**2.2 Laboratory tests**

Arterial oxygen pressure (PaO2) is a marker for diagnosis of NRDS. PaO2 less than 50 mmHg with cyanosis in room air or need for supplementary oxygen for maintaining O2 level above 50 mmHg are indicators for NRDS [14]. Metabolic and respiratory acidosis are measured through a blood sample.

Gastric aspirate shake test (GAST) is another laboratory measure with reported sensitivity of 100% and specificity of 92% for diagnosis of NRDS [18]. GAST identifies presence or lack of surfactant in the gastric fluid aspirates [19].

Recently published studies have mentioned a new factor for early detection and prediction of NRDS in premature infants. Transforming growth factor β1 (TGF-β1) is a cytokine, which has the responsibility for regulating and differentiating different cell lines [20, 21]. These studies have marked the role of TGF-β1 in development of various acute and chronic lung injuries and concluded that this factor can be used as a diagnostic and prognostic one [22]. The same role has been considered for interleukin-6, which is a glycoprotein secreted mostly from T cells and mononuclear macrophages causing inflammatory reactions [23, 24].
