Surveillance and measures of disease Chapter 3

Question 1

What is surveillance?

Answer 1

Ongoing systematic collection, analysis, and interpretation of health data essential to the planning, implementation, and evaluation of public health practice closely integrated with the timely dissemination of these data to those who need to know.

Question 2

Active surveillance

Answer 2

A system in which project staff are specifically recruited to carry out a surveillance program. They are recruited to make periodic field visits to health care facilities such as clinics, primary health care centers, and hospitals in order to identify new cases of a disease or diseases or deaths from the disease that have occurred (case finding). e.g. interview physicians, review medical records, survey village and towns.

Question 3

Passive surveillance

Answer 3

Passive surveillance denotes surveillance in which available data on reportable diseases are used, or in which disease reporting is mandated or requested by the government or the local health authority, with the responsibility for the reporting often falling on the health care provider or district health officer. e.g. STD, HIV

Question 4

Natural history of disease

Answer 4

Primary prevention done before the biological onset of the disease
2. 1. Biological onset of a Disease [ Pre-clinical (before accurate diagnosis) - Secondary prevention]
2. Pathological evidence if sought [Pre-clinical (before accurate diagnosis) -Secondary prevention] e.g. during routine screening, mamogram or hpv test cancer found.
3. **Signs and symptoms **of Disease [Clinical - tertiary prevention]
4. Seek medical care [Clinical - tertiary prevention]
5. Diagnosis [Clinical - tertiary prevention]
5. Treatment [Clinical - tertiary prevention]
6. Outcome
* Cure,
* Contro
* Disability
* Death

Question 5

Ratio

Answer 5

Division of one number by another, numbers dont have to be related

Question 6

Rate

Answer 6

The most important; Time is an intrinsic part of the denominator a type of ratio

Question 7

Proportion

Answer 7

Numerator is a subset of the denominator, expressed in percentage

Question 8

Incidence rate

Answer 8

NUMBER OF OUTCOMES/PERSON-YEAR AT RISK

Quantifies the number of new cases of disease that develop in a **population at risk **during a specified time period. The incidence rate of a disease is defined as the number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease. THE PERSON DID NOT HAVE THE DISEASE PREVIOUSLY.
-transition from non-disesed to diseased
-it is a measure of risk of developing a disease among the person at risk

Question 9

Cumulative incidence rate

Answer 9

PERSONS WITH OUTCOME / PERSONS AT RISK

-** period of time in which all individuals in the population can be at risk for the outcome**
- any number in the denominator must have the potential to be part of the numerator [ proportion]
e.g new cases of uterine cancer at a point in time/people with uterus at a point in time
-Assumes complete uniform follow-up in a fixed population (entire pop is followed up for the entire period) ; we cannot follow everyone in the population.. people drop out get lost..

Cumulative incidence per 1000 = new cases at a specified period of time /pop at risk at that time * 1000

Question 10

Incidence Rate (person-time)/Incidence density

Answer 10

It is not possible to follow ALL individuals at risk for a given time period, this is when we introduce person-time approach. When different individuals are observed for different lengths of time, we calculate an incidence rate (also called an incidence density), in which the denominator consists of the sum of the units of time that each individual was at risk and was observed. This is called
person-time and is often expressed in terms of person-months or person-years (py) of observation.

Incidence rate = number of new cases during a particular time/TOTAL person-time who have been observed during the specified time period *1000

Question 11

When is prevalence useful?

Answer 11

1. When Onset of disease is unclear [since we take the new+existing disease, we dont know when the disease started, one year ago a week ago?]
2. When it is impossible to ascertain population at risk since it Does not measure the risk of the disease[ as the numerator contains people with different duration of the disease]
3. When we are interested in time trends and geographical distribution
4. Planning for health resources and facilities as it** tells us about the burden of disease**

Question 12

Prevalence

Answer 12

Quantifies the **number of existing cases of disease in a population at a point or during a period of time → a snapshot **of who has and does not have a disease (i.e.,disease status) at a specific time. What Proportion of the population is affected by the disease at that time?

Question 13

Prevelance formula

Answer 13

number of new +existing cases/poulation at risk at the specified time X 1000

Question 14

Point , period, lifetime Prevelance

Answer 14

1. **Point: **at a certain point in in time
2. Period: Range of time , month, year
3. Lifetime: entire life duration

Question 15

Steady State - Relationship between Prevalence and Incidence

Answer 15

𝑃(1−𝑃)=𝐼𝑅∗𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑑𝑢𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑑𝑖𝑠𝑒𝑎𝑠𝑒
Prevalence is a function on of IR and the duration of the disease.
Disease duration - if cured or death short duration ; if new treatment that prolongs life but does not cure disease duration increases

Prevalance decreases - Death or cure
Prevalance increases - Prolongation of life but not cure

**P = IR **
* IR is constant over time (no new cases)
* Disease duration is stable (either by death or prolongation of life due to treatment)

When disease is very rare P is very small
Rare means no new cases to add to the pool increasing the Prevalence.

Question 16

Factors influencing prevalence

Answer 16

Increased by:
* ** Longer duration
* Prolongation of life
* Increase in new cases
* Out-migration of healthy**

Decreased by:
* **Shorter duration
* High case-fatality
* Decrease in new cases
* In-migration of healthy*

Question 17

Sentinel sites

Answer 17

Specific location, facility, or healthcare provider that is strategically chosen to monitor and report on particular health conditions or diseases within a given population. hospitals, clinics, and healthcare facilities. The Canadian Primary Care Sentinel Surveillance Network (CPCSSN

Question 18

All types of surveillance (5)

Answer 18

1. Active - house to house in search to identify suspects
2. **Passive - **patients reporting to health facilities
3. **Enhanced **= Active + passive
4. Sentinel = certain sites selected for monitoring clinics, centers.The Canadian Primary Care Sentinel Surveillance Network (CPCSSN
5. **Syndromic **- identification and management of specific syndroms rather then pathogens E.G. if there is a surge in Anthrax they look at the symptoms of the disease first

Question 19

Qualities of ideal surveillance

Answer 19

Timeliness: the speed between steps in a public health surveillance system.
*
**Simplicity: **should be as simple as possible while still meeting objectives.
*
**Flexibility: **can adapt to changing information needs or operating conditions with little additional time, personnel, or allocated funds.
*
**Acceptability: **reflects the willingness of persons and organizations to participate in the surveillance system.
*
**Sensitivity: **refers to the proportion of cases of a disease detected by the surveillance system OR to the ability to detect outbreaks, including the ability to monitor changes in the number of cases over time.
*
Positive predictive value: proportion of reported cases that actually havethe health-related event under surveillance.
*
Representativeness: accurately describes the occurrence of a health-related event over time and its distribution in the population by place and person.