Indicators of Health and Disease Frequency Measures

Introduction

A health indicator is "a construct of public health surveillance that defines a measure of health (i.e. the occurrence of a disease or other health-related event) or a factor associated with health (i.e. health status or other risk factor) among a specified population". Health indicators represent summary measure that capture relevant information on different health attributes and dimensions, and the performance of the health system. Seen together, these measures attempt to reflect and monitor the health status of a population.

Health indicators are required to answer how healthy is a given community. Health indicators measure the health status of a community; compare the health status of one country with that of another; assess the health care needs; allocate scarce resources; monitor and evaluate health services, activities and programs. Health indicators should have following characteristics:

• Should be valid
• Should be reliable and objective
• Should be sensitive
• Should be specific
• Should be measurable
• Should be policy relevance
• Should be easy to use and interpret by analyst
• Should be understandable by information user

Important attributes to ensure quality of indicators

1. Integrity (no missing data)
2. Internal consistency (realistic, coherent and do not contradict)

Types of Health Indicators

Health cannot be measured in exact measurable terms. Health is multidimensional and each dimension is influenced by numerous factors. Health indicators are therefore numerous:

1. Mortality indicators
2. Morbidity indicators
3. Disability rates
4. Nutritional status indicators
5. Health care delivery indicators
6. Utilization rates
7. Indicators of social and mental health
8. Environmental indicators
9. Socio-economic indicators
10. Health policy indicators
11. Indicators of quality of life
12. Others indicators: social indicators, basic need indicators, health for all indicators.

1. Mortality: The total number of death due to the disease is known as mortality. Mortality data provides the starting point in many epidemiological studies. The death rate or crude mortality rate is calculated as,

CMR (CDR) = Number of deaths in a specified period / Average total population during that period × 10ⁿ

Mortality rates are also measured as specific death rates (disease specific, age specific, sex-specific), case fatality rates, proportional mortality rate, survival rate. The mortality data has following limitations:

1. The incomplete reporting of death
2. Lack of accuracy
3. Lack of uniformity
4. Choosing a single cause of death
5. Changing fashions in diagnosis
6. Disease with low fatality

The mortality data are employed in explaining trends and differentials in overall mortality, indicating priorities for health action and allocations of resources, in designing intervention program and in the assessment and monitoring of public health problems and programs.

2. Morbidity: Morbidity is defined as “any departure subjective or objective, from a state of physiological well-being’. Morbidity (sickness, illness, disability) measures:

• Person who were ill
• The illnesses
• The duration illness

The morbidity is measured by incidence and prevalence. The morbidity data have values as follows:

• Describe extent and nature of the disease load in the community
• Provide more comprehensive and more accurate and clinically relevant information on patient characterization.
• Serve as astarting point for etiological studies and play aimportant role in disease prevention.
• Essential for monitoring and evaluation of disease control activities.

Incidence: Incidence rate is defined as “the number of new cases occurring in a defined population during a specified period of time “. There are two measures of disease incidence:

a. Risk (Cumulative incidence)- It is defined as the probability of disease-free individuals developing a given disease over a specified period, condition on that individualnot dying from any disease during that period. Risk usually refers to the first occurrence of the disease for each disease fee person.

Cumulative incidence = number of new cases of specific disease / Population at risk during that period of time ×1000

Both numerator and denominator include only those individuals who at the beginning of the period are free from the disease and therefore at risk to get it. The incidence rates include attack rate, secondary attack rate, hospital admission rate, etc. Cumulative incidence is,therefore the proportion of the individuals in the disease-free state at the beginning of the period that moves to the disease state during the period.

Characteristics of cumulative incidence

1. It is a proportion.
2. It has no dimension.
3. It varies between zero and one.

The concept of risk requires a period reference that describes the time span over which the new cases are detected since cumulative incidence is proportional to the length of the observation period. The length period must always be reported in interpreting any reported value of cumulative incidence.

b. Incidence rate or incidence density (Person-time concept)- Incidence rate of disease occurrence is the instantaneous potential for change in disease status per unit of time, relative to the size of candidate population at that time.

In some situations, the whole population may not be equally at risk throughout the same time interval. They may join the community at different times and duration, and the duration of exposure is an important determinant of risk (the longer exposure time, the higher presumed risk), therefore, the duration of exposure is an important factor in determining the population at risk (denominator) and thus to obtain the accurate incidence density, to take this into account, exposure units are defined as “person time”, such as:

• Person week of exposure at risk
• Person mouth of exposure at risk
• Person year of exposure at risk

and these can be used as the denominator or the appropriate morbidity rates.

Incidence rate

Or incidence density= Number of new cases of disease during a calendar period / Total person-time exposure

Prevalence: The proportion of all current cases of the disease in a point of time or over a period of time in a given population is called prevalence. Like all proportions, it is dimensionless and can take avalue between zero and one.

Prevalence= Number of individuals having the disease at a particular point in time / Number of individuals in the population at that point in time

Since, thenumerator of prevalence comprises all those who have the disease at that instant, regardless of whatever it was contracted recently (new) or long ago old), therefore, disease of long duration tends to have a higher prevalence than shorter illness.

Prevalence of two types: Point prevalence and Period prevalence

Prevalence helps to estimate the existing magnitude of health and disease problem in the community. Prevalence rate is useful in administrative and planning purposes because it measures the need for treatment, hospital beds and equipments, health facilities and manpower needs.

Relationship between prevalence and incidence

Incidence is a proxy for “risk” whereas prevalence is the best for assessing disease burden or case load in a geographic area. There is a well known relationship between incidence and prevalence.

Prevalence = Incidence × Duration

P = I × D

Assumptions:

• Low prevalence
• Steady state

Difference in characteristics between incidence and prevalence

3.Disability rates: The disability rate may be event type or person type indicators.

a. Event type- The examples of event type disability rates include number of activity restricted days, bed disability days, work loss days, etc.

b. Person type- The limitation of morbidity (confined to bed), limitation of activities (eating, washing), etc. are some examples.

4.Nutritional status indicators: Weight and height, mid arm circumference, height of school children, prevalence of low birth weight (less than 2.5 kg) are some widely used indicators to measure nutritional status.

5 .Health care delivery indicators: The doctor-population ration, doctor-nurse ratio, population bed ratio, population health center ratio are some examples.

6. Utilization rates: To obtain information on health status, the extent of uses of health services are often investigated. For examples, proportion of infants who receive antenatal care, proportion of pregnant women who receive antenatal care.

7. Indicators of social and mental health: The social indicators include suicide, crime, road traffic accidents and drug abuse, smoking, etc. These social indicators provide a guide to social action for improving the health of the people.

8. Environmental indicators: These reflect the qualities of physical and biological environment in which disease occurs and which the people live. These indicators include pollution of air and water, solid wastes, noise, exposure to toxic substances on food or drink.

9. Socio-economic indicators: These indirect indicators are of great importance in the interpretation of indicators of health care. For examples, rate of population increase, per capita GNP, level of unemployment, dependency ratio, etc.

10. Healthy policy indicators: The most important indicator is “allocation of adequate resources” i.e. proportion of GNP agent on health services, proportion of total health resources devoted to primary health care.

11. Indicators of quality of life: The physical quality of life, infant mortality, life expectancy, literacy rate etc. are indicators of quality of life.

12. Other indicators:

a. Social indicators: Population, family population, distribution of income, etc.

b. Basic need indicators: These include calorie consumption, access to water etc.

c. Health for all indicators

References

Atlas, RM and R Bartha. Microbial Ecology:Fundamentals and Applications. The Benjamin Cummins Publication co. Inc., 1998.

Gordis, L. Epidemiology. third edition. 2004.

Maier, RM, IL Pepper and CP Gerba. Environmental Microbiology. Academic press Elsevier Publication, 2006.

park, K. Park's Text Book of social and prevention Medicine. 18th edition. 2008.