20000 women underwent a newly developed cervical smear screen. 2000 of them had a smear
result which shows premalignant changes and had further investigations and biopsies. Only
200 women who had undergone biopsies had premalignancy changes seen on histology. What
is the single best term to describe the cervical smear test?
Answer Description
Low specificity
Remember that for a test to be considered a screening test, it has to be cheap and also it has
to have high sensitivity. As described in this question there are many false positives for the
newly developed cervical smear screen (only 200 out of the 2000 women were seen to have
premalignant changes on histology). So this is test is considered to have a low specificity.
Study Notes
Answers
Low sensitivity 0%
Low specificity Correct! 0%
High false negative rate Wrong 0%
High true negative rate 0%
High true positive rate
Answer Description
Low specificity
Remember that for a test to be considered a screening test, it has to be cheap and also it has
to have high sensitivity. As described in this question there are many false positives for the
newly developed cervical smear screen (only 200 out of the 2000 women were seen to have
premalignant changes on histology). So this is test is considered to have a low specificity.
Study Notes
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Answers
Low sensitivity 0%
Low specificity Correct! 0%
High false negative rate Wrong 0%
High true negative rate 0%
High true positive rate 0%
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Definitions in Epidemiology
True positive
Is the amount of sick people correctly identified as sick
False positives
Is the amount of healthy people incorrectly identified as sick
True negative
Is the healthy people correctly identified as healthy
False negative
Is the sick people incorrectly identified as healthy
High sensitivity
Means few false negatives
Low sensitivity
Means many false negatives
High specificity
Means few false positives
Low specificity
Means many false positives
Mnemonic:
seNsitivity :- Look at false Negatives
sPecificity :- Look at false Positives
Relative risk (RR)
Is used to compare the risk in two different groups of people.
For example, consider the risk for blindness in a patient with diabetes over a 5-year
period. If the risk for blindness is 4 in 100 (4%) in a group of patients treated
conventionally (control group) and 3 in 100 (3%) in patients treated with a new drug
(experimental group), the relative risk is the ratio of the two risks: - 3% / 4% = 75% ( or
0.75)
Exam tip
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Risk ratios are an important topic in PLAB as it is one of the main epidemiologic questions
asked.
A good formula to memorize for PLAB part 1 for Relative risk (RR) is:
Relative risk (RR) = Risk of disease in exposed / risk of disease in unexposed
Other clinical study related definitions
Case series and case reports.
These are reports of cases and use no control groups or patient groups to compare
outcomes in between them. They are comparable to writing a report on a topic that you
have read.
Case-control study.
This is a study in which patients who already have a definite condition are compared with
people who do not have that particular condition. These study designs are observational,
meaning that researches study but do not modify what occurs. They look back in the past
to assess if there was a significant alteration in the rates of exposure to a defined risk
factor between the groups. Example, group X consist of patients with liver cancer and
group Y consist of patients without liver cancer. Information is collected observing at
back at their alcohol history. The hypothesis would be that drinking alcohol would incur a
higher risk of liver cancer
Cohort study.
These studies identify a group of people who share a defining feature. This group is then
compared with a similar group of people with the same defining feature. Cohort studies
are simply observational and may be Prospective(going forward in time) or retrospective (
going back in time). Because these studies are observational, the researchers (similar in
case-control studies) do not interfere with the groups of people. Example of a prospective
cohort study, group X are patients who drink alcohol, group Y are patients who do not
drink alcohol. They are followed in time to determine who develops liver cancer.
Difference between a case-control study and a retrospective cohort study are:-
In case-control, one starts with the outcomes and studies the exposure.
In retrospective cohort studies, one already has determined the exposure but now needs
to study the relationship of exposure to disease outcome.
Randomised controlled clinical trial.
These are experiments that introduce a treatment or exposure to study its effect on real
patients. It generally compares an active group with a control group and employs
methods such as randomization and blinding to reduce bias. Normally, new drugs are
tested this way. Example, a group of patients with disease X who meet specific criteria,
are randomly assigned to receive either the experimental treatment or the control
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treatment (either the standard treatment for disease X or a placebo). Both groups are
then followed in time, and the outcomes are observed.
Cross-sectional study.
This is a study that describes the relationship between diseases and other factors at one
point in time in a defined population. They are often used for comparing diagnostic tests.
A good example would be researchers conducting a survey on a population to determine
the relationship between increased levels of serum cholesterol and electrocardiographic
evidence of coronary artery disease. Both the exposure and the disease outcome are
taken at the same point of time. It is similar to a snapshot of a population at a specific
time.
Systematic review.
This focuses on a clinical topic and answers a specific question that surfaces
Meta-analysis.
This is a study that examines a number of studies on a topic and mathematically
combines the results to report the results as if it were one large study.