Study objective: The cost and inconvenience of polysomnography make simplified techniques of screening desirable in the strategy of diagnosis of sleep apnea syndrome (SAS). We have evaluated, in a prospective study of 301 consecutive patients referred for suspected sleep disorders, an index (delta index) that detects apneic events by quantifying arterial oxygen saturation (SaO2) variability.
Setting: Regional sleep laboratory taking referrals from general practitioners and specialists.
Design: Classic polysomnography was the gold standard, with 15 apneas plus hypopneas per hour (RDI) being used as a threshold for definition of obstructive sleep apnea (OSA). Oximetry was recorded over the same night. Signal variability was quantified as a function of time, using digital processing of oximetric data. Sensitivity, specificity, and positive and negative predictive values of oximetry testing were calculated. A receiver operating characteristic (ROC) curve was constructed representing the comparative courses of sensitivity and 1-specificity at different thresholds of delta index.
Results: Three hundred one patients were included (age, 56 +/- 12 years). Their RDI was 30 +/- 24. For a delta threshold at 0.6, the sensitivity of oximetry for the diagnosis of OSA was 98% and the specificity was 46%. The positive and negative predictive values for diagnosing SAS were 77% and 94%, respectively. The three false-negative cases had a relatively high awake SaO2 (97 vs 93.9 +/- 2.8%), a moderate RDI (23.3 +/- 1.6), and were less obese than the other patients (body mass index: 25 +/- 3 vs 33 +/- 8). The 58 false-positive cases had an RDI of 8 +/- 4, an awake SaO2 of 93.1 +/- 3.6 vs 94.1 +/- 2.6 for the rest of the population (p = 0.01). Finally, the false-positive cases had more airways obstruction (FEV1/VC = 72 +/- 13 vs 77 +/- 15%; p = 0.026). Using a delta value of 0.8 leads to a sensitivity of 90% with 19 false-negative cases but with a higher specificity of 75%.
Conclusions: A nocturnal oximetry test with a delta index below 0.6 is helpful in ruling out the diagnosis of SAS in patients being screened for this condition, as this yielded only three negative test results in 301 screening procedures.