title: The impact of obstructive sleep apnea variability measured in-lab versus in-home on sample size calculations.
abstract: BACKGROUND
  When conducting a treatment intervention, it is assumed that variability associated with measurement of the disease can be controlled sufficiently to reasonably assess the outcome. In this study we investigate the variability of Apnea-Hypopnea Index obtained by polysomnography and by in-home portable recording in untreated mild to moderate obstructive sleep apnea (OSA) patients at a four- to six-month interval.
  
  
  METHODS
  Thirty-seven adult patients serving as placebo controls underwent a baseline polysomnography and in-home sleep study followed by a second set of studies under the same conditions. The polysomnography studies were acquired and scored at three independent American Academy of Sleep Medicine accredited sleep laboratories. The in-home studies were acquired by the patient and scored using validated auto-scoring algorithms. The initial in-home study was conducted on average two months prior to the first polysomnography, the follow-up polysomnography and in-home studies were conducted approximately five to six months after the initial polysomnography.
  
  
  RESULTS
  When comparing the test-retest Apnea-hypopnea Index (AHI) and apnea index (AI), the in-home results were more highly correlated (r = 0.65 and 0.68) than the comparable PSG results (r = 0.56 and 0.58). The in-home results provided approximately 50% less test-retest variability than the comparable polysomnography AHI and AI values. Both the overall polysomnography AHI and AI showed a substantial bias toward increased severity upon retest (8 and 6 events/hr respectively) while the in-home bias was essentially zero. The in-home percentage of time supine showed a better correlation compared to polysomnography (r = 0.72 vs. 0.43). Patients biased toward more time supine during the initial polysomnography; no trends in time supine for in-home studies were noted.
  
  
  CONCLUSIONS
  Night-to-night variability in sleep-disordered breathing can be a confounding factor in assessing treatment outcomes. The sample size of this study was small given the night-to-night variability in OSA and limited understanding of polysomnography reliability. We found that in-home studies provided a repeated measure of sleep disordered breathing less variable then polysomnography. Investigators using polysomnography to assess treatment outcomes should factor in the increased variability and bias toward increased AHI values upon retest to ensure the study is adequately powered.
authors.affiliation: Advanced Brain Monitoring Inc., Carlsbad, CA 92008, USA. dan@b-alert.com
authors.forename: Daniel
authors.initials: D
authors.lastname: Levendowski
authors.forename: David
authors.initials: D
authors.lastname: Steward
authors.forename: B Tucker
authors.initials: BT
authors.lastname: Woodson
authors.forename: Richard
authors.initials: R
authors.lastname: Olmstead
authors.forename: Djordje
authors.initials: D
authors.lastname: Popovic
authors.forename: Philip
authors.initials: P
authors.lastname: Westbrook