workshop epidemiology

OSCE-Aid Revision Workshops: Epidemiology

© 2015 www.osce-aid.co.uk

Epidemiology & evaluation of evidence

Overview: This is a role-play exercise based on an OSCE communication station related to epidemiology and evaluation of evidence. In a typical scenario, the students will be asked to read the station and then carry out the scenario. Some UCL OSCE stations examine ‘professional skills’, and one of these relates to ‘evaluation of evidence’. These stations usually take 10 minutes and will involve explaining evidence from a scientific paper to a patient. Students will need to use communication skills and their knowledge of statistics to pass the station. Format:

Each role play will involve a student and an actor; each played by a student. You will act as the examiner and timer

Give the student and actor a brief and ask the student to read their brief to the whole group

Give every student in the group a copy of the student’s brief so that they can look at the data while the OSCE is being played out

There will be four OSCE stations available for this lesson but it is likely you will only cover 2

Each station should take 10 mins, followed by 1 min of feedback/discussion

If there is added time at the end, there are some topics for discussion included in the notes below

Inform the group at the end that a copy of these exercises and notes on epidemiology will be available on the website for them to access



Scenario 1

Student brief You are a foundation doctor on a medical ward. Mrs Roberts has recently been admitted to hospital, where she is on treatment for a chest infection after presenting with chest pain and shortness of breath. Her neighbour has recently been treated for a pulmonary embolism and has lent her a scientific paper about D-dimer blood tests. Mrs Roberts is due to be discharged today, but she is now worried that she may be having a PE. She is requesting a D-dimer blood test and would like to ask you some questions. Please address any concerns she may have.

D-dimer assay: Results based on number of

positive CTPA results

N = 41 (initial results)

Sensitivity 95.1 (82.2-99.2)

Specificity 16.0 (11.1-22.5)

Positive predictive value 21.0 (15.5-27.7)

Negative predictive value 93.3 (76.5-98.8)



Scenario 1 Actor brief You are Mrs Roberts, a 70-year-old ex-teacher who has been having a cough productive of green sputum for 1 week. You are a smoker. You started having chest pain and shortness of breath yesterday, which is why you presented to A&E. You now feel much better after having fluids and antibiotics for 24 hours, and you are due to be discharged today. You are worried because your neighbour, Meryl, had a pulmonary embolism recently, and she told you she had similar symptoms to you. Please ask the doctor the following questions:

o I would like a D-dimer blood test to test me for a pulmonary embolism, like my neighbour had. She told me this could diagnose a PE. If a D-dimer is raised, does it mean I have a pulmonary embolism?

o Are there any other reasons why my D-dimer would be raised? o I’ve read this paper, but I don’t understand some of the terms in it. Look at

this table. What does the sensitivity of D-dimer mean? o What does the specificity mean? o What is the PPV and NPV? What do these mean for me?



Scenario 1 Model answer

- Having a raised D-dimer does not mean that you have a PE. PE is a clinical diagnosis usually, but the gold standard to test for it is using imaging called a CTPA. D-dimer is a test that’s used when other scoring systems tell us that the likelihood of a PE is low, to help us to guide our management and to decide whether other investigations are needed.

- Though venous thromboembolism is one cause of a raised D-dimer, there are numerous other common causes. These include: ◦ increasing age ◦ pregnancy ◦ smoking ◦ atrial fibrillation ◦ pneumonia ◦ vasculitis ◦ superficial phlebitis ◦ severe infection ◦ trauma ◦ inflammatory disorders ◦ disseminated intravascular coagulation ◦ vaso-occlusive sickle-cell crisis ◦ acute cerebrovascular accident ◦ acute myocardial infarction ◦ unstable angina ◦ many cancers including lung, prostate, cervical, and colorectal

- Your D-dimer would most likely be raised if we tested it; this is because you have a pneumonia. You are also elderly and a smoker, which also tends to raise your D-dimer. We don’t have any clinical suspicion of you having a PE, as your symptoms are likely caused by pneumonia, therefore it will not give us any more information and may lead to unnecessary tests.

- Sensitivity means the proportion of people who actually have a disease who are found to have it via a test (correctly identified as positive), which would ideally be 100%. The sensitivity of D-dimer is 95%, which means 95% of patients with a PE will have a positive D-dimer.

- Specificity means the proportion of people who do not have a disease who are found to not have it via a test (correctly identified as negative), which would ideally be 100%. The specificity of D-dimer is 16 %, which means 16 % of patients tested who do not have PE will have a negative D-dimer.

- Positive predictive value (PPV) means the proportion of people testing positive for disease who actually have the disease. The PPV of D-dimer is 21%, which means 21% of patients who have a positive D-dimer will have a PE.

- Negative predictive value means the proportion of people testing negative for disease who actually do not have the disease. The NPV of D-dimer is 93.3%, which means 93.3% of patients who have a negative D-dimer will not have a PE.

- These values mean that this is a good test for ruling out PE, but not for diagnosing it as a raised value does not imply a PE. This is because it is a non-specific test, though it is very sensitive. If you are suspicious of a PE, it will be useful to use D-dimer to rule this out.



Scenario 2

Student brief You are a foundation doctor in a GP surgery. Mr Roberts has recently been started on Ramipril for hypertension. Mr Roberts is an amateur statistician and has read that evidence is assessed on the basis of p-values, and that the cut off for this value is less than 0.05. He has found the following table from a paper and he is concerned that these values show that Ramipril is not effective. He would like to ask you some questions about this.

Outcome Ramipril group N=4645 Number (%)

Placebo group N=4653 Number (%)

Relative Risk (95% CI)

Cardiovascular event (including death)

651 (14)

826 (17.8)

0.78 (0.70-0.86)

Death from non CV cause

200 (4.3) 192 (4.1) 1.03 (0.85-1.26)

Death from any cause 482 (10.4) 569 (12.2) 0.84 (0.75-0.95)



Scenario 2 Actor brief You are Mr Roberts, a 56 year-old amateur statistician. You normally use herbal remedies and worried about using Ramipril. You are hoping that this study you have found shows that Ramipril is not an effective treatment, and will not decrease your risk of death. Please ask the doctor the following questions:

o What does the Relative Risk mean? Is it the same as a p-value? o What is the 95% Confidence Interval? When is a Confidence Interval

significant for a Relative Risk? o Does this study mean that Ramipril will increase my life expectancy? o What do these results suggest? Which values are significant and which ones

are insignificant?



Scenario 2 Model answer - Relative risk measures the increased or decreased probability you are to have a

disease if you have been exposed to a certain factor

if RR=1, then the risk is the same in the exposed and unexposed groups

RR>1, then there is an increased risk in the exposed group

RR<1, then there is a decreased risk in the exposed group

- Relative risk is not the same as a p value. A p value is always between 0 and 1. It expresses the weight of evidence for or against a null hypothesis. Values are usually 0.05 or 5%. The lower this value is, the less proof there is towards the null hypothesis. You are right that if the p value is <0.05 then it is considered statistically significant, which is evidence against the null hypothesis being true. If the p value is equal to or >0.05 then it is considered to not be statistically significant; there is therefore not enough evidence available to disprove the null hypothesis.

- Confidence interval is how confident we are that the true population value lies within a particular range. It is usually 95%, and refers to a binomial distribution curve. It is a margin of error around the estimate, that indicates how precise the estimate is - the smaller the interval, the more precise the estimate. It can be used to deduce statistical significance by checking if the interval contains the null hypothesis value. If the CI discludes the null hypothesis value, then the estimate is statistically significant at the 5% level. In this case: null hypothesis values for relative risk = 1. If a 95% CI does not include the null value, then p <0.05 automatically.

- This study shows that using Ramipril will decrease your risk of death from any cause. Patients using Ramipril will be 0.84 times as likely to die from any cause as someone not on Ramipril.

- These results suggest that there is a significant reduction in mortality from cardiovascular

death and from death from any cause while on Ramipril. These figures show that the effect is no longer significant when you look at only non-cardiac causes of death. This is because the RR is more than 1 in this bracket, while it is below 1 in the other two categories. Furthermore, the 95% CI do not contain 1 in the latter, while they do contain 1 in the former aka. They contain the null hypothesis.



Scenario 3

Student brief You are an F2 in a GP surgery. Mrs Hall is considering starting a new antidepressant medication, which has recently come on the market. She has read a paper about the drug, where she saw that the Number Needed to Treat for response vs. placebo was 8 (95% CI 6-16) and for remission was 14 (95% CI 8-55). She also read that the Number Needed to Harm value vs. placebo for diarrhoea was 6 (95% CI 5-8). She doesn’t know what this means, and wishes to discuss these findings with you.



Scenario 3 Actor brief You are Mrs Hall, a 40 year-old house wife with depression. You have been reading about a new type of antidepressant which you would like to try. You’re not sure if these values you’ve read are good or not. Please ask the doctor the following questions:

o What is Number Needed to Treat? What is Number Needed to Harm? o How is it calculated? o What does this mean for this antidepressant medication? Should I take it?



Scenario 3 Model answer - Number Needed to Treat (NNT) is the number of people you need to treat in order that

one additional patient has a positive outcome/to prevent one additional person having a bad outcome. A lower number suggests a more effective treatment. In this trial, the NNT was 8 for response to the new antidepressant, which means that 8 people need to be treated to see a significant response.

- Number Needed to Harm (NNH) is the number of people you need to treat in order that one additional patient has a negative outcome. In this trial, 6 people would need to take the treatment before the negative outcome of diarrhoea is recorded.

- Calculated by: 1 divided by the absolute risk reduction between two groups

e.g.: RCT showed 0.014 adverse outcomes in treated group, and 0.044 adverse

outcomes in the non-treated group

NNT = 1 / (0.044-0.014) = 1/ 0.03 = 33

33 people required to treat so that one additional person has a positive outcome

- This means that 1 in 8 people will experience a response from this antidepressant, while 1 in 6 will experience a side effect. It is your decision if you take this treatment. But the risk of you developing a side effect slightly outweighs the chance of it being an effective treatment for you, especially when it comes to remission rather than just response, where the NNT rises to 12. However, the likelihood that you will experience a side effect is relatively high. What is important to consider here is the ratio of NNT:NNH.



Scenario 4

Student brief You are an F2 in a GP surgery. Miss Nicholls is a young lady who is thinking of starting the combined oral contraceptive pill. She has been reading about the side effects of this treatment and has ‘Googled’ some trials. She’s become quite anxious about all the different trials, and feels confused. She’d like to know the difference between these trials and what they mean.



Scenario 4 Actor brief You are Miss Nicholls, a 21 year-old student who wishes to start the oral contraceptive pill for birth control. You are worried about the side effects and have become confused while ‘Googling’ all the different trials. You feel you have seen conflicting information, and you’re not sure which trial to trust. Please ask the doctor the following questions:

o What are the different types of clinical trial? What is a randomized controlled trial? What is a case-control trial? What is a cross-sectional study? What is a cohort study?

o Which trial design is considered the gold standard? Why? When asked about the oral contraceptive pill and whether you would like to take it, please explain you would like to go home and research this before deciding.



Scenario 4 Model answer

- RCT is gold standard trial. This is because it tests between two variables, allowing direct comparison of two groups, and controlling for confounding factors through randomisation. This test also reduces bias as it can be blinded

Type of study

Details

Case-control study

Observational study

Investigates the cause of a disease when comparing people with and without that disease

starts with a disease of interest and selects patients with that disease for study – the 'cases'

then selects group without the disease to compare – the 'controls'

often sample sizes of the two groups are similar

cases and controls are then compared to discover possible causal factors

retrospective study

Cohort study

An observational study that aims to investigate causes of a disease

longitudinal, starting with an unselected group of subjects who are then followed up for a set period of time

usually prospective, where risk factor data is collected before the person gets the disease

aims to discover if there are particular aetiological factors causing a disease in the future

Cross-sectional study

Observational study

A sample group is chosen and data from an individual is collected at one point in time only

e.g. surveys

Randomised control trial

Experimental study

prospective and longitudinal

assesses efficacy of treatment or intervention

compares outcomes in a group of patients treated with a new therapy, and those in a comparable group of patients with a control therapy (e.g.: a placebo)

patients in both groups randomised, enrolled, treated and followed over same time period

gold standard test – tests between two variables



Notes for discussion Effect measures – table adapted from http://www.medicine.ox.ac.uk/bandolier/painres/download/whatis/what_are_conf_inter.pdf Measure of effect

Abbreviation Description No effect Total success

Absolute risk reduction

ARR Absolute change in risk: the risk of an event in the control group minus the risk of an event in the treated group; usually expressed as a percentage

ARR=0% ARR=initial risk

Relative risk reduction

RRR Proportion of the risk removed by treatment: the absolute risk reduction divided by the initial risk in the control group; usually expressed as a percentage

RRR=0% RRR=100%

Relative risk RR The risk of an event in the treated group divided by the risk of an event in the control group; usually expressed as a decimal proportion, sometimes as a percentage

RR=1 or RR=100%

RR=0

Odds ratio OR Odds of an event in the treated group divided by the odds of an event in the control group; usually expressed as a decimal proportion

OR=1 OR=0

Number needed to treat

NNT Number of patients who need to be treated to prevent one event; this is the reciprocal of the absolute risk reduction (when expressed as a decimal fraction); it is usually rounded to a whole number

NNT=1/initial risk

References: Definitions and answers adapted from Celine Lakra and Charlie Vickers on OSCE-Aid http://www.ncbi.nlm.nih.gov/pubmed/22284853 - reference for antidepressant http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3305359/ - reference for d-dimers and CTPA

Resources: http://www.medicine.ox.ac.uk/bandolier/painres/download/whatis/what_are_conf_inter.pdf

http://www.medicine.ox.ac.uk/bandolier/painres/download/whatis/what_are_conf_inter.pdf

http://www.ncbi.nlm.nih.gov/pubmed/22284853

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3305359/

http://www.medicine.ox.ac.uk/bandolier/painres/download/whatis/what_are_conf_inter.pdf



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