Theory
The test is based on differences between maternal and fetal hemoglobin. Maternal blood contains adult hemoglobin composed of two alpha and two beta subunits (aka hemoglobin A or HbA; i.e., normal adult hemoglobin). Fetal blood contains fetal hemoglobin composed of two alpha and two gamma subunits (aka hemoglobin F or HbF; i.e., normal fetal hemoglobin). This difference in composition gives the different types of hemoglobin different chemical properties (in addition to the higher affinity HbF has for dissolved blood oxygen over HbA allowing baby to compete for oxygen better than mother). Fetal hemoglobin is resistant to alkali (basic) denaturation, whereas adult hemoglobin is susceptible to such denaturation. Therefore, exposing the blood specimen to sodium hydroxide (NaOH) will denature the adult but not the fetal hemoglobin. The fetal hemoglobin will appear as a pinkish color under the microscope while the adult hemoglobin will appear as a yellow-brownish color.
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