High Quality Content by WIKIPEDIA articles! Hypomagnesemia (or hypomagnesaemia) is an electrolyte disturbance in which there is an abnormally low level of magnesium in the blood. Usually a serum level less than 0.7 mmol/L is used as reference. The prefix hypo- means low (contrast with hyper-, meaning high). The middle `magnes` refers to magnesium. The end portion of the word, -emia, means `in the blood.`