<prompt> The reagent used in Apt test is: D. REPEAT Feb 2014Sodium bicarbonateKCLSodium hydroxideSodium chloride  <response>: Ans: C Sodium hydroxide Ref: Nelsons Textbook of Pediatrics, 19th editionExplanation:The Alkali Denaturation Test Apt-Downey Test or Apt TestUsed to detect differentiate Fetal or neonatal blood from maternal blood Qualitative.Clinical Uses:o Vaginal bleeding in late pregnancy is tested to rule out Vasa previa.o Neonatal bloody vomitus or bloody stool to rule out swallowed maternal blood.o To detect the presence of fetal blood in the maternal circulation in cases of suspected fetal-maternal hemorrhage.Positive test indicates that blood is of fetal origin.Negative test indicates that the blood is o maternal origin.PrincipleFetal hemoglobin alpha 2 gamma 2 subunits is resistant to alkali basic denaturation, whereas Adult hemoglobin alpha 2 beta 2 subunits is susceptible to such denaturation.Exposing the blood specimen to sodium hydroxide NaOH will denature the adult but NOT the feta! hemoglobin.The fetal hemoglobin will appear as a pinkish color under the microscope while the adult hemoglobin will appear as a yellow- brownish color.MethodThe blood is mixed with sterile water to cause hemolysis of the RBCs, yielding free hemoglobin.The 5 ml pink hemoglobin-containing supernatant is then mixed with 1 mL of 1 NaOH.Fetal hemoglobin will stay pink and adult hemoglobin will turn yellow-brown.Adult hemoglobin changes colour because it is less stable and will convert to hematin. 