when disease is rare (<10%) and is the standard measure in case-control studies.

Attributable risk (risk difference) = incidence in exposed − incidence in unexposed; population attributable fraction = (Ipopulation − Iunexposed) ÷ Ipopulation.

Herd immunity threshold = 1 − 1/R0; for measles (R0 12–18) this is roughly 92–95%, for polio ~80–85%, for influenza ~50–70%.

Crude death rate = deaths in a year ÷ mid-year population × 1000; age-standardized rates are needed before comparing populations with different age structures.

Screening test criteria: the condition must be important, have a detectable latent stage, an acceptable test, and effective early treatment; lead-time bias and length bias inflate apparent survival.

National Health Policy (India, 2017) targets raising government health expenditure to 2.5% of GDP and eliminating catastrophic out-of-pocket spending; the 2005 National Rural Health Mission was subsumed into the National Health Mission in 2013.

Ayushman Bharat has two components: Health and Wellness Centres (upgraded sub-centres/PHCs delivering comprehensive primary care) and Pradhan Mantri Jan Arogya Yojana, which covers ₹5 lakh per family per year for secondary and tertiary care.

IPHS norms: a sub-centre serves about 5,000 population (3,000 in hilly/tribal areas); a PHC serves 30,000 (20,000 in hilly/tribal areas); a CHC serves 120,000 (80,000 in hilly/tribal areas) with 30 beds.

The three-tier rural system is sub-centre → PHC → CHC → district hospital; the first point of contact for curative care is the sub-centre/Health and Wellness Centre, with referral upward.

Universal Immunization Programme schedule: BCG, OPV-0 and hepatitis B birth dose at birth; OPV/ pentavalent at 6, 10 and 14 weeks; measles-rubella at 9–12 months and 16–24 months; DPT and OPV boosters at 16–24 months.

National programmes to recall: NTEP for tuberculosis (DOTS, now shorter oral regimens), NVBDCP for vector-borne diseases, NPCDCS for non-communicable diseases, and RMNCH+A for reproductive, maternal, newborn, child and adolescent health.

Cold chain: vaccines are kept at +2 to +8 °C in refrigerators; OPV is stored at −20 °C in the freezer; the vaccine vial monitor changes colour as cumulative heat exposure rises and indicates when a vial must be discarded.

Sampling: simple random, systematic (every kth unit), stratified (random within strata), cluster (whole groups), and multistage; a sampling frame is the list from which the sample is drawn.

Type I error (alpha) is rejecting a true null hypothesis; type II error (beta) is failing to reject a false null hypothesis; power = 1 − beta, conventionally 80%.

Normal distribution: about 68% of observations lie within mean ± 1 SD, 95% within ± 1.96 SD, and 99% within ± 2.58 SD.

Chi-square tests association between two categorical variables; the t-test compares means of two groups; ANOVA compares means of three or more groups; the paired t-test handles before-after measurements in the same subjects.

Confidence interval = estimate ± Z × standard error; a 95% CI that excludes the null value corresponds to p < 0.05 for a two-sided test.

Bias types: selection bias (non-representative sampling), information/measurement bias, recall bias (case-control), and confounding (a third variable associated with both exposure and outcome, controlled by matching, stratification or regression).

Outbreak investigation steps: verify the diagnosis, confirm the existence of an outbreak, define and find cases, describe by time-place-person, form a hypothesis, test it, and institute control measures.

A spot map plots cases geographically; an epidemic curve plots cases by date of onset and its shape (single peak, propagated, or continuous common source) suggests the transmission pattern.

Safe drinking water standards: residual free chlorine 0.2–0.5 mg/L after at least 30 minutes of contact; turbidity below 5 NTU (ideally <1 NTU); pH 6.5–8.5; most probable number for coliforms should be zero per 100 mL.

Water purification sequence: storage/sedimentation → coagulation-flocculation (alum) → rapid sand filtration → disinfection (chlorination or boiling); the Horrocks apparatus and the multiple-tube (MPN) test assess water quality in the field and laboratory.

Sanitation: a sanitary latrine separates excreta from human contact, has a water seal against flies and odour, and prevents groundwater pollution; the Sulabh twin-pit pour-flush design allows alternating pit use until contents are safe.

Air pollution: PM2.5 penetrates alveoli and is the pollutant most consistently linked to cardiorespiratory mortality; the Air Quality Index runs from 0–50 (good) to 401–500 (severe).

Occupational health: pneumoconioses include silicosis (silica dust), asbestosis, byssinosis (cotton dust), bagassosis (sugarcane dust) and farmer's lung; lead exposure causes anaemia, abdominal colic and a blue line on the gums.

Noise: the permissible 8-hour occupational limit is 90 dBA; a 3 dB increase halves permissible exposure time; community noise limits are stricter at night.

Vector control: anopheline mosquitoes breed in clean stagnant water and transmit malaria; Aedes aegypti breeds in domestic containers and transmits dengue, chikungunya and Zika; integrated vector management combines source reduction, larviciding and adulticiding.

Maternal mortality ratio = maternal deaths ÷ live births × 100,000; the leading direct causes globally are haemorrhage, hypertensive disorders, sepsis and obstructed labour.

Antenatal care: at least four visits with the first in the first trimester; iron-folic acid supplementation (60 mg elemental iron + 500 µg folic acid daily) and calcium (500 mg twice daily) are recommended in pregnancy.

Infant mortality rate = infant deaths under 1 year ÷ live births × 1000; neonatal mortality rate covers deaths in the first 28 days; the perinatal period spans 22 completed weeks of gestation to 7 days after birth.

Growth monitoring: weight-for-age detects underweight, height-for-age detects stunting (chronic), weight-for-height detects wasting (acute); mid-upper arm circumference below 11.5 cm in children 6–59 months indicates severe acute malnutrition.

Nutritional deficiencies: vitamin A deficiency causes night blindness and xerophthalmia; iodine deficiency causes goitre and cretinism; iron deficiency causes microcytic hypochromic anaemia; vitamin D deficiency causes rickets.

Breastfeeding: exclusive breastfeeding for the first 6 months, complementary feeding from 6 months with continued breastfeeding to 2 years; colostrum is rich in immunoglobulin A and should not be discarded.

Health education models: the health belief model (perceived susceptibility, severity, benefits, barriers), the transtheoretical model (precontemplation to maintenance), and the PRECEDE-PROCEED planning framework.

Tuberculosis: diagnosis by sputum smear microscopy, CBNAAT/GeneXpert and culture; the standard regimen for new drug-sensitive cases is 2 months of HRZE followed by 4 months of HRE; adherence is supported by DOTS.

Malaria: Plasmodium falciparum causes severe disease and cerebral malaria; diagnosis is by rapid diagnostic test or thick and thin blood films; chloroquine remains effective for P. vivax in most settings, with primaquine for hypnozoites.

Dengue: caused by a flavivirus with four serotypes; warning signs include persistent vomiting, abdominal pain, fluid accumulation and mucosal bleeding; platelet count falls and haematocrit rises in the critical phase.

Rabies: post-exposure prophylaxis comprises wound washing with soap and running water for 15 minutes, rabies vaccine by the intradermal or intramuscular schedule, and rabies immunoglobulin for category III exposures.

Non-communicable diseases share modifiable risk factors: tobacco, harmful alcohol use, unhealthy diet, physical inactivity and obesity; WHO targets a 25% reduction in premature NCD mortality by 2025.

Diabetes diagnosis: fasting plasma glucose ≥126 mg/dL, 2-hour plasma glucose ≥200 mg/dL on an oral glucose tolerance test, or HbA1c ≥6.5%; hypertension is defined as systolic ≥140 or diastolic ≥90 mmHg.

Disease elimination and eradication: eradication means permanent global reduction to zero (smallpox, 1980); elimination means interruption of local transmission in a defined area (e.g., polio, leprosy, measles goals); control means reducing burden to an acceptable level.

Planning cycle: situational analysis → priority setting → objective and target setting → strategy and resource planning → implementation → monitoring and evaluation, with feedback to the next cycle.

Health management functions (POSDCORB): planning, organizing, staffing, directing, coordinating, reporting and budgeting; supervision should be supportive and problem-solving rather than punitive.

Health indicators: life expectancy at birth, under-5 mortality rate, maternal mortality ratio, disability-adjusted life years (DALYs) and quality-adjusted life years (QALYs); DALYs combine years of life lost with years lived with disability.

Cost analysis: cost-effectiveness ratio = cost ÷ health outcome (e.g., cost per DALY averted); cost-benefit analysis expresses both costs and outcomes in monetary terms; cost-utility uses QALYs.

Health care financing: out-of-pocket payments are the least equitable and most impoverishing; risk pooling through insurance and tax-funded systems spreads financial risk across populations.

Health systems framework: six WHO building blocks - service delivery, health workforce, health information, access to essential medicines, financing, and leadership/governance.

Monitoring versus evaluation: monitoring is continuous and tracks process and outputs against plans; evaluation is periodic and judges relevance, effectiveness, efficiency and impact.