*2.3.4 Auxiliary diagnostic techniques*

Several auxiliary tests are required to support the diagnosis, to identify the complications, and to aid in the differential diagnosis in OSA patients. Blood tests are helpful in identifying the diseases that might lead to OSA and their complications. Hemograms can play a role in idiopathic polycythemia when establishing a preliminary diagnosis of OSA. Thyroid hormone profile (T3, T4, TSH) is a routine test to discriminate between OSA and hypothyroidism [34, 35]. Lung X-rays can help identify certain lung diseases and their complications that accompany OSAS. OSArelated respiratory function tests have a ratio of greater than 1 for "forced expiratory flow 50% /forced inspiratory flow 50%" and saw tooth pattern in flow-volume curve. Individuals with OSA have normal arterial blood gas levels during daytime; in cases having chronic hypercapnia during daytime while they are awake, coexistence of chronic obstructive pulmonary diseases or diseases leading to alveolar hypoventilation like neuromuscular insufficiency should be considered [36]. In an obese patient who is hyper-somnolent and who snores, a diagnosis of OSA should be eliminated. In an individual having findings of right heart failure and pulmonary hypertension on echocardiogram, if these findings cannot be explained, possible coexistence of OSA should be considered [37, 38]. In OSA, sleep is interrupted due to apnea episodes frequently recurring during the night, and the patients are excessively sleepy the next day. In the identification of sleepiness, the most commonly used method is "Epworth Sleepiness Scale" [39]. A diagnostic method for objective demonstration of excessive sleepiness during daytime is "Multiple Sleep Latency Test (MSLT)". Despite the fact that it does not have a direct role in the diagnosis of OSA, it helps to differentiate diseases like central hypersomnia and narcolepsy that cause similar symptoms. Having short sleep latency is a non-specific sign for sleep pressure during daytime and is often seen for OSA patients [40].
