2.1 - 3: Epidemiology intro, diagnostic testing and screening

Question 1

Define epidemiology:

Answer 1

• The study of the distribution and determinants of disease in human populations
• Studies of distribution are largely descriptive (e.g. geography, time, age, gender, social class, ethnicity, occupation)

Question 2

Define determinants:

Answer 2

• The factors that precipitate disease (aetiological or causal agents)
• e.g. biological (cholesterol), environmental (pollutants), social/behavioural (smoking)

Question 3

Types of population:

Answer 3

• Target population: Population you are drawing inferences from
• Study population: Population you are collecting data from

Question 4

Sources of data in epidemiological investigation:

Answer 4

• Routinely collected data: vital registrations like birth/death/infectious disease data, hospital databases
• Purposely collected data: Surveys, recruitment and follow-up

Question 5

3 Key limitations of routinely collected data:

Answer 5

• Coverage: lack or variance in reporting of certain diseases, patchy sickness certification, cases who did not present to hospital
• Accuracy: Incorrect diagnoses of cause of death/illness
• Availability: Data may be barred by GDPR etc -> Research Ethic Committee approval can be obtained with justification

Question 6

Mortality and morbidity:

Answer 6

• Mortality: Death due to disease in questions
• Morbidity: Being ill with disease in question

Question 7

Issue of disease severity and reporting:

Answer 7

• ‘Disease iceberg’
• Less severe: more cases but less well recorded so unnoticed

Question 8

Incidence and prevalence:

Answer 8

• Incidence: Number of new cases of the disease within a specified period of time
• Prevalence: Number of existing cases of a disease at a particular point in time -> strongly impacted by duration of illness
• Both are typically measured on a relative scale (e.g. incidence is measured in person-years)
• Prevalence measures are susceptible to survival bias

Question 9

CI:

Full name, notation (not confidence intervals)

Answer 9

• Cumulative incidence risk
• Number of people who get disease during a period / number of people free of disease at start point
• Denoted H(t)

Question 10

Crude mortality rate:

Answer 10

• Number of deaths in a specified period of time, divided by average population at risk during that period multiplied by length of study period
• Often not that informative without considering confounders -> Standardised rate required

Question 11

Sensitivity and specificity: (Definitions and notation)

Answer 11

• Sensitivity = probability of diagnosing a true case as diseased
• Specificity = probability of diagnosing a truly non-disease person as non-diseased
• D and D-bar: Diseased status
• T and T-bar: Positive and negative test results
• FC 11
• Values are typically represented as a percentage

Question 12

PPV and NPV:

Answer 12

• Positive predictive value: proportion of persons who are in fact diseased among those who test positive
• Negative predictive value: Proportion of persons who are in fact non-diseased among those who test negative

Question 13

Balancing sensitivity and specificity via cutoff point:

Answer 13

• Receiver operating curve (ROC)
• y: Specificity, x: one - sensitivity
• Typically choose top-left-most point on curve

Question 14

PPV expressed using Bayes thm:

Answer 14

• FC 14

Question 15

Likelihood ratio of a positive test result:

Answer 15

• LR = sensitivity / (1-specificity)

Question 16

Weighting for spec and sens.:

Answer 16

• M = w x sensitivity = (1-w) x specificity
• w can be maximised with respect to study or population criterion
• Study: w = within-sample disease prevalence
• Population: Population disease prevalence

Question 17

Principle and aim of population screening for disease:

Answer 17

• Testing populations at risk who are asymptomatic for a disease
• Aiming to detect disease early -> better prognosis
• Generally cheap procedures followed up with more specific ones
• Often based on bio-markers from blood or urine samples (e.g. PSA, carcinoembryonic antigen)

Question 18

Common screening programmes for cancer:

Answer 18

• Pap smear (cervical)
• Mammography ( breast)
• Colonoscopy (bowel/colorectal)
• Faecal occult blood test (bowel/colorectal)
• Derma check (melanoma)

Question 19

Common screening programmes for foetal abnormalities:

Answer 19

• Alpha-fetoprotein
• Blood tests
• Ultrasound

Question 20

Lead- and length-time and selection bias:

Answer 20

• Lead-time bias: Survival time since diagnosis is longer with screening -> need to compare mortality in screened / non-screened groups
• Length-time bias: Less severe cancers may be screen detected which may not be otherwise implicated prior to death so benefits seem greater
• Selection bias: Sub-groups may be more likely to attend for screening such as those with a family-history