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Investigation of an Epidemic Screening Studies

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موضوع مغلق
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قديم 06-07-2012, 12:00 AM
الصورة الرمزية الكفاح
 


الكفاح will become famous soon enoughالكفاح will become famous soon enough


Investigating an Epidemic

Here are the major steps that the investigator of an Epidemic follow. Some of the steps may occur at the same time, or some steps may precede depending of the situation, nonetheless it is useful to know the general approach

Investigating an Epidemic

Step 1: Verify Diagnosis
As the cases come in to the ER or to the attention of the authorities, it is essential to know what is the disease in question we are facing. To do so clinical and laboratories test can be done to confirm diagnosis
Exp: An outbreak of hepatitis was first misdiagnosed as leptospirosis
Exp: 1967, an epidemic of “gonorrhea” among girls in a grade school turn out to be a faulty report

Step 1: Verify Diagnosis
Thus it is essential to establish diagnostic criteria for cases, which can rely on symptoms and/or lab test

Step 2: Verify that there is truly an Epidemic
One thing to do is to compare the rates to past rates Another thing is to consider if the increase in numbers can be explained: such as in increase in population in a university town, change in diagnosis, change in ways of diagnosing (new more accurate tests)

Step 3: Describe the Epidemic (Time, Place, Person
Plot cases by time they were reported, and by place of occurrence. Calculate the rates of the diseases by person characteristics (age, gender, occupation, food consumption)
example: John Snow study on London Water
Example: NJ, 1960, epidemic of hepatitis among males. Thru Investigation it was found that the source was contaminated clams at the bay

Step 4: Formulating and testing hypothesis
Does the Epidemic has a common source or a propagated source
To answer this question, we can look at the plot of the cases by time

Also, compare the ill to the none ill with regard to the possible exposure
In a food outbreak, all those attended the party would be asked what they ate, and then compare the attack rate among those who ate ice cream and those who did not
Use test statistics to test the hypothesis

Step 5: Control and Prevent
Once the cause of the epidemic is known then it is the researcher responsibility to work with proper authorities to stop the epidemic
Example: Mayo use in the hospital cafeteria has gone bad. Throw it away, and educate the cafeteria personnel of proper way to handle food
Prepare a report on the Epidemic, discuss factors of the Epidemic, evaluate measures to control and prevent

EXAMPLE OF A FOODBORNE OUTBREAK

1940, NY Oswego county
46 people came to the ER with gastrointestinal symptom
It was found that they had attended a Church supper the previous evening.
The person in charge started collected information from the ill people and also those who attended but did not become ill. He/She collected information on person, time, and food ate


Specific Control Measures:
The Reservoir: The nature of the reservoir will guide the method to control it. Example a pet in the house will be immunized
Bird Flue and irradiation of chicken in China
If the reservoir are humans, the situation is much more complex. Nonetheless there are ways to prevent transmission of the agent

Interrupt the transmission:
Purification of water, pasteurization of milk, safe food handling
Of major importance in hospitals to prevent nosocomial infections

Reduce Host susceptibility:
Immunization against DTP is a good example.
Also there is something called passive immunization, Mother transmit some protection to her baby, Antibiotic given to prevent tuberculosis, gonorrhea, syphilis, and recently Small pox

Surveillance:
regular collection, summarization, and analysis of data on newly diagnosed cases for the purpose of identifying high-risk groups, change in rates and trends, understanding the model of transmission and reducing or eliminating the disease

Active surveillance; The data is collected by the agency itself from clinics, hospitals, and other health care centers
Passive surveillance: data is generated without soliciting, ie the physicians, or the nurse would report the case.

Surveillance of nosocomial infection
Hospitals are place where both people with infection and vulnerable individual comes and thus the risk of transmission of agents should be assessed carefully
Health care professionals deal with a large number of people and can become the vehicle for the agent
Instruments used in the hospital also should be considered for risk of transmitting the disease (needles, … )
Regular report on the infectious has been established in most of the hospitals, the team would include physicians, nurses, and epidemiologist

Screening Studies
Definition: application of a test to people who are asymptomatic (no symptoms yet) for the purpose of classifying them with respect to their likelihood of having a particular disease. Those who test positive are then evaluate more to make sure that they do have the disease


Assumption: early detection will lead to a better favorable prognosis, since treatment will start earlier than usual and thus the outcome would be more favorable.
In some instance there are risk or cost associated with the Screening that must be weighted against the benefits

Screening Studies

Which diseases should be screened for?

Serious, early treatment should give better outcome in terms of reduction in mortality and morbidity of the disease, and the preclinical prevalence should be high among the screened population

The most important aspect as far as we are considered probably is whether early treatment is favorable or not:
Consider the natural history of a disease:


Which diseases should be screened for?
Example: uterine cancer takes a long time tobecome symptomatic (around 10 years), pap smear can detect the disease early on. Early treatment in this case has better outcome
Lung cancer on the other hand, has poor prognosis regardless of early or late detection
Nonmelatonic skin cancer is virtually completely curable even after it becomes symptomatic
hypertension meets all criteria for screening. Serious disease, early treatment reduce risk of morbidity and mortality, (mostly very frequent)

Evaluating the test:
The test should be inexpensive, easy to administer, impose minimal discomfort, and have results that are valid, and reliable
reliability: Is that the same person should receive same result if the test was done over and over (regardless if the result is correct or not)
Validity: whether the test is correctly identifying those with and without the disease

Sensitivity = P(T+|D+) = a/a+c
Specificity = P(T-|D-) = d/d+b
Positive Predicitve Value = P(D+|T+) = a/a+b
Negative Predictive Value = P(D-|T-) = d/c+d


Sensitivity = P(T+|D+) = 132/177 = 74.6%

Specificity = P(T-|D-) = 63,650/64,633 = 98.5%
Positive Predictive Value = P(D+|T+) = 132/1115 = 11.8%
Negative Predictive Value = P(D-|T-) = 63650/63695 = 99.9%

Though we would like to have a screening test with high sensitivity and specificity, this is not possible because these two test are related and a trade off exist


Evaluating screening programs:

Feasibility and Efficacy

Screening Studies

Feasibility:
Is assessed through looking at aspects of the screening program such as: acceptability by screenees, cost effectiveness, subsequent diagnosis required for those who tested positive

pap smear has great acceptability
Sigmoidoscopy (to detect colon cancer) involves great discomfort and thus was not accepted greatly by the population at risk
PPV and NPV are used to reflect on feasibility

Feasibility:
PPV is greatly affected by the prevalence of the disease, with rare disease the PPV is small

Effectiveness:
Measure the impact of the screening program on the public health
To be effective a program must used a good screening test and have a good program component
Efficacy can be tested by comparing the mortality rate of those that underwent the screening and those who did not

Effectiveness:
To be aware of are three biases that affect the evaluation of the efficacy:
Self selection Bias
Lead time bias
Length Bias

Self selection bias:
Those who come for screening are usually more health aware, involves in healthier life style than those who do not come from screening.
have a better adherence to the treatment and as such have a better prognosis, and mortality rate, thus the difference in mortality rate is not due to the early detection rather than the fact that those screened have better self characteristics

Lead time:
Is when the survival rate for a disease appear to have increase not because prognosis became better, but rather because the time of diagnosis was done earlier

Lead time:
One way to check it is to compare age specific death rate rather than survival time
Another way, if possible to calculate the lead time, is to account for it in the analysis


Length bias
overrepresentation among the screen detected cases of those with a long preclinical phase of the disease and this a more favorable outcome
Preclinical period vary from one person to another
Those with longer preclinical period are more likely to be detected by screening, just because they have more time to be detected while still preclinical


Length bias
Thus most of those with longer preclinical period will be collected among the screened group, and thus bias the survival rate
IT is very difficult to account for this bias, though the literature has some ways to account for it
Dr. Hani
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من مواضيعي : الكفاح
التعديل الأخير تم بواسطة الكفاح ; 06-07-2012 الساعة 12:31 AM.
موضوع مغلق

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الكلمات الدليلية (Tags)
epidemic, investigation, screening, studies


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