Richard, a hemoglobin check makes perfect sense. The JW population belong to a rare blood group that cannot have the part of blood that corrects for anemia. A belief, a phbia, a conviction...whatever, they can't have that part of blood. Early detection of an anemic condition, with a simple blood test for iron levels, is practical.
Iron supplements and diet can sometimes correct hemoglobin deficiencies in a person's blood, whether they appear healthy or not