Risk and frequency: incidence and prevalence

Raj Bhopal, Bruce and John Usher Professor of Public Health,

Public Health Sciences Section, Division of Community Health Sciences,

University of Edinburgh, Edinburgh EH89AGRaj.Bhopal@ed.ac.uk

Risk and frequency: incidence and prevalence- educational objectives

You should understand: Risk is the likelihood of an individual

developing a disease/problem Epidemiology measures risk (actual or absolute

measures) and how this compares with other populations (relative measures).

A risk factor is a characteristic associated with disease.

The meaning of the words rate, ratio and proportion in everyday and epidemiological language.

The principal measures of risk- the incidence and prevalence rates.

Risk

Risk in everyday language and in epidemiology

Risk markers Associations Risk factors Causes

Numbers, proportions, ratios and rates

Epidemiology needs the number and characteristics of disease cases, of people with risk factors, and of the population from which the above people derive.

Numbers of cases, or people with the risk factors comprise the numerator-the top half of the fraction- the population from which they come is the denominator-the bottom part in of factions

The fraction, numerator divided by the denominator, is usually called the rate in epidemiology

A ratio is one number in relation to another, and a rate is a ratio.

Measures in Epidemiology

Numbers of cases Proportional mortality Proportional mortality

ratio Actual/Crude prevalence

and incidence rates Specific prevalence and

incidence rates Standardised rates Standardised ratios

Relative risk Odds ratio Attributable risks Numbers needed to

treat and prevent Life years lost Disability adjusted life

year (DALY) Quality adjusted life

year (QALY)

Incidence rate Dictionary definition: the act of happening, and, the

occurrence or the extent or frequency of occurrence

Count of new cases over a period of time in a population size defined by characteristics (age, sex, etc), and place and time boundaries

Obtain from disease register or cohort study or trial The two key formulae are: New cases

Population-at-risk, or New cases

Time spent by the study population at-risk

Denominator

What might be your denominator for a study defining the incidence of (a) infant mortality (b) the sudden infant death syndrome (‘cot’ death) and (c) myocardial infarction?

What information would you need to make a rational choice?

Incidence rates: a variety of names

Incidence density is also known as incidence rate, person-time incidence rate, instantaneous incidence rate, hazard rate and force of morbidity or mortality

Cumulative incidence is usually simply referred to as incidence rate (or rarely, cumulative proportion)

Incidence: shifting population at risk

Assume that the incidence rate of a disease is 20% per year and we follow up 100 people

How many people at an six months, on average, will develop the disease?

For diseases that occur only once how many are at risk after six months, or a day, or in the first hour?

How many are at risk after one month? How should we take into account this shifting

denominator in large studies? Incidence density is a measure of the occurrence

of disease over a period of time approaching zero

Figure 7.1

time

very short time span approaching zero

Incidence density

Figure 7.2

Numerator:

time

study start - year 1 - year 2 - year 3 - year 4

0 cases 1 2 3

Denominator:(years of observation assuming each case occurs at mid-point of interval and contributes 0.5 years)

10 19.5 280 10 19.5 35.528

Incidence density and a person-time denominator

If the incidence is constant over time then the incidence density is estimated by the probability that a person well at that time will develop the disease in a moderate interval of time, i.e. not a period approaching zero.

When might this assumption of constant incidence be true? In this circumstance using a person-time denominator

estimates the average value of the incidence density. The person-time denominator is the amount of time that the

study population has spent at risk (disease-free, or alive, in the case of mortality studies)

When would this approach be either inappropriate or unnecessary?

Person denominator (cumulative incidence rate)

Ranges from 0 to 1 Measures absolute risk (probability) of disease

e.g. cases/10,000 people = 5% Can be used to construct relative risks Incidence rates can be calculated with

population estimates, e.g., from a census, and disease from a register

Can only be used with cohort studies where study participants are enrolled at about the same time

Time denominator (estimate of incidence density)

Ranges from zero to infinity Not clearly interpreted as a measure of absolute

risk e.g. 50 cases per 1,000 person-years Can be used to construct relative risks Migration loss to follow-up and migration data are

not usually available in population estimates so person-time cannot be calculated

Can be used either when enrolment is at about the same time or when enrolment is spread over time

Prevalence rate

Count of cases (new and old) at a point in time in a population size defined by characteristics (age, sex, etc) and place

Obtained from cross-sectional studies or disease registers

The formula is: All cases

Population-at-risk

Types of prevalence

For prevalence, unlike incidence, include those people who have the disease in the denominator

The point prevalence rate comprises all the cases of a disease that exist at a point in time

Period prevalence is all cases whether old, new or recurrent, arising over a defined period, say a year or two. The denominator is the average population over the period (or mid-point estimate)

Lifetime prevalence is the proportion of the population who have ever had the disease

Figure 7.4

Incident cases

Deaths, emigrations and recovery

Prevalent cases

Figure 7.7

Incident cases

Prevalent cases

Recoveries

Emigrant cases, unmeasured cases occurring abroad, and deaths

RecoveriesPopulation reservoir Immigration

Births

Emigrant and non-measured cases,

deaths

Incidence and prevalence and preferences For studies of the causes of disease the incidence

rate is preferred. Why? For studies of the burden of diseases of short

duration e.g. measles incidence is also preferred. Why?

The prevalence rate is generally preferred as the measure of burden for long-lasting diseases. Why?

For health behaviours and other disease risk factors prevalence is the preferred measure (even in studies of disease causation). Why?

Overall and specific rates

The rate can be subdivided by any characteristic of epidemiological interest eg age, sex, place and time.

Such rates are called specific rates, e.g. age or sex specific rates

Specific rates permit rational and easy comparison of disease patterns in different places and times for they can be directly compared with each other

Why is this not true for overall rates?

Prevalence and incidence In fixed populations, the prevalence is equal to the

incidence rate x average duration of disease It follows that incidence rate = point prevalence rate

duration; and duration = point prevalence rate ÷ incidence rate.

In a dynamic population, however, the prevalence of a disease cannot be predicted from knowledge of the incidence (or vice versa) because of migration into and out of the population, deaths, changing disease rates, changes in prognosis and error in measuring the incidence (or prevalence) accurately.

In practice, either the prevalence and incidence are both measured or a choice of one is made.

Entering diagnosis on a death certificate: exercise

A person who has a feverish illness diagnosed on laboratory tests as influenza, develops cough and shortness of breath shown to be pneumonia, followed by a deep venous thrombosis. The doctors suspect that pulmonary embolus has occurred but before it can be confirmed by tests, the patient collapses and dies unexpectedly. Assume that there is no post-mortem because the relatives refuse permission

Complete the specimen death certificate in table 7.5 for the above person. Take care to order the causes as instructed and note that as the death certificate states that the underlying cause of death goes on line 1(c)

Table 7.5 Specimen death certificate: exercise

I (a) Disease or condition directly leading to death (b) Other disease or condition, if any, leading to I(a) (c) Other disease or condition, if any, leading to I(b)

II Other significant conditions CONTRIBUTING TO THE DEATH but not related to the Disease or condition causing it

CAUSE OF DEATHThe condition thought to be the "Underlying Cause

of Death" should appear in the lowest completed line of Part

Death certificate

Compare your completed certificate with mine below.

1a Pulmonary embolus b Pneumonia c Influenza 11 Deep venous thrombosis

Coding of diagnosis: exercise

Based on your completed death certificate code the causes of death (see table 7.6).

Reconsider your choice of order of causes of death after reading the coding rule from the ICD (see table 7.6).

Death certificate: coded

Compare your completed certificate with mine below.

1a Pulmonary embolus (I26) b Pneumonia (J18) c Influenza (J10.0) 11 Deep venous thrombosis (I80.1)

Lower limb amputation: measurement

To calculate the frequency of disease we need rules to judge whether a case is a case, whether it is new or old and to decide whether to include recurrences

Each study will need to make this decision in the light of its aims

Decisions are not easy except for the incidence of mortality, diseases which are irreversible e.g. amputation of a limb, disease which usually occur only once e.g. measles, or diseases agreed by definition or convention to be lifelong diseases, e.g. diabetes

So how will we define lower limb amputation and measure its prevalence? Where will the data come from?

Operating theatre recordsN = 192

Limb fitting centreN = 66

Hospital discharge data

N = 165

85

1517

75

65 268

Figure 7.1

Class exercise: angina

A health authority (or an equivalent body such as an insurance agency or a managed care organisation) serving 500,000 people wishes to cost and plan a service for the medical and surgical management of angina of the population, with particular emphasis on the numbers of cases requiring surgery.

You are invited to assist. Consider the general principles that you would apply to the task.

Consider the relative merits of incidence, point prevalence, period prevalence and lifetime prevalence.

Summary Risk is the possibility of harm In epidemiology risk is the likelihood of an individual in

a defined population developing a disease or other adverse health problem

In epidemiology the association between risk of disease and both individual and social characteristics (risk factors) is often the starting point for causal analysis

Measures of risk include incidence and prevalence rates.

The incidence rate focuses on new cases prevalence on all cases

In a fixed population prevalence rate is equal to the incidence rate multiplied by the duration of the disease