Study of serum cystatin C levels in patients with obstructive sleep apnea

Background Serum cystatin C is a sensitive indicator for early renal impairment in patients with obstructive sleep apnea (OSA). In patients with OSA without known chronic kidney disease (CKD), serum cystatin C was found to be elevated, indicating latent renal disease, which may be caused by recurrent episodes of hypoxemia and reoxygenation that trigger inflammatory process. Objective To evaluate serum levels of cystatin C in otherwise healthy patients with OSA to predict the future risk of latent renal impairment in these patients. Patients and methods A total of 30 patients were classified into two groups: group I included 10 healthy controls, and group II included 20 patients with OSA. The following were done: full history taking, clinical examination, pulmonary function tests, subjective evaluation of daytime sleepiness using the Epworth Sleepiness Scale, and overnight polysomnography. Finally, serum cystatin C levels at the end of polysomnography and glomerular filtration rate was calculated using modification of diet in renal disease formula and CKD-Epidemiology Collaboration cystatin equation. Results In patients with OSA, the mean level of serum cystatin C was statistically significantly higher than that in controls. These higher levels were sex, age, and BMI independently. Moreover, serum levels of cystatin C in patients with OSA were significantly correlated with OSA severity (apnea-hypopnea index) and degree of nocturnal hypoxia through positive correlation with total sleep time less than 90%, total sleep time less than 80%, and respiratory disturbance index and negative correlation with average SPO2%, lowest SPO2%, and estimated glomerular filtration rate by CKD-Epidemiology Collaboration cystatin equation. Conclusions Higher serum levels of cystatin C were detected in patients with OSA without known comorbidities, which indicates that these patients are at a higher risk of developing CKD. Intermittent hypoxia seems to have the main role in the progression of this process.