community diagnosis

• III. Measures of Morbidity:

• Morbid means disease. Morbidity is an important part of community health. It gives an idea about disease status in that community.
• The most important measures of morbidity are incidence, prevalence & attack rate.
Incidence: is the most important measure of morbidity. It is the occurrence of an event or a characteristic over a period of time.

No. of new events occurring during a period of time

Incidence rate = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x 1000
Total population at risk of getting that event

Notes:

. Population should be free of disease at beginning of study.

. The population at risk should be susceptible and exposed to the disease, (everyone in denominator must have the potential to be part of the numerator).

. It is a measure of new events therefore it’s a measure of risk of disease but must be for a specific time period.

. Time is an integral part of incidence. If it is not mentioned it will not be a rate, it will be a proportion.


• Uses of incidence:
• Describe the rate of diseases occurrence overtime.
• Assess patient survival from diseases.
• Compare the risk of disease between two or more populations (relative risk).
• Since the incidence rates are affected by any factor that affects the development of a disease, they can be used to detect the etiologic factors.
• But note that, the incidence rate is obtained from cohort studies (follow up studies).

• E.g. a study examined the occurrence of TB in a district. A total of 10000 persons who have no TB were followed for the year 2010. It was seen that 15 persons developed the disease in that year.
• No. of new cases of TB in year 2010
• Incidence rate of TB = ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x 1000
• in the village Total persons at risk of having TB
• during 2010
• 15
• Incidence rate = ـــــــــــــــ x 1000 = 1.5 cases of TB /1000
• 10000 population in that year

• Prevalence: is the presence of the disease in the community whether old or new cases in a well defined population.
• No. of new and old events occurring
• at a point of time
• Prevalence rate = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x k
• Total population at risk of getting that event
Uses of prevalence:
1. Describes the burden of a disease in a community, thus it's important for health care planner to put and implement the effective health programs for the control and prevention of any disease.
2. Define the rate of clinical characteristics in subjects with specified diseases.
3. Estimates the probability of having a disease given the result of a diagnostic test result.
But note that, prevalence rate is obtained from cross-sectional studies (community studies).


• There are two types of prevalence:
• Point prevalence: is the no. of events that are present at a point in time whether old or new cases. This is the most used form of prevalence.
• No. of new and old events occurring
• at a point in time
• Point Prevalence rate= ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x1000
• Total population at risk of getting that event
• 2. Period prevalence: is the no. of cases of disease whether old or new
• that are present within a period of time (usually a calendar year).
• No. of new and old events occurring
• during a period of time
• Period Prevalence rate= ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x 1000
• Total population at risk of getting that event

1

2

3

4


5

P1 p2

Incidence : cases 1 , 2 & 4

Point prevalence p1 : cases 1,2, 3 & 5 , p2 : cases 2 , 4 & 5

Period prevalence : cases 1 , 2 , 3 , 4 & 5

Period of time

1 year

• Relation between incidence & prevalence:

• Prevalence rate is influenced by both the incidence of disease and its duration. It sometimes approximates the incidence rate if the course of disease is short.
• Prevalence = Incidence × Duration of disease (P = I×D)
• Example:
• There were 120 cases of typhoid fever in a small city (50000 pop.) at 1st Jan.2002. At the end of this year there were 30 added cases of that disease. At April of that year 60 cases (old and recently diagnosed) .
• Calculate the point prevalence (at April,2002) and period prevalence for the year 2002.

• No. of new and old cases of typhoid fever at April

• Point Prevalence = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــx 1000
• rate Total population at risk of getting that event
• 60
• Point Prevalence = ـــــــــــــــــــــــ x 1000 = 1.2 cases of typhoid fever
• rate 50000 per 1000 pop at April, 2002.
• No. of cases present at the beginning of 2002 + new
• events occurring during that period of time
• Period Prevalence= ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x1000
• rate Total population at risk of getting that disease
• 120 + 30
• Period Prevalence = ـــــــــــــــــــــــــــ x 1000 = 3 cases of typhoid fever/1000
• rate 50000 population during 2002


• Attack Rate: is an incidence rate during a limited time period. For infectious diseases, the attack rate is the number of secondary cases occurring within the accepted incubation period divided by the number of susceptible individuals in a closed group exposed to the primary (index) case.
• No. of new events occurring during a limited time period
• Attack rate = ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x 1000
• Total population at risk of getting that event

• Example

• A study was done to assess the health status of a community with a total population of 50000. It was seen that in 1 year:
• Live births = 2000
• Total deaths = 500
• Stillbirths = 90
• Neonatal deaths = 300
• Deaths due to leukemia = 15
• Cases of leukemia = 45
• Calculate the main fertility, morbidity, and mortality measures of this community.

• Answer

• Fertility measures:
• No. of live births during one year
• *Crude Birth Rate = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x 1000
• Midyear population
• 2000
• Crude Birth Rate = ــــــــــــــ x1000 = 40/1000 pop.
• 50000
• Mortality measures:
• No. of stillbirths during one year
• *Stillbirth rate = ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــx1000
• Total births (stillbirths + live births)
• 90
• Stillbirth rate = ـــــــــــــــــــــــــــــ x1000 = 43.1/1000 total births.
• 90+2000


• Deaths among neonates(<4 weeks of age)
• during one year
• *Neonatal Mortality Rate = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x1000
• Total live births
• 300
• Neonatal Mortality Rate = ــــــــــــ x 1000 = 150/1000 live births.
• 2000
• Deaths due to leukemia during one year
• *Cause specific = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x1000
• mortality rate Total population at risk of getting the disease
• 15
• Cause specific = ــــــــــــــــــــــــ x1000 = 0.3/1000 pop. died due to leukemia.
• mortality rate 50000

• Deaths due to leukemia during one year

• *Case fatality rate = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x 100
• Total no. of leukemia cases in that year
• 15
• Case fatality rate =ـــــــــــــــ x 100 = 33.4% of leukemia cases died in that year.
• 45
• No. of deaths from leukemia
• During one year
• *Proportional mortality = ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــx100
• Total no. of deaths in that year
• 15
• Proportional mortality = ــــــــــــــــx100 = 3% of all deaths were due to leukemia.
• 500


• Total deaths (all ages, both sexes, all causes)
• *Crude Death Rate = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x1000
• Midyear population
• 500
• Crude Death Rate = ــــــــــــــــــــ x 1000 = 10/1000 pop.
• 50000
• Morbidity measures:
• No. of new cases of leukemia occurring during a
• period of time
• *Incidence rate= ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــx1000
• Total population at risk of getting the disease
• 45
• Incidence rate= ــــــــــــــx1000 = 0.9/1000 pop. is the incidence of leukemia.
• 50000