Question:

Researchers conduct a randomized controlled trial to determine whether prophylactic administration of tranexamic acid (TXA) in addition to prophylactic oxytocin in women undergoing vaginal delivery has an effect on the incidence of postpartum hemorrhage (PPH), defined as blood loss ≥500 mL. Study participants are women in labor who have a planned vaginal delivery of a live singleton at ≥35 weeks gestation. They are randomly assigned to receive 1 g TXA or placebo intravenously in addition to prophylactic oxytocin after delivery. The relative risk of PPH among women receiving TXA in addition to oxytocin is 0.80 (95% confidence interval of 0.66-0.96). Which of the following is the most appropriate conclusion about the effect of TXA in addition to prophylactic oxytocin after delivery on the risk of PPH?

Adding TXA is associated with a 20% increase in the risk of PPH

Receiving TXA in addition to prophylactic oxytocin decreases the risk of PPH by 80%

The risk of PPH is reduced by 20% when TXA is added to prophylactic oxytocin

The risk of developing PPH in the TXA treatment group is 0.8%

TXA prophylaxis has no significant effect on the risk of PPH

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Explanation:

Relative risk (RR) is a measure of association between exposure to a risk modifier (eg, treatment) and an outcome (eg, disease course) that is commonly used in cohort and experimental studies. RR is the risk of an outcome (eg, postpartum hemorrhage [PPH]) in the exposed group (eg, tranexamic acid [TXA]) divided by the risk of the same outcome or disease in the nonexposed group (eg, placebo):

RR = (risk of disease in exposed group) / (risk of disease in nonexposed group)

The RR may be interpreted as follows:

RR <1.0 indicates that the exposure decreases the risk of disease.
RR = 1.0 (null value) indicates that the exposure has no effect on the risk of disease.
RR >1.0 indicates that the exposure increases the risk of disease.

The confidence interval (CI) of RR indicates, with a certain confidence level (eg, 95%), whether a given RR is statistically different from the null value (ie, RR = 1.0). A CI that includes the null value (ie, RR = 1.0) is not statistically significant, and a CI that excludes the null value is statistically significant.

In this study, the RR of PPH (ie, disease) among women receiving TXA (ie, exposed) compared to women receiving placebo (ie, nonexposed) is 0.80. Because this RR is <1.0, the exposure decreases the risk of disease. The RR of 0.80 indicates that adding TXA decreases the risk of PPH by 20% (ie, 1 − 0.80 = 0.20) (Choices A and B).

(Choice D) In this study, 0.80 represents the relative risk (not absolute risk) of PPH among women receiving TXA compared to those receiving placebo. For instance, the actual risk of PPH could be 1.6% with TXA compared to 2.0% with placebo, producing the RR of 0.80 (ie, RR = 1.6 / 2.0 = 0.80).

(Choice E) The reported CI for RR in this study is 0.66-0.96. It excludes the null value (ie, RR = 1.0); therefore, adding TXA has a statistically significant effect on the risk of PPH.

Educational objective:
Relative risk is the ratio of the probability of an event (eg, disease outcome) occurring in the exposed group (eg, treatment group) versus the probability of the event occurring in the unexposed group (eg, control group).