Measuring Morbidity
Learning objectives:To list the frequently used measures of morbidity
To compare between incidence and prevalence rates
Methods of measurement
Frequencies of event: One of the methods of measuring is just simply counting.Ex 1: How many cases of asthma occurred in a village during a year?
Ex2: How many cases of DM report to the hospital every day?
Ex3: How many cases of Kala-Azar occurred in May 2010..............etc.
☻ While frequencies are useful in many ways; there is a great danger of misleading by just going with frequencies.
☻ Be careful in comparing the absolute number of cases of certain disease in two populations.
Rate, Ratio and proportion:
The key in epidemiology is relating the frequency {the numerator}, to an appropriate population {the denominator}.
Proportion: A measurement in which those who are included in the numerator must also included in the denominator {the numerator is derived from the denominator}, but not require unit of time {Part/Whole}.
Ex1: The proportion of women over 50 years who had hysterectomy = Women who had hysterectomy in that year / Women over 50 years [in the same year] X Constant.
Ex2: The proportion of successful students in a class = Successful students / total students in the class.
Rate: A measurement in which those who are included in the numerator must also included in the denominator {the numerator is derived from the denominator}, but require unit of time.
Ex1: The rate of development of TB in a village in one year= No. of TB cases in one year / No. of population in that year X Constant.
Ratio: A measurement that obtained by dividing one quantity by anther without implying any relation between the numerator and the denominator and also require unit of time.
Frequently used measures of morbidity
Incidence RateAttack Rate
Secondary Attack Rate
Point Prevalence
Period Prevalence
Incidence rate
Incidence measures the number of new cases of a disease (or other health -related phenomenon) that occur during a specified period of time in a population at riskThe numerator of an incidence rate should reflect new cases of disease which occurred or were diagnosed during the specified period. The numerator should not include cases which occurred or were diagnosed earlier.
The population at risk.
This means that persons who are included in the denominator should be able to develop the disease that is being described during the time period covered.Factors affecting incidence
New risk factorOral contraceptives and increase in thromboembolism;
Food additives and cancer
New virus (HIV and AIDS)
Changing habits
Increased smoking and lung cancer
Fluoridated water and decrease in dental caries
Changing virulence of causative organisms
Drug-resistant bacteria and deaths from infection (TB)Influenza virus mutation Increase influenza
Drug resistance to malaria prophylaxis and increase in malaria
Changing of intervention programmes
vaccination against measles ( measlesPolio eradication campaigns ( polio
Chemoprophylaxis ( meningitis, Rheumatic diseases
Selective migration of susceptible persons to an endemic area( incidence
Population patternAging ( Degenerative diseases
Reporting
Increase reporting ( incidenceScreening
Early detection of cases ( incidence
New diagnostic tools
New diagnostic tools ( detection of casesAttack Rate
An attack rate is a variant of an incidence rate, applied to a narrowly defined population observed for a limited time, such as during an epidemic.The attack rate is usually expressed as a percent.
Example
Of 75 persons who attended a picnic, 46 subsequently developed gastroenteritis. Calculate the attack rate of gastroenteritisAttendees = 75 ILL = 46
Attack rate = (46 75) X 100 = 61%
Secondary Attack Rate
A secondary attack rate is a measure of the frequency of new cases of a disease among the contacts of known cases.Prevalence
Prevalence measures the number of cases (new and old) of the disease (or other health-related phenomenon) at a point or period in time.The numerator for prevalence includes All persons ill from a specified cause during a specified interval (or at a specified point in time) regardless of when the illness began.
Relationship between incidence and prevalence
Factors affecting Prevalence:
Changes in incidencePrevalence= Incidence x duration.
Changes in disease duration and chronicity
Chronic diseases are accumulating so increase the prevalence
Acute diseases of a high recovery rate or high case fatality rate decrease prevalence
Intervention programs
If management programs lead to cure decrease prevalence
If only increase survival without cure increase prevalence
Selective attrition
selective migration of cases, or susceptible or immune persons
Changing classifications:
the data coding according to various disease categories often changes, and variations in prevalence may be reported due to misclassification).Divergence between incidence and prevalence:
Disease in which incidence is stable and prevalence is decreasingInterpretation
Rapid recovery from disease for example,A new drug has been discovered.
2. The disease is becoming more fatal for example,
an increase in disease virulence,
increasing failure of treatment, or decreasing application of effective treatment.
3. Selective out migration of cases (perhaps seeking treatment elsewhere).
Disease in which incidence is stable And prevalence is increasing
Interpretation
Slow Recovery (the disease is becoming more chronic) due to
less effective drugs or
Poor compliance (drugs are less frequently used), or
resistance to the drugs is increasing.
The disease is becoming less fatal due to,
Use of a newly discovered, potent drug or
the organism is becoming less virulent.
Early detection of diseases
There is selective immigration of cases to the area.
Incidence is increasing over time, but the prevalence is decreasing
Interpretation
The disease is becoming significantly shorter in duration:New agent, more frequent exposure, more acute.
The disease is becoming more fatal.
Calculate the following rates:
Point prevalence on October 1, 1990
Calculate the incidence rate from October 1, 1990 to September 30,1990,
Period prevalence, October 1, 1990 to September 30, 1991
(Using the midpoint population as the denominator.)
EX: n a nursery of 100 children, 60% are immunized against measles. On child travelled and return with measles and infected other 25 children. What is the secondary attack rate?
26039 X 1100 =67%
Example 2: TB Cases: Raw Data
Newly Diagnosed No TBWeek with TB Diagnosed
------------------------------------------------------------------------------------------------------
0 0 1,025
1 105 920
2 180 740
3 390 350
4 325 25
a. For weeks 14, calculate weekly prevalence and incidence rates, and express them as decimals.
TB: Incidence and Prevalence rates
Individuals Weekly
Total Cases at Risk at Incidence Prevalence
Week New Cases So Far Start of Week Rate per 100 Rate per 100
1
2
3
4
Individuals Weekly
Total Cases at Risk at Incidence Prevalence
Week New Cases So Far Start of Week Rate Rate
--------------------------------------------------------------------------------------------------
1 105 105 1,025 10.2 10.2
2 180 285 920 19.6 27.8
3 390 675 740 52.7 65.9
4 325 1,000 350 92.9 97.6
Example 1: In a longitudinal study investigating the relationship between air pollution and the occurrence of asthma, 10,000 individuals were examined in January 2000. Of the 10,000 persons, 7000 were exposed to air pollution and 3000 were not. At this examination, 100 cases of asthma were discovered, 70 among those exposed to air pollution. All the persons initially examined who did not have asthma were available for subsequent repeated examinations over the next five years. These revealed 300 new cases of asthma in the total group, including 40 among those not exposed to air pollution.
Calculate the prevalence rate of asthma in January 2000.
100/10,000 X 1000 = 10 cases of asthma per 1000 persons in January 2000Calculate the Prevalence rate among those exposed to air pollution
70/7000 x 1000 = 10 cases of asthma per 1000 persons in January 2000
Calculate the prevalence rate among those not exposed to air pollution
30/3000 x 1000 = 10 cases of asthma per 1000 persons in January 2000Calculate the incidence rates of asthma over the 5 years.
300/9900 x 1000 = 30.3 or 30 cases per 1000 persons during the 5-year periodCalculate the average incidence rate of asthma in 1year.
30/5 = 6 cases per 1000 personsCalculate the incidence rate in those exposed to air pollution
260/7000-70 x 1000 = 37.5 or 38 cases per 1000 persons during the 5-year periodCalculate the incidence rate in those not exposed to air pollution
40/3000 - 30 x 1000 = 13.5 or 14 cases per 1000 persons during the 5-year periodInterpret the finding.
Example 2:In the first week of October, 2010, 100 out 800 college students developed Influenza A virus infection. All of the students were at risk for developing Influenza.
In the second week, 50 students also get the infection besides 400 subjects among the student families (average 5 members per family).
Calculate the attack rate and secondary attack rate of Influenza A virus in first week.
Which measurement can determine the contagiousness of the disease?
The attack rate = 100 / 800 X 100 = 12.5% or 13%
The secondary attack rate = 400/ (5X100) X 100 = 80% among family members
Secondary attack rate
Al Kindy College of Medicine
Community Medicine Department Lab. 4 Practical Lab. Of General Epidemiology د يوسفHYPER13PAGE HYPER15
10Incident (new) cases
Prevalent (existing) cases