PSU45 THE IMPACT OF TIME-DEPENDENT BIAS ON EVALUATION OF INPATIENT COST AND LENGTH OF STAY IN HOSPITAL-ASSOCIATED VENOUS THROMBOEMBOLISM AMONG ARTHROPLASTY PATIENTS

Abstract

We explored the impact of time-dependent bias through a cohort study which estimated the impact of hospital-acquired venous thromboembolism (HA-VTE) on inpatient cost and length of stay (LOS) among patients receiving total hip or knee arthroplasty. The impact of HA-VTE on inpatient cost and LOS was estimated using three different approaches with Veterans Health Administration data. First a naïve approach ignored the time-varying nature of the exposure by defining exposed subjects as those with an HA-VTE at any time in the hospital stay. Second, the time-varying exposure was addressed by restricting the cohort to subjects whose hospital stays crossed two calendar months and exposure was defined as the occurrence of HA-VTE at the beginning of the 2nd month; this enabled isolation of costs in the period before versus after HA-VTE. Third, exposure incidence density sampling was used to account for time-dependent bias. In the naïve analysis, HA-VTE increased inpatient costs by $21,551 (95% CI $16,862-$26,239) and LOS by 6.57 (95% CI 4.83-8.32) days. After correcting for time-dependent bias in the restricted cohort, these estimates were $17,113 (95% CI $3,372-$30,853) and 4.74 (95% CI -3.33-12.82) days, respectively. In the exposure incidence density sampling analysis, findings were much closer to the naïve analysis at $20,552 (95% CI $15,822-$25,283) and 6.32 (95% CI 4.58-8.07) days. Compared to estimation approaches that attempted to overcome time-dependent bias, naïve approaches led to estimates of the cost attributable to HA-VTE that were 4.86% to 25.93% higher and estimates of the LOS attributable to HA-VTE that were 3.94% to 38.64% higher. This study reveals that ignoring time-dependent bias results in the potential for considerable inflation of inpatient cost and LOS estimates attributable to HA-VTE. Researchers should take the time-dependent nature of an exposure into consideration when estimating the impact of time-varying exposures on health utilization outcomes.