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Ob/ Gyn Journal Club Notes

Learn about the measurement of disease through incidence and prevalence, as well as the calculation of absolute risk, relative risk, and odds ratios. Understand the relative risk and absolute risk of venous thromboembolism in gynecologic surgeries and how to communicate this information to patients.

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Ob/ Gyn Journal Club Notes

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  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 • 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

  4. How do we measure disease? Prevalence • 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 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

  9. Relative Risk

  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

  13. 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

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