1. Ob/Gyn Journal Club Notes
Martha A. Wojtowycz, PhD
January 11, 2019

2. Learning objectives Distinguish between incidence and prevalence
Calculate Absolute Risk, Relative Risk, and Odds Ratios
Distinguish between Relative Risk and Absolute Risk
Explain relative risk and absolute risk of an outcome to your patients

3. How do we measure disease? Incidence
# of new cases of disease
# of individuals at risk for developing disease
Over a given time frame (usually 1 year)
Usually expressed per 1,000 but can express as appropriate (eg per 100,000 for rare diseases)
Incidence of disease in a population is the absolute risk
Limitations: natural history- need to be able to define onset of disease (new case)- in some diseases, like TB infection, obesity-may be challenging to define onset of disease

4. How do we measure disease? Prevalence
# of cases of disease (old and new)
# of individuals at risk for developing disease
At a given time (point prevalence v. period prevalence)
Usually expressed per 1,000 but can express as appropriate (e.g., per 100,000 for rare diseases)

5. Incidence and Prevalence: Acute, self-limited disease
Incidence (new cases going in)
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Prevalence
Examples: GI illness,
Influenza, pancreatic cancer
Cases leaving prevalence pool- cure or death

6. Incidence and Prevalence: Chronic disease
Incidence (new cases going in)
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Examples: HIV, DM
Prevalence
Cases leaving prevalence pool - cure or death

7. Relative Risk (RR)
Ratio of the risk of disease (or incidence rate) in the exposed group to the risk of disease (incidence rate) in the nonexposed group.
RR= (Risk in exposed)/(Risk in nonexposed)
RR= (Incidence in exposed)/(Incidence in nonexposed)

8. Relative Risk Interpretation
RR > 1.0
Risk in exposed group > risk in nonexposed group; positive association; may be causal
RR = 1.0
Equal risks; no association
RR < 1.0
Risk in exposed group < risk in nonexposed group; negative association; may be protective

9. Disease Develops Does Not Develop Total Incidence Rates Exposed a b
Relative Risk
Disease
Develops
Does Not Develop
Total
Incidence
Rates
Exposed a b
a + b
a/(a+b)
Not exposed c d
c + d
c/(c+d)

10. Relative Risk Calculation
Incidence in exposed = a/(a+b)
Incidence in nonexposed = c/(c+d)
RR = (a/(a+b)) / (c/(c+d))

11. Odds Ratio (OR) Another measure of strength of association
Used when we do not have information on the incidence in the exposed or nonexposed
Case control study
Approximation of Relative Risk
OR= (Odds that person with disease was exposed)/ (Odds that person without disease was exposed)
OR can be <1.0, = 1.0, >1.0

12. Odds Ratio
Disease
(+)
(-)
History of Exposure a b
No history of exposure c d

13. Odds that person with disease was exposed = a / c
Odds Ratio
Odds that person with disease was exposed = a / c
Odds that person without the disease was exposed = b / d
OR = (a/c) / (b/d) = ad / bc

14. Jorgensen ET Al., “incidence of Venous Thromboembolism after different modes of gynecologic surgery”, Obstetrics and Gynecology, vol. 132, no. 5, November 2018.

15. Absolute Risk of Venous Thromboembolism
Absolute risk (or incidence) of venous thromboembolism is 0.2% or 2 per 1,000 gynecologic surgeries
0.7% or 7 per 1,000 hysterectomies
0.2% or 2 per 1,000 myomectomies
Venous thromboembolism is a rare event
Patients should be provided with the absolute risk information so that they are aware that this is rare

16. Relative risk of venous thromboembolism
Exposure is minimally invasive gynecologic surgery (laparoscopy or vaginal)
Unexposed group underwent laparotomy
RR for laparoscopy vs laparotomy
Incidence for laparoscopy = 0.19% or 1.9 per 1,000
Incidence for laparotomy = 0.85% or 8.5 per 1,000
RR = .0019/.0085 = 0.22
Shows that laparoscopy has a protective effect
Compared with laparotomy, laparoscopic surgery is associated a 78% lower risk of venous thromboembolism

17. Relative risk of venous thromboembolism
RR for vaginal vs laparotomy
Incidence for vaginal = 0.06% or 0.6 per 1,000
Incidence for laparotomy = 0.85% or 8.5 per 1,000
RR = .0006/.0085 = 0.07
Shows that vaginal surgery has a protective effect
Compared with laparotomy, vaginal surgery is associated a 93% lower risk of venous thromboembolism

18. What to tell your patient
Relative risk allows for a comparison of the absolute risks but caution patients not to overestimate the absolute risk
For example, suppose we have a rare event where:
Absolute risk for all surgeries is 1 per 1,000 surgeries
Absolute risk for surgery type A is 3 per 1,000 surgeries
Absolute risk for surgery type B is 0.3 per 1,000 surgeries
Relative risk = .003/.0003 = 10.0
Compared with surgery type B, patients who undergo surgery type A are 10 times more likely to experience this rare event

19. Sample size/Study design issues
Despite the fact that this study included over 40,000 surgical encounters, the authors were unable to:
Run all of the analyses for myomectomies because of the small numbers of myomectomies
Simultaneously adjust for potential confounders because there were very few cases with a venous thromboembolism
Case control study design often used to study rare events but is more susceptible to bias