Cohort studies — MCQs

You are attempting to quantify the degree of infectivity of a novel respiratory virus. You assess 1,000 patients who have been exposed to the virus and find that 500 ultimately are found positive for the virus within a 1-year follow up period. Conversely, from a 1,000 patient control group who has not been exposed to carriers of the virus, only 5 became positive over the same 1-year period. What is the relative risk of a contracting this virus if exposed?

A rheumatologist is evaluating the long-term risk of venous thromboembolism in patients with newly diagnosed rheumatoid arthritis by comparing two retrospective cohort studies. In study A, the hazard ratio for venous thromboembolism was found to be 1.7 with a 95% confidence interval of 0.89–2.9. Study B identified a hazard ratio for venous thromboembolism of 1.6 with a 95% confidence interval of 1.1–2.5. Which of the following statements about the reported association in these studies is most accurate?

A prospective cohort study was conducted to assess the relationship between LDL-C and the incidence of heart disease. The patients were selected at random. Results showed a 10-year relative risk (RR) of 2.30 for people with elevated LDL-C levels compared to individuals with normal LDL levels. The p value was 0.04. This study is most likely to have which of the following 95% confidence intervals?

A cohort study was done to assess the differential incidence of diabetes in patients consuming a typical western diet, versus those consuming a Mediterranean diet. A total of 600 subjects were included with 300 in each arm. Results are as follows: Diabetes development No-diabetes development Western diet 36 264 Mediterranean diet 9 291 What is the odds ratio of developing diabetes for a given subject consuming the western diet as compared to a subject who consumes the Mediterranean diet?

In a community of 5,000 people, 40 people from 40 different households develop an infection with a new strain of influenza virus with an incubation period of 7 days. The total number of people in these households is 150. Ten days later, 90 new cases of the same disease are reported from these same households. Twenty-five more cases are reported from these households after a month. The total number of cases reported after a month from this community is 1,024. What is the secondary attack rate for this infection?

A popular news outlet recently published an article that discussed the size of low-density lipoprotein (LDL) cholesterol particles: type A and type B. Type B is thought to be more harmful to arterial walls. A group of researchers wants to determine whether patients who have an elevated level of type B LDL cholesterol are more likely to develop cardiovascular events. A study is designed with 3418 adult participants. Initial levels of type B LDL are obtained and participants are separated into normal and elevated levels of type B LDL. Socio-demographics including age, gender, education level, and smoking status are also recorded. The primary outcome is incidence of cardiovascular events over 10 years. Secondary outcomes include all-cause death, death by cardiovascular events, stroke, and hospitalizations. For this study, which of the following analyses would be the most appropriate measure to determine the association between type B LDL and cardiovascular events?

A multicentric, ambidirectional cohort study (i.e. a study that combines elements of both prospective and retrospective cohort studies) was designed in order to evaluate the relationship between nasal colonization with methicillin-resistant Staphylococcus aureus (MRSA) and exposure to patients in intensive-care units of several tertiary hospital centers. The sample included 1,000 physicians who worked in the hospital environment and who willingly underwent swabbing of their nasal vestibule and nasopharynx for active surveillance. Data of their working location was obtained from hospital administrative services. Of those who worked in the intensive care unit, 350 were colonized with MRSA, while 250 were not. Whereas in those that worked in other hospital wards, 100 were colonized with MRSA, and 300 were not. What is the relative risk of MRSA colonization in relation to working in the intensive-care unit?

The study is performed in an attempt to determine whether there is an association between maternal exposure to 2nd-hand smoke and low birth weight. A total of 1,000 women who have given birth to at least 1 child are placed into 1 of 2 groups according to the birth weight of their 1st child. Each group includes 500 women whose 1st child either weighed < 2,500 g (5.5 lb) or > 2,500 g (5.5 lb). In the 1st group, 250 subjects admitted to living with or being in close proximity to a smoker. In the 2nd group, 50 subjects admitted to living with or being in close proximity to a smoker. Which of the following is the strongest measure of association that can be calculated from this study?

You have been asked to quantify the relative risk of developing bacterial meningitis following exposure to a patient with active disease. You analyze 200 patients in total, half of which are controls. In the trial arm, 30% of exposed patients ultimately contracted bacterial meningitis. In the unexposed group, only 1% contracted the disease. Which of the following is the relative risk due to disease exposure?

A new treatment for hemorrhagic stroke, which is a life-threatening clinical condition that occurs when a diseased blood vessel in the brain ruptures or leaks, was evaluated as soon as it hit the market by an international group of neurology specialists. In those treated with the new drug, a good outcome was achieved in 30%, while those treated with the current standard of care had a good outcome in just 10% of cases. The clinicians involved in this cohort study concluded that the newer drug is more effective and prompted for urgent changes in the guidelines addressing hemorrhagic stroke incidents. According to the aforementioned percentages, how many patients must be treated with the new drug to see 1 additional good outcome?