Overnight urinary uric acid: Creatinine ratio for detection of sleep hypoxemia: Validation study in chronic obstructive pulmonary disease and obstructive sleep apnea before and after treatment with nasal continuous positive airway pressure

During hypoxia ATP degradation to uric acid is increased in animal models and humans. To assess the reliability of an overnight increase in uric acid excretion as a marker of nocturnal hypoxemia, we selected 10 normal volunteers (7 males and 3 females), 29 COPD patients (26 males and 3 females), and 49 subjects with obstructive sleep apnea (OSA) (43 males and 6 females). The patients underwent standard polysomnography, which was repeated in 14 subjects with nasal continuous positive airway pressure (CPAP), and were subdivided into two groups: Group D included desaturating subjects who spent at least 1 h at Sa(O₂) <90% and 15 min below 85%, and Group ND were nondesaturating subjects. The overnight change in the uric acid:creatinine ratio (ΔUA:Cr) was negative in normal subjects (-27.5 ± 9.1 [mean ± SD]) and ND groups: -19.7 ± 14.3 in COPD, -16.1 ± 13.0 in OSA. In both COPD and OSA Group D, the ratio was usually positive: ΔUA:Cr was 17.9 ± 31.4 in Group D COPD (p <0.001 versus ND) and 10.1 ± 30.7 in Group D OSA (p <0.001 versus ND and versus normal subjects) despite 4 of 15 false negative results in COPD and 8 of 20 in OSA. CPAP effective treatment induced a marked reduction ((p = 0.0024) in ΔUA:Cr, leading to a negative value. We conclude that ΔUA:Cr seems to be a promising index of significant nocturnal tissue hypoxia, with good specificity but poor sensitivity (about 30% false negative), which might be useful for the long-term follow-up of outpatients on nasal CPAP with a positive ratio at baseline.