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Detailed statistical analysis plan for the difficult airway management (DIFFICAIR) trial

Anders Kehlet Nørskov12*, Lars Hyldborg Lundstrøm1, Charlotte Vallentin Rosenstock1 and Jørn Wetterslev2

Author Affiliations

1 Department of Anaesthesiology, Nordsjællands Hospital, Copenhagen University Hospital, Hillerød, Capital region of Denmark 3400, Denmark

2 Copenhagen Trial Unit, Center for Clinical Intervention Research, Copenhagen University Hospital, Rigshospitalet, Copenhagen 2100, Denmark

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Trials 2014, 15:173  doi:10.1186/1745-6215-15-173

Published: 15 May 2014



Preoperative airway assessment in Denmark is based on a non-specific clinical assessment left to the discretion of the responsible anesthesiologist. The DIFFICAIR trial compares the effect of using a systematic and consistent airway assessment versus a non-specific clinical assessment on the frequency of unanticipated difficult airway management.

To prevent outcome bias and selective reporting, we hereby present a detailed statistical analysis plan as an amendment (update) to the previously published protocol for the DIFFICAIR trial.


The DIFFICAIR trial is a stratified, parallel group, cluster (cluster = department) randomized multicenter trial involving 28 departments of anesthesia in Denmark randomized to airway assessment either by the Simplified Airway Risk Index (SARI) or by a usual non-specific assessment. Data from patients’ preoperative airway assessment are registered in the Danish Anesthesia Database. An objective score for intubation grading the severity, that is the severity of the intubations, as well as the frequency of unanticipated difficult intubation, is measured for each group.

Primary outcome measures are the fraction of unanticipated difficult and easy intubations.

The database is programmed so that the registration of the SARI is mandatory for the intervention group but invisible to controls.

Data recruitment was commenced in October 2012 and ended in ultimo December 2013.


We intend to increase the transparency of the data analyses regarding the DIFFICAIR trial by an a priori publication of a statistical analysis plan.

Trial registration NCT01718561.

Statistical analysis plan; Cluster randomized trial; Airway management; Cluster analysis; Difficult intubation