John A. Heit, MD; Kent R. Bailey, PhD; L. Joseph Melton, MD, MPH
Potential Financial Conflicts of Interest: None disclosed.
Heit JA, Bailey KR, Melton LJ. Incidence of Pregnancy-Associated Venous Thromboembolism. Ann Intern Med. 2006;144:454-455. doi: 10.7326/0003-4819-144-6-200603210-00020
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Published: Ann Intern Med. 2006;144(6):454-455.
Dr. MacCallum and coworkers argue that we incorrectly reported our venous thromboembolism incidence rates for the first and second postpartum weeks as 3.6% and 1.5%, respectively, because of incorrect denominator calculations. The denominator calculations were based on the total number of live births in Olmsted County (n = 50 080) and “period at risk” definitions of 9 and 3 months for pregnancy and the postpartum period, respectively. A 1-week period of risk is 1/52 of a year (0.019), such that the woman-years at risk during the first postpartum week were 0.019 times the number of live births (0.019 × 50 080 = 951 woman-years). Thirty-four women had documented incident venous thromboembolism during this week, providing an incidence of 0.036 (34 of 951), or 3.6 per 100 woman-years (i.e., 3.6%). The incidence rate for the second postpartum week was calculated similarly. However, although the incidence rates are correct as reported, we incorrectly compared these rates with the proportion of patients developing venous thromboembolism after total hip replacement. According to data reported from the California Patient Discharge Data Set, among 56 720 patients undergoing elective total hip replacement between 1 January 1992 and 30 September 1996, 1358 developed symptomatic venous thromboembolism within 3 months (0.25 year) after surgery (1). Thus, there were 14 180 person-years at risk (56 720 × 0.25), and the venous thromboembolism incidence was 0.096 (1358 of 14 180), or 9.6 per 100 person-years. While this rate is similar to our incidence rates for the first and second postpartum weeks, at least some of the patients who had hip replacement probably received prophylaxis such that the incidence in the absence of prophylaxis could have been higher.
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