Best method for the diagnosis of lung maturity is?

Phosphtidyl glycerol estimation in amniotic fluid REF: Williams Obstetrics 22nd ed Chapter 29 Several procedures have been proposed for its evaluation but two of them (L/S ratio, "shake test" and phosphatidylglycerol in amniotic fluid) have been widely acclaimed. According to Williams estimation of phosphatidylcholine or phosphatidylglycerol is better and more sensitive and specific for lung maturity because phosphatidylglycerol is not detected in blood, meconium, or vaginal secretions; these contaminants do not confuse its interpretation The same view is suppoed by nelson.