Association Between Day and Time of Admission to Critical Care and Acute Hospital Outcome for Unplanned Admissions to Adult General Critical Care Units: Cohort Study Exploring the ‘Weekend Effect’

Arulkumaran N, Harrison DA and Brett SJ

British Journal of Anaesthesia 2017;118 (1): 112-22.

Aim of Study

  • Does the day and time of admissions to ICU influence overall mortality?
  • ICU must operate as a 24/7 speciality so answering the question of the existence of the weekend effect is an important clinical investigation.
  • It is imperative to ascertain whether out-of-hours services are adequate to cover emergency care without any adverse impact on outcome for patient

Design and Location

  • Prospectively collated cohort study
  • Data from UK Intensive Care National Audit and Research Centre (ICNARC) Case Mix Programme (CMP) database
  • Not including specialist units (neuro, cardiac) nor standalone HDU
  • Day split into “routine hours” (0800-1759) and “out of hours” (1800-0759)
  • Missing physiological data presumed normal
  • Potential confounders identified in advance and adjusted for in analysis:
    • Age
    • Severe conditions in past medical history
    • Prior functional dependency
    • Number of days from hospital admission to critical care admission
    • location before admission
    • CPR in 24 hours prior to critical care admission
    • Primary reason of admission to critical care
    • Acute severity of illness score (ICNARC and APACHE scores used)
  • Stats:
  • 3 multilevel logistic regression models used:
    • Unadjusted with single covariant of day/time of admission
    • Adjusted model adjusting for key potential confounders:
    • Adjusted model including delay/no delay covariate
  • 216 General/mixed Intensive Care Units in the UK that submitted data for the ICNARC CMP
  • April 1 2013-March 31st 2015



  • Inclusions:
    • Age >16
    • Unplanned admission
  • Exclusions:
    • Planned ICU admission (including post theatre, transfers and repatriation)
    • Readmissions – only the first episode was counted
    • Organ donation
    • Missing data (primary outcome or key confounders)
  • 300,469 admission screened, 195,428 included in final analysis
  • Baseline characteristics:
    • mean age of patients admitted was 60 yr
    • mean APACHE II = 17
    • ~1/4 had some degree of prior dependency
    • Overall, 4% of patients received in-hospital CPR during the 24 h before admission and a further 4% had out-of-hospital CPR
    • ~1/4 admitted with respiratory pathology
    • No significant differences between the two cohorts seen in any of the major criteria, including age, severity of illness, and co-morbidities

Weekend days cohort

  • Day of admission analysis: Patients admitted on Saturday or Sunday
  • Time of admission analysis: During routine working hours (0800-1759) and out of (1800 0759)

Week days cohort

  • Day of admission analysis: Patients admitted on Monday, Tuesday, Wednesday, Thursday or Friday
  • Time of admission analysis: During routine working hours (0800-1759) and out of (1800 0759)

  • Primary outcome:
    • Total ICU mortality: 18.8%
    • Total hospital mortality: 26.6%
    • Using Monday as reference day, no significant difference seen between odds of admission out of hours or at a weekend in adjusted analysis (P value =0.61)
  • Secondary outcome:
    • 8,295 (4.2%) patients experienced a delay (documented decision to admit to time of admission) of at least 1 hour prior to admission to ICU. 2,097 (1.1%) had delay >4hrs
      • Both groups associated with an increased risk of death (OR 1.08 for < 4hrs, 1.17 for >4 hrs, P=0.04)
      • Adjusting to account for delay did not change lack of effect of day/time of admission


multilevel logistic regression models.

The output of each model is presented as odds ratios with 95% confidence intervals (CIs) and P-values.

Hypothesis testing.

Results of the hypothesis tests are reported as the P-value (Wald test) and the absolute risk difference with 95% CI (based on linear combinations of the marginal predicted mortalities).


  • After risk adjustment with detailed clinical data, there was no difference in acute hospital mortality for unplanned admissions to ICU between weekdays and weekends, or daytime and nighttime
  • A small proportion of patients experienced delays in admission to ICU and this was associated with an increased risk of death

Stated Limitations from the Study


  • Important question
  • Large database
  • Multi centre
  • Detailed risk adjusted analysis
  • Secondary analysis an important question


  • Without delving into the inner workings of each ICU, shows association not causation
  • Difference exists between units in the ICNARC CMP
  • Delayed admission is a complex entity; may represent increased complexity of patient and further investigations or treatments.
  • UK based study

Discussion from Journal Club Meeting (?Change of Practice)

Ensure no delay to admissions to ICU

Ensure access to ICU specialist consultant at all times

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