Final answer:
The Direct Coombs test is the chemistry test used to diagnose or monitor Hemolytic Disease of the Newborn (HDN). It detects antibodies bound to the newborn's red blood cells, which can lead to cell destruction and jaundice.
Step-by-step explanation:
The chemistry test done to diagnose or monitor Hemolytic Disease of the Newborn (HDN) is the Direct Coombs test. This test aims to detect if the red blood cells of the newborn have been coated with antibodies from the mother, which could lead to their destruction. Presence of these antibodies suggests the immune system is mistaking the baby's red blood cells for foreign invaders and initiating their breakdown, resulting in high levels of bilirubin and the associated jaundice.
The Direct Coombs test looks for nonagglutinating antibodies or complement proteins attached to red blood cells in vivo. The Indirect Coombs test (Indirect Antiglobulin Test) is used to screen a pregnant woman's serum for antibodies that might lead to HDN, though it's not used to diagnose HDN in the newborn itself. These antibodies are against red blood cell antigens that are unbound, and uninvolved with ABO blood group antigens. Determining ABO blood types and Rh antigens is also crucial in the management of HDN, but for initial diagnosis and monitoring, the Direct Coombs test holds the key role.