Objective: To determine whether observed and predicted mortality for intensive care unit (ICU) transfer admissions is different from non-ICU transfer admissions and how that might affect ICU performance evaluation.
Design, setting, and patients: We retrospectively analyzed the charts of 3,416 patients admitted to our tertiary referral ICU from January 1995 to December 2001 and evaluated the effect on our performance (based on the Simplified Acute Physiology Score II risk model) of accepting patients transferred from another hospital's ICU.
Main results: During the study period, 597 patients (17%) had been transferred from a non-ICU setting in another hospital (hospital transfer) and 408 (12%) from another hospital's ICU (ICU transfer). ICU mortality and standardized mortality ratios were significantly higher for ICU-transfer patients than for hospital-transfer or directly admitted patients: 34% vs. 23% vs. 17% (p < .0001) and 0.95 (95% confidence interval, 0.83-1.08), 0.82 (95% confidence interval, 0.71-0.95), and 0.62 (95% confidence interval, 0.55-0.68), respectively. ICU-transfer patients had 3.6-fold longer mean ICU stays and 1.9-fold longer durations of mechanical ventilation than directly admitted patients. Hospital-transfer (odds ratio = 1.89) and ICU-transfer patients (odds ratio = 2.41) had significantly higher mortality rates, even after adjustment for case mix and disease severity. Consequently, a benchmarking program adjusting only for these latter variables, but not admission source, would penalize our ICU by 39 excess deaths per 1,000 admissions as compared with another ICU admitting no transfer patients. Finally, patients transferred from the ward of another hospital had significantly higher mortality rates (odds ratio = 1.56) as compared with patients directly admitted from the ward of our hospital, confirming the "transfer effect" for this homogeneous patients' subgroup.
Conclusions: Admission source remains a strong and independent predictor of ICU death, despite adjustment for case mix and disease severity at ICU admission. Specifically, accepting numerous ICU-transfer patients, for whom the probability of ICU death is the most underestimated by a system adjusting only for case mix and disease severity, can adversely affect the evaluation of referral centers' performance. Future benchmarking and profiling systems should evaluate and adequately account for the ICU-transfer factor to provide healthcare payers and consumers with more accurate and valid information on the true performance of referral centers.