A 18 weeks pregnant, Rh negative multigravida with previous history of a child born with chromosomal abnormality underwent cordocentesis. The test used to differentiate between maternal and fetal blood in a given sample is:

Apt test or hemoglobin alkaline denaturation test is a quick bedside technique to differentiate fetal blood from maternal blood. It is based on the ability of fetal hemoglobin to resist denaturation in alkaline conditions. To detect the ratio of fetal RBCs to maternal RBCs, a Kleihauer-Betke test is performed. Osmotic Fragility of Erythrocytes (Dacie's Method) measures the relative resistance of red cells to haemolysis in a hypotonic medium. Amniotic fluid phospholipid is determined by bubble stability test. Also know: Fetal blood cells are (140 fL) are larger than maternal cells (80 fL). Ref: Obstetrics: Normal and Problem Pregnancies By Steven G. Gabbe, Jennifer R. Niebyl, Henry L Galan, Eric R. M. Jauniaux, Mark B Landon, Joe Leigh Simpson, Deborah A Driscoll, 2012, Page 214.