Phosphorus is not typically on a metabolic panel. It has been run by my nephrologist (I have chronic kidney disease) in the past). The critical low (1.0mg/dl) was found when I was in the ER as part of the cardiac work up. They often draw a magnesium level (along with troponin levels, d-dimer, etc) and the ER doctor ran the phosphorus along with those. Now that I am stabilized and home they are having me do a 24 hour urine collection to get more accurate measurement.