I dont know what a CMP test is. As far as Im aware, blood volume isnt generally tested and something which is more so tested in research labs (so there must be a way to actually test this). Anyone who has ME should suspect they "may" have this issue but one thou can come to an fair idea if low blood volume may be partly involved in causing symptoms by various indications of it. Some of these indications would be: - thirst to which another cause cant be found or that someone drinks far more then usual to feel better some - orthostatic intollerance eg headaches and dizziness when standing (not enough blood getting to brain), finding that one improves some on laying (blood gets to head better then). - Finding standing is worst then walking. (less blood gets to the head when standing then when moving). - POTS (in ME is usually caused by low blood volume). Other orthostastic autonomic issues may indicate too. - finding that treatments which raise blood volume help - those who have issues with low blood volume are usually affected by heat more then those who dont have this issue. So feeling worst on hot days is another indication.