A "precursor cell marker TdT test" refers to a laboratory test that detects the presence of an enzyme called "Terminal deoxynucleotidyl transferase (TdT)" in a patient's blood or bone marrow sample, which is primarily used to identify immature blood cells (precursor cells) and is particularly helpful in diagnosing acute lymphoblastic leukemia (ALL) as these leukemia cells often express high levels of TdT.