What is the most common electrolyte abnormality in patients on total parental nutrition?

Depends. Lots of potential problems. Abnormalities in sodium, potassium, magnesium are not uncommon. Abnormalities in glucose (not an electrolyte) are common. Careful monitoring, and appropriate changes in formulas, or Insulin dosing can help keep these down to a minimum. Some patients have been able to be maintained on long-term TPN (>10-15 yrs) in good health. Infection always a risk too.