Acute pulmonary embolism (PE) is a major health problem. In 2004, 122,000 patients were discharged with a primary diagnosis of PE from U.S. hospitals, with an average 30-day mortality of approximately 9%.^{5, 6} The diagnosis of PE is complex, requiring the timely access to sophisticated radiographic procedures such as spiral computed tomography, ventilation-perfusion lung scanning, and venous ultrasonography.⁷ In addition, the quality of anticoagulant therapy, the mainstay of treatment for this condition, may depend on provider experience and the availability of specialized anticoagulation clinics.⁸ Timely access to thrombolytic therapy and other measures of intensive care may also affect patient outcomes. Since it is likely that the availability and quality of these diagnostic and therapeutic procedures differ for each day in the week, it is possible that an association exists between weekend admission and outcomes in patients with PE. Given the high incidence and mortality related to this condition, even small differences in mortality between weekend and weekday admissions could translate into a substantial number of additional deaths in the population.

Using a statewide database of 15,531 patients discharged with PE, we sought to assess whether there were associations between weekday and weekend admission and mortality and LOS for patients hospitalized with PE. We hypothesized that patients admitted on weekends would have a higher short-term mortality and an increased LOS compared to patients admitted on weekdays.

There are several possible explanations for the association between weekend admission and increased 30-day mortality, particularly among the most severely ill patients. Fewer medical providers and professional staff tend to work in hospitals on weekends than on weekdays,^{13, 14} and those who do work on weekends may have less clinical experience.^{15, 16} There are also fewer supervisors on weekends, and they are often responsible for supervising the work of staff members they do not know as well.¹⁷ Understaffing in the emergency and radiology departments, numerically and in terms of expertise, could potentially result in delayed diagnosis and treatment of PE, with an unfavorable impact on patient prognosis. Provision of care by covering physicians and/or more junior physicians may lead to the underuse of recommended processes of care for PE that are associated with improved patient outcomes. These processes of care include the use of validated algorithms for diagnosing PE and anticoagulation-related procedures such as an overlap of heparin and warfarin therapy of 4 days or more before heparin is stopped.^{7, 18} Moreover, specialized services with positive impact on anticoagulation quality and outcomes,⁸ such as anticoagulation clinics, may be temporarily unavailable during weekends. Inadequate professional staffing and medical coverage during the weekend may also delay the detection of potentially fatal, early complications (e.g., cardiogenic shock, anticoagulation-related bleeding). The relative contributions of these possible explanations to higher mortality among weekend admissions are not known but the impact falls most heavily on the most severely ill patients with PE. Further research is warranted on the associations between professional staffing, medical coverage, processes of care, and outcomes for patients with PE. If understaffing or medical coverage contributed to lower quality of care or worse outcomes, possible solutions would be to increase staffing and medical coverage during weekends or regionalize PE care in higher-volume hospitals where continuous coverage can be provided by personnel experienced in the management of PE.¹⁹

We found that the survival curves of patients admitted on weekends and patients admitted on weekdays began to diverge early and continued to separate over time. One possible explanation for this finding is that suboptimal care in the early phase of PE may also bring dire consequences later in the course of the illness. In a prior study, patients who did not reach a therapeutic activated partial thromboplastin time within 24 hours had a three-fold higher risk of recurrent venous thromboembolism throughout a three-month follow-up period of than those who did.²⁰

Our findings have important public health implications. Extrapolating from our data, we estimate that, each year, about 26,000 patients with a primary diagnosis of PE are admitted on weekends in the United States.⁵ Based on an excess 30-day mortality of 1.4% among weekend admissions with PE compared to patients admitted on weekdays, about 364 additional deaths would be potentially attributable to weekend admissions in the United States annually, or about 3 additional deaths per 1000 admissions for PE.

Our results are consistent with a prior Canadian study of 11,686 patients with PE that found a significantly higher in-hospital mortality among patients who were admitted on weekends relative to patients admitted on weekdays during the 1988-1997 period (OR 1.19, 95% CI: 1.03-1.36), after adjusting for age, sex, and the Charlson comorbidity index.²¹ However, a small Scottish study enrolling of 137 patients with PE did not find a significant association between weekend admission and in-hospital mortality or LOS,²² possibly due to a lack of power.

Our study has several limitations. First, patients in our sample were identified using ICD-9-CM codes for PE rather than standardized radiographic criteria, and patient eligibility may therefore be subject to study selection biases due to hospital coding procedures. In prior studies, up to 96% of patients with specific codes for PE had objectively documented disease on the basis of chart review criteria,^23-25 but little is known about the sensitivity of these codes for detecting this condition. In one previous study, the ICD-9-CM codes missed 13% of patients with PE.²⁶ Thus, we cannot entirely exclude the possibility that the potential for variation in the sensitivity of coding across study centres represents a threat to the validity of our findings (misclassification bias). Second, although we used several techniques to adjust for severity of illness, it is possible that the observed mortality difference between weekend and weekday admissions may be due to unmeasured, residual confounding. Third, we could not assess whether differences in the duration of symptoms, appropriateness and timeliness of diagnosis or anticoagulation-related processes of care would explain differences in outcomes between patients admitted on weekends and weekdays. Moreover, we had no information on physician-level (e.g., experience, specialty training and annual volume of PE cases per physician) and system-level (e.g., staff volume and availability of anticoagulation clinics) factors with a potential impact on the recommended processes of care and outcomes of PE. Thus, we could not explore whether these factors are associated with the higher observed mortality among patients admitted on weekends. Fourth, our study does not account for deaths declared by paramedics before hospital admission.²¹ Because these deaths may be more common on weekends than on weekdays,²¹ our study may have underestimated the difference in mortality. Finally, because our study patients were hospitalized during the 2000-2002 period, we cannot exclude the possibility that changing practice patterns, such as earlier diagnosis of PE using spiral computed tomography or increasing use of low-molecular-weight heparins rather than unfractionated heparin could have influenced our results.