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

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

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

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)

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

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

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)

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

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)

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

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

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

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

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

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

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

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

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

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