The aim of this assessment is to construct a simple decision tree to assess the cost-effectiveness of a new cervical screening test when compared to the current screening test. The assessment task requires you to complete a series of structured stages, and then to write a brief health technology assessment report with the specified headings summarizing the project, methods and outcomes. All stages contribute to the assessment.
Your completed assessment task should have
1. An executive summary that represents a brief health technology assessment report based on your answers, summarizing the project, methods and outcomes. This part should be no more than 500 words.
The required headings for the executive summary are
- Objectives
- Methods
- Data
- Results
- Key areas of uncertainty
- Discussion
- Recommendation
2. An attachment that shows your answers to Parts One-Six of the task, including workings. (Contribution to overall marks are shown for each part)
Part One: Calculating the accuracy of the test
Screening tests for cervical cancer aim to identify pre-cancerous changes in the cervix that could develop into cervical cancer. If the pre-cancerous tissues is identified early and removed, then cervical cancer can be prevented for developing.
The Comparator - In the conventional Pap smear, the doctor collecting the cells smears them on a microscope slide and applies a fixative. This slide is then sent to a laboratory for evaluation. Studies of the accuracy of conventional (current) Pap smear tests report:
- Sensitivity 72%
- Specificity 94%
The New Test - The new test works in exactly the same way as the current test, however the manufacturer believes that the sensitivity of the new test is better. Below are the results of a cohort study that tested a new cervical screening test. Note that all women were 30 years of age when tested.
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New Test
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Disease Status
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Total
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Cervical cancer (+ve)
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Cervical cancer (-ve)
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Test (Positive)
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44
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36
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|
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Test (Negative)
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6
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564
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|
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Total
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|
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A) For the new cervical screening test define the following (include the number of individuals in each group)
- True positive
- False positive
- True negative
- False negative
B) Calculate the sensitivity and specificity for the new test
C) Compared with the current test, the new test was evaluated using a different cohort of women and in a different laboratory. How might this inadvertently influence the sensitivity and specificity of the new test?
Part Two: Construct a decision tree
Your task is to assess the cost-effectiveness of screening women when they reach the age of 30. We also assume that everyone who is invited to participate in screening program receives a cervical screening test (i.e. the uptake rate of the test is 100%).
Draw a decision tree to determine whether the new cervical screening test is more cost-effective than the current test. To do this you need to create a decision node with the option to accept the new test or the current test. For each test the terminal nodes should reflect the possible outcomes of the test result (e.g. True positive etc...).
Part Three: Estimating the benefit of testing
To populate the decision tree we need to estimate the benefits and costs of each test option. The benefits of screening are measured in terms of quality adjusted life years (QALYs) gained (i.e. quality-of-life multiplied by the number of years in that health state).
-Utility score - A time-trade off study conducted on the same cohort of women that received the new test demonstrated that;
- The average utility in the non-cancer group (test negative) was 0.92.
- The average utility in the non-cancer group (test positive) was 0.91 (slight reduction in utility due to further investigations and concern of possible cancer)
- The average utility in the cancer group (not detected by the test) was 0.45 (This reduced utility is due to the side-effects of treatment and the impact of the disease).
- The average utility in the cancer group (detected by the test and treated early) is 0.86 (there is a slight reduction in quality of life due to early treatment.
-Survival - Long-term registry data were used to estimate the additional survival (note that this is the additional survival beyond 30 years of age, which is the age when a person would be screened in this model)
- The average survival of a 30 year old woman with cervical cancer (not detected early) is an additional 10 years.
- The average survival of a 30 year old woman with cervical cancer that is detected early and treated (i.e. detected with a positive test results) is an additional 40 years.
- For all other 30 year old women (no cancer) the average survival is an additional 40 years.
A) Calculate the average additional QALYs gained for individuals with the following possible test outcomes:
- True positive
- False positive
- True negative
- False negative
B) In this model all outcomes (costs and benefits) are undiscounted. Why do we discount future costs and benefits? Why might discounting costs and benefits at the same rate penalize preventative health programs?
Part Four: Estimating Costs
The tables below were taken from a longitudinal cohort study of women that participated in the current screening program. The unit costs are provided in Table 1. Table 2, contains an inventory of all the resources used, on average, by an individual depending upon their test result.
- For example, an individual identified as being 'true positive' (using the current test) would require the following resources - 1xcurrent test, 2 x GP visits, 1 x further examination - early treatment. Therefore their treatment would cost - 1x$50 + (2x$35) + 1 x $1000 = $1,120
Combine the information from Tables 1 and 2 to generate the total cost of each screening outcome, do this for both the current test and the new test scenarios?
Table 1: Unit costs
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Description
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Cost
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Current test
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$50
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New Test
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$400
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GP appointment
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$35
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Further examination - No treatment
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$500
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Further examination - Early treatment
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$1000
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Delayed treatment
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$50000
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Table 2: Resources use for each possible alternative
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|
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Current test
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New Test
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GP visit
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Further exam - no treat
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Further exam - early treat
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Delayed treatment
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|
Current test
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True Positive
|
1
|
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2
|
|
1
|
|
|
False positive
|
1
|
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2
|
1
|
|
|
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True negative
|
1
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1
|
|
|
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False negative
|
1
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1
|
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1
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|
New Test
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True Positive
|
|
1
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2
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1
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|
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False positive
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|
1
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2
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1
|
|
|
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True negative
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1
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1
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|
|
|
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False negative
|
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1
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1
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|
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1
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Part Five: Cost-utility analysis
You should now have the following information:
- Accuracy of the current and new cervical screening tests
- A decision tree that reflects the possible outcomes of both tests
- An estimate of the QALYs gains for each alternative
- An estimate of the resource use (cost) of each alternative
The final information that you need to complete to complete the analysis is the prevalence of cervical cancer in this population. In this example we are screening women 30 years of age; the prevalenceof cervical cancer in this cohort is 9 in 10,000 or (0.0009)
A) You should now have enough information to complete table3: Model Parameters
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Parameter description
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Current Test
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New Test
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Prevalence of cervical cancer
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0.0009
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0.0009
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Sensitivity of test
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0.72
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|
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Specificity of test
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0.94
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|
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Cost - True Positive
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|
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Cost - False Positive
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|
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Cost - True Negative
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Cost - False negative
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QALYs - True Positive
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QALYs - False Positive
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QALYs - True Negative
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QALYs - False negative
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B) You now need to combine this information into your decision tree to determine the cost-effectiveness of the new test relative to the current test. Provide your answer as an incremental cost-effectiveness ratio (ICER) (i.e. cost/QALY gained)
Hint: Remember that you need to calculate the expected value (costs and QALYs) of each alternative before you can estimate the cost-effectiveness. It is easier to calculate the expected value if you start at the end of the tree, rather than the beginning (i.e. you need to roll-back the decision tree - see lecture notes for example)
C) If the decision maker has set an explicit threshold of $50,000 / QALY gained, would you say the new test is cost-effective.
Part 6: Sensitivity Analysis
The decision maker would like you to determine the cost-effectiveness of the new test in a population of women with a family history of cervical cancer. In this high risk cohort of women the prevalence of cervical cancer is 1 in 100 (0.01)
A) Calculate the ICER of the new test relative to the current test in this population of women
B) Why do you think the cost-effectiveness of the new test is sensitive to prevalent risk of cervical cancer in the population?
C) In the original model (prevalence = 0.0009), we assumed a 20 min GP appointment costs $35. However, an audit of General Practices conducting the new test shows that 80% of GPs charge patients a double appointment (2x20mins). How does this change you ICER? Explain your answer.
D) What would be the ICER for the new test in the high risk cohort (prevalence of cervical cancer is 0.01), if 80% of GPs charge patients a double appointment (2x20mins).
E) What type of sensitivity analysis was carried out in sub-question (D)? What is the advantage of this over what was conducted in sub-question (C)?