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:
**Core Concept**
The test used to differentiate between maternal and fetal blood in a given sample during cordocentesis involves the detection of a specific enzyme present in fetal red blood cells, which is not present in maternal blood. This enzyme is crucial for the diagnosis of fetal blood in cases where the maternal and fetal circulations are mixed.
**Why the Correct Answer is Right**
During cordocentesis, it is essential to differentiate between maternal and fetal blood to avoid contamination and ensure accurate diagnosis. The enzyme used for this purpose is **Kleihauer-Betke test**. This test detects the presence of fetal hemoglobin by observing the resistance of fetal red blood cells to acid elution. Fetal red blood cells contain **HbF** (fetal hemoglobin), which is resistant to acid elution, whereas adult red blood cells contain **HbA** (adult hemoglobin), which is not. By using this test, the presence of fetal blood can be confirmed, and the risk of contamination can be minimized.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Kleihauer-Betke test is not used for differentiating between maternal and fetal blood. The Kleihauer-Betke test is used to detect the presence of fetal red blood cells in maternal circulation.
**Option B:** This option is incorrect because the Apt test is used to differentiate between fetal and adult hemoglobin by observing the resistance of hemoglobin to alkali denaturation. However, this test is not used for cordocentesis.
**Option C:** This option is incorrect because the Sabin-Feldman dye test is used for the diagnosis of toxoplasmosis, not for differentiating between maternal and fetal blood.
**Option D:** This option is incorrect because the Coombs test is used to detect the presence of antibodies against red blood cells, not to differentiate between maternal and fetal blood.
**Clinical Pearl / High-Yield Fact**
In cases of cordocentesis, it is essential to use the Kleihauer-Betke test to differentiate between maternal and fetal blood. This test is crucial for accurate diagnosis and avoiding contamination.
**Correct Answer: D. Apt test.**