Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1038 page)

   After counting 2,000 red cells, the results are reported as a percentage of fetal red cells present in the maternal circulation. This result can be multiplied by the maternal blood volume in order to determine the volume of fetal blood (mL) present in the maternal circulation.

   
Normal range
(adult RBCs): <1% fetal Hb.

   Interpretation

   The presence of fetal RBCs in maternal blood indicates fetal–maternal hemorrhage.

   Limitations

False-Positive Results

   Fetal Hb–containing RBCs may be found in approximately 50% of pregnant women (but in only 1% of pregnancies is the infant anemic).

   Certain hematologic disorders in adults, such as leukemias or myelodysplastic syndromes, may increase the level of fetal-type hemoglobin.

   Lymphocytes may take up stain in varying degrees.

False-Negative Results

   A major blood group incompatibility between mother and infant can cause elimination of fetal RBCs due to hemolysis.

LACTATE DEHYDROGENASE

   Definition

   LD occurs in the cytoplasm of all cells; there are five isoenzymes. The highest concentrations are found in the heart, liver, skeletal muscle, kidney, and the RBCs, with lesser amounts in the lung, smooth muscle, and brain. LD catalyzes the interconversion of lactate and pyruvate.

   
Normal range:
110–240 IU/L.

   Use