Taking into consideration the above screening test, we randomly pick 1000 people from the general population. Considering the prevalence of a major depressive disorder using DSM-IV in the general population as 10%, calculate the positive predictive value of the 2-item screening test in the population?
D. In Question 75, we discussed how the prevalence of a condition can vary according to the population tested. Using the same screening test for depression in the general population of 1000 subjects (N), we are asked to calculate the positive predictive value. The prevalence rate or pre-test probability is 10% (A + C/N). We need to make a fresh 2 × 2 table in order to answer the question. We know that sensitivity and specificity remains constant for the disease. From the given data the prevalence = A+C/N = 10%
As N = 1000 now, we can say A+C = 100
Sensitivity (A/A+C) = A/100 = 0.91; so, A = 91.
Specificity (D/B+D) = 67.74%; D/900 = 0.677; D = 610.
Using the formula for positive predictive value, PPV = A/A+B = 91/290 = 31%.
Reference:
Taking into consideration the above screening test, we randomly pick 1000 people from the general population. Considering the prevalence of a major depressive disorder in the general population using DSM-IV as 10%, calculate the new negative predictive value of the two-item screening test in the population?
E. See the table in Answer 81. Using the formula for negative predictive value, NPV = D/C+D = 98.36%. Note that the same answer can be derived using pretest odds and likelihood ratios. Please see question 6.
The table below shows the adverse events reported during an RCT on sertraline for the prevention of relapse in detoxicated alcohol-dependent patients with a comorbid depressive disorder. Answer Questions 83–86 based on the data presented in the table:
What proportion of patients develops dyspepsia after exposure to the sertraline?
A. This question pertains to the risk of the development of dyspepsia in the trial. As mentioned earlier, a 2 × 2 table with the exposure (drug/placebo) to the left and the outcome (dyspepsia) on top can be drawn to make calculations easier.
This question looks at the chances of developing dyspepsia with sertraline. It is otherwise called the ‘experimental event rate’ (EER). This is calculated as A/(A + B); that is, 6/44 = 0.136 or 13.6%. Similar to the above question, the chances of developing dyspepsia with placebo, or the ‘control event rate’ (CER) is C/(C + D), or 2/39 = 0.05 or 5%.
References:
What proportion of dyspepsia will be eliminated if sertraline was not administered?
C. This is otherwise called the ‘attributable risk’ or the ‘risk difference’ or ‘absolute risk reduction’ (ARR). It is calculated as the difference in the absolute risks of developing a headache between sertraline and placebo, that is 13.6 – 5 = 8.6%
How many times is a person on sertraline more likely to develop dyspepsia than a person on placebo?
B. This question asks for the ‘relative risk’ or ‘risk ratio’ of dyspepsia with sertraline. It is an estimate of how much greater is the risk of developing dyspepsia with sertraline than with placebo. It is the ratio of the absolute risks or ratio of event rates, i.e. EER/CER = 13.6/5 = 2.7. This means that the risk of dyspepsia with sertraline is 2.7 times that of placebo. If there is no difference between sertraline and placebo, the relative risk would be 1. Expressed otherwise, relative risk values that are more than 1.0 represent increases in risk. Relative risk values that are less than 1.0 represent decreases in risk. If 95% confidence intervals are given, and if the range includes the value 1, then the elevation in risk can be considered as statistically insignificant. The relative risk is used as a primary summary measure in RCTs and cohort studies.
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