The incidence of a particular disease is greater in men than in women, but the prevalence shows no sex difference.
The most probable explanation is that:
Correct Answer C:
Incidence is defined as the number of new cases of a disease. Prevalence is the number of existing cases of a disease at a particular point in time. If the duration of the disease is longer in women, then there would be no difference in prevalence, even if the incidence is higher in men (choice C). The increased number of males with the disease would die sooner and therefore the overall number would not affect the prevalence.
Which one of the following descriptors of a diagnostic test is directly influenced by the prevalence of the disease being tested for:
Correct Answer D:
Positive predictive value (PPV) is defined as: of all the people with a positive test for a disease, the number that actually have the disease being tested. It is related (directly proportional) to the prevalence of the disease. The higher the prevalence, the higher the PPV will be.
A new drug treatment is shown to reduce the incidence of a complication of a disease by 50%. If the usual incidence of this complication were 1% per year, how many patients with this disease would have to be treated with this medication for 1 year to prevent one occurrence of this complication?
Considering relative risk reduction without also considering the absolute rate can distort the importance of a therapy. A useful way to assess the importance of a therapy is to determine the “number-needed-to-treat” for that therapy. To calculate this number, the absolute risk reduction would be 0.5% (0.5 x .01). Thus, the number-needed-to-treat for the example cited would be 200 (100/0.5).
More detailed explanation:
NNT = 1/ARR
Where ARR = CER (Control Event Rate) - EER (Experimental Event Rate).
The ARR is the amount by which your therapy reduces the risk of the bad outcome. For example, if your drug reduces the risk of a bad outcome from 50% to 30%, the ARR is:
ARR = CER - EER = 0.5 - 0.3 = 0.2 (20%)
In this specific case, the new drug reduces the incidence by 50% - from 1% to 0.5%).
Therefore ARR = 1/100 - 0.5/100 = 0.005
Then NNT = 1/0.005 = 200
A recent study of cholesterol-reducing medication shows that healthy patients taking the investigational drug over 5 years had a 5% incidence of myocardial infarction, whereas 7% of control subjects not taking the drug suffered from a myocardial infarction.
How many patients must be treated with the new drug for 5 years in order to prevent one myocardial infarction?
Correct Answer E:
The number needed to treat (NNT) refers to the number of people who need to be treated in order to prevent one undesirable outcome. This statistic has become increasingly useful for interpretation of clinical research results, and it is an understandable form in which to present information to patients. It is calculated as the reciprocal of the absolute risk reduction (ARR). The ARR is the rate in the control group minus the rate in the study group. Therefore, the calculation in this example is: NNT = 1/0.07 - 0.05 = 1/0.02 = 50.
You are considering how useful a new treatment might be in preventing stroke. A well designed study is reported with 200 patients in the treated group and 200 patients in the untreated group. The study finds a 5-year risk of stroke of 3% in the treated group versus 5% in the untreated group.
Assuming this study is valid and applicable to your patient population, how many patients would you have to treat for 5 years to prevent one stroke (number needed to treat, or NNT)?
The relative risk reduction (RRR) is the proportional decrease in disease incidence in the treated group relative to the incidence in the control group. In this example, the 3% incidence in the treated group is 40% less than the 5% incidence in the control group: (5% - 3%)/5% = 40%. The absolute risk reduction (ARR) is the difference between the incidence of disease in the treatment group and the incidence in the control group. In this example the ARR is 5% minus 3%= 2%. The number needed to treat (NNT) equals the reciprocal of the ARR: 1/.02 = 50.
The RRR is not a very useful clinical statistic in clinical practice. It amplifies small differences and makes clinically insignificant findings appear significant because it essentially ignores the baseline risk “How much will I decrease my patient’s risk of an adverse outcomes by this treatment?”. The NNT is also very useful for clinicians, as it answers the question, “How many patients will I need to treat to prevent one adverse outcome?”
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