4th Epid.pract

MEASURESOF MORBIDITY

By:Dr.Yossra K.Al-RobaiaayAssistant professorFICMS (FM)

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.
How many cases of asthma occurred in a village during a year?
How many cases of DM report to the hospital every day?
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}.
Eg: 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.

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.
Eg: 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.
• Expressed as X:Y
• Eg: Male Female ratio
Eg: 15.7 million individuals with diabetes
250.4 million individual without diabetes = 1 : 15.9
•


• Table 3.3 Frequently Used Measures of Morbidity
• Measure
• Numerator
• Denominator
• (attack rate)
• Number of new cases of disease during specified time interval
• Population at start of time interval
• Secondary attack rate
• Number of new cases among contacts
• Total number of contacts
• Incidence rate(or person-time rate)
• Number of new cases of disease during specified time interval
• Summed person-years of observation or average population during time interval
• Point prevalence
• Number of current cases (new and preexisting) at a specified point in time
• Population at the same specified point in time
• Period prevalence
• Number of current cases (new and preexisting) over a specified period of time
• Average or mid-interval population

3. measures of morbidity

Incidence Rate
Attack 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 risk

The 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 factor
Oral 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


• Factors affecting incidence
Screening
Early detection of cases  incidence

New diagnostic tools

New diagnostic tools  detection of cases

• Factors affecting incidence

Selective migration of susceptible persons to an endemic area incidence
Population pattern
Aging  Degenerative diseases

Reporting
Increase reporting  incidence

EXAMPLE

Among the inpatients of Al Yarmouk Teaching Hospital during the year 2006, a total of 120 were admitted with a primary diagnosis of UTI. For the same year, the hospital had a total of 24000 patients.

The incidence of UTI in that year was = 120/24000 X1000 = 5 per 1000 patients per year.

Attack 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 gastroenteritis
Attendees = 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

Inciedent cases

• Factors affecting Prevalence:

• Changes in incidence
Prevalence= 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

• Factors affecting Prevalence:

• 3. Intervention programs
If management programs lead to cure decrease prevalence
If only increase survival without cure increase prevalence
• 4. Selective attrition
selective migration of cases, or susceptible or immune persons
•
• 5. 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:

A. Disease in which incidence is stable and prevalence is decreasing

Interpretation
1. 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).

Divergence between incidence and prevalence:

A. Disease in which incidence is stable and prevalence is decreasing






Divergence between incidence and prevalence:
• B. 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.

Divergence between incidence and prevalence:

• B. Disease in which incidence is stable And prevalence is increasing
•
•




Divergence between incidence and prevalence:
C. Disease in which 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.
•

Divergence between incidence and prevalence:

C. Disease in which Incidence is increasing over time, but the prevalence is decreasing





Exercise 1: In 2009, 87,151 new cases of amoebiasis were reported among Iraqi civilian population. The 2009 mid-year civilian population in Iraq was estimated to be 29,552,000. Calculate the incidence rate of amoebiasis in Iraq in 2009.
Calculate the incidence rate:

Exercise 1: In 2009, 87,151 new cases of amoebiasis were reported among Iraqi civilian population. The 2009 mid-year civilian population in Iraq was estimated to be 29,552,000. Calculate the incidence rate of amoebiasis in Iraq in 2009.
Calculate the incidence rate:
incidence rate= No. of new cases * K (in a period of time)
Population at risk
Calculate incidence rate: 87,151/ 29,552,000 × 100,000
= 297.4/100,000



Exercise 2: Two surveys were done of the same community 12 months apart. Of 5,000 people surveyed the first time, 25 had mycoplasma. Twelve months later, 35 had the disease, including the original 25.
Calculate the prevalence at the second survey, and compare the prevalence with the 1-year incidence.
•
•

1. Prevalence at the second survey:

x = cases at second survey = 35(the all cases old +new)
y = population = 5,000
x/y × 10n = 35/5,000 × 1,000 = 7 per 1,000
2. Incidence during the 12-month period:
x = number of new cases during the 12-month Period = 35 − 25 = 10
y = population at risk = 5,000 – 25(old cases should be eliminated) = 4,975
x/y × 10n = 10/4,975 × 1,000 = 2 per 1,000


Exercise 3: In 2009, 33,151 new cases of gonorrhea were reported among the Iraqi population. The 2009 mid-year Iraqi population was estimated to be 29,552,000. For these data we will use a value of k=100000. Calculate the 2009 gonorrhea incidence rate for the Iraqi population using these data.


Exercise 3: In 2009, 33,151 new cases of gonorrhea were reported among the Iraqi population. The 2009 mid-year Iraqi population was estimated to be 29,552,000. For these data we will use a value of k=100000. Calculate the 2009 gonorrhea incidence rate for the Iraqi population using these data.

33151 / 29,552,000 × 100,000 =112/100000



Exercise 4: In a survey of patients at a sexually transmitted disease clinic in San Francisco, 180 of 300 patients interviewed reported use of a antibiotics at least once during the 2 months before the interview :calculate The period prevalence of antibiotics use in this population over the last 2 months
•
•

Exercise 5:

In 2010, a US state reported an estimated 253,040 residents over 20 years of age with diabetes. The US Census Bureau estimated that the 2010 population over 20 year in that state was 5,008,863. Calculate the prevalence of diabetes

Exercise 5:

In 2010, a US state reported an estimated 253,040 residents over 20 years of age with diabetes. The US Census Bureau estimated that the 2010 population over 20 year in that state was 5,008,863. Calculate the prevalence of diabetes

The prevalence of diabetes=

253,040/ 5,008,863 *100,000 = 5.05/100,000

Exercise 6:

of 75 persons who attended a summer picnic, 46 subsequently developed gastroenteritis. calculate the attack rate?

Exercise 6: of 75 persons who attended a summer picnic, 46 subsequently developed gastroenteritis. calculate the attack rate?
To calculate the attack rate of gastroenteritis we first define the numerator and denominator:
x = Cases of gastroenteritis among persons who attended the picnic = 46
y = Number of persons at the picnic = 75
Then, the attack rate for gastroenteritis is
46/75 × 100 = 61%


Exercise 7:
In a particular community, 115 persons in a population of 4,399 became ill with a disease of unknown etiology. The 115 cases occurred in 77 households. The total No. of persons living in these 77 households was 424.
• Calculate the incidence rate
• Calculate the primary and secondary attack rate in the affected households, assuming that only one case per
household was a primary (community-acquired) case.

Exercise 7: In a particular community, 115 persons in a population of 4,399 became ill with a disease of unknown etiology. The 115 cases occurred in 77 households. The total No. of persons living in these 77 households was 424.
Calculate the incidence rate
Calculate the primary and secondary attack rate in the affected
households, assuming that only one case per household was a primary (community- acquired) case.
Incidence rate =115/4399 *10.000 = 261/10.000
Primary Attack rate = 77/424 * 100= 18.1%
Secondary attack rate= No. of cases among contacts * K
No. of contacts
= 115-77/424-77*100 = 10.9%

Exercise 8: Seven cases of hepatitis A occurred among 70 children attending a child care center. Each infected child came from a different family. The total number of persons in the 7 affected families was 32. One incubation period later, 5 family members of the 7 infected children also developed hepatitis A. We will calculate the attack rate in the child care center and the secondary attack rate among family contacts of those cases.

Answer :

1. Attack rate in child care center:
x = cases of hepatitis A among children in child care center = 7
y = number of children enrolled in the child care center = 70
Attack rate = y/ x × 100 = 70/ 7 × 100 = 10%
2. Secondary attack rate:
x = cases of hepatitis A among family contacts of children with hepatitis A = 5
y = number of persons at risk in the families (total number of family members—children
already infected) = 32 - 7 = 25
Secondary attack rate =y/x × 100 = 25/5 × 100 = 20% 75

QUIZ:

A study examined depression among persons with dementia. The study recruited 201 adults with dementia. Of those 201, 91 had a prior diagnosis of depression. Over the first year, 7 adults developed depression.

Calculate one year incidence rate of depression among adults with dementia

QUIZ answer:
Incidence = 7/110 (201-91) = 0.063
The one year incidence of depression among adults with dementia is 6.36%

Can also be expressed as 63.6 (64) cases per 1,000 persons with dementia