How is fetal blood differentiated from maternal blood:

Apt test or Singer&;s alkali denaturation test can be performed to confirm a fetal bleed and thereby vasa pre.Apt test is based on the principle that fetal hemoglobin is more resistant to alkali denaturation.Therefore,when water and the blood are mixed with sodium hydroxide,it remains pink for longer if fetal in origin.if it is maternal blood.it turns yellow brown in 2 min,as adult hemoglobin is denatured easily. TEXT BOOK OF OBSTETRICS,Sheila Balakrishnan,2nd edition,page no.138