Nile blue sulphatase test in amniotic fluid is for the following:

Centrifuged cells in amniotic fluid stained with 0.1% Nile blue sulphatase shows orange blue colouration of the cells. The discolouration is due to presence of exfoliated fat containing cells from sebaceous glands of the skin of the fetus. It is an indicator of skin maturity of the fetus. This test usually used for the diagnosis of premature rupture of membranes. Ref: Textbook of Obstetrics by D C Dutta, 6th edition, Page 317.