Question

Oral contraceptives and myocardial infarction [new]. A cohort study was conducted to determine the effect of oral contractive use on heart disease risk in 40- to 44-year old women (fictitious data). This study found 13 new cases among 5000 OC users over 3-years of follow-up. In contrast, among 10,000 non-users, 7 developed a first myocardial infarct. Using the data above:

1. construct a 2x2 table.

2. what is the measurement you want to calculate?

3. calculate the measurement.

4. what does the result mean?

5. what should you do next?

Answer 1

1. Answer of question 1

2 x 2 table is constructed by taking exposure and non exposure in row and outcome (disease and non disease) in 2 columns.

Here exposure is " Oral Contaceptive(OC) use"

And Outcome " New Myocardial infraction (MI)"

Next for Oral contraceptive we would use OC and for Myocardial Infraction we would use MI. These 2 are common medical abbreviations.

Total OC users = 5000

MI in OC users = 13

No MI in OC users = 5000- 13 = 4987

Total non OC users evaluated = 10,000

New MI in Non OC users = 5

No new MI in Non OC users = 10,000- 5 = 9995

here ia the 2× 2 Table

	New MI detected	New MI non detected	Total
OC users	13	4987	5000
Non OC users	5	9995	10000
	18	14982	15000

2. Answer of question 2

we would measure Incidence Rate of newer MI, Risk of MI in OC users, Risk of MI in non OC users, Relative Risk, Odds of having MI in Non OC users, Odds of having MI in OC users and Odd Ratio.

3. Answer of question 3

Incidence rate = total new MI / Total subjects followed up

18 / 15000 = 0.0012 (in % 0.0012× 100= 0.12%)

Risk of MI in OC users = MI in OC users / total OC users

13 / 5000 = 0.0026

Risk of MI in Non OC users =

MI in Non OC users/ Total non OC Users

5/10000 = 0.0005

Relative Risk = Risk in OC users / Risk in non OC users

0.0026/0.0005 = 5.2

Odds of having MI in Non OC users =

MI in Non OC users / No MI in Non OC users

5 / 9995 = 0.0005

Odds of having MI in OC users =

MI in OC users /no MI in OC users

13 / 4987 = 0.00261

Odd Ratio =

Odds of Having MI in OC users / Odds of Having MI in Non OC users

0.00261/ 0.0005 = 5.22

4. Answer of question 4

A. . Incidence rate is 0.0012

Means 0.0012 out of 1 incidence in 3 years

Further multiply by 10,000 then

12 incidence of newer MI in women age group of 40 to 44 years in studied area in 3years.

In percentage (0.0012 × 100)

0.12 % new MI cases in women of 40 to 44 years seen in 3 years

B. Relative risk

Relative risk is 5.2

Means there is 5.2 times more risk to have MI in women who use OC than who do not use OC.

In another term it can be converted in precentage ( 5.2 x 100 = 520%) risk of MI in Non OC users is considered 100%.

There is 420% more risk of MI in OC users as compare to women who do not use OC.

C. Odd Ratio = 5.22

When the incidence is lower than 5% relative risk and odd ratio result in same value. In our case incidence rate is 0.12% so odd ratio and relative risk are almost same 5.22 and 5.2 respectively.

5. Answer of question 5

there is 420% more risk of Myocardial Infraction with Oral Contraceptives so practice to use OC must be reduced and using other contraceptive methods should be suggested.