<prompt> The Kleihauer Betke test for detecting foetal erythrocytes is based on the fact thatAdult erythrocytes are larger than those of foetusHbA has higher oxygen affinity than HbFHbF is more resistant acid elution than HbAHbA takes up erythrosin stain less than HbF  <response>: The most commonly used quantitative test for fetal red cells in the maternal circulation is the acid elution or Kleihauer Betke test. Fetal erythrocytes contain HbF, which is more resistant to acid elution than HbA. 