Some infusion companies have patients keep records of their oral food intake, and conduct tests to determine if the formula meets the patient's calorie and protein needs. Physicians may have to adjust the prescription when these needs change. Since lipids produce about twice the number of calories per gram as carbohydrates, a formula with a minimum amount of dextrose and the remaining required calories in lipids, will mean less volume to infuse. Also, the more carbohydrates, the more carbon dioxide is produced as a by-product of glucose metabolism. Glucose-based formulas may also create problems with high blood sugar and elevated liver enzymes.
Most formulas contain both lipids and dextrose along with protein, vitamins, minerals, etc.
One of the major drawbacks of TPN is its cost. Some insurance companies may not cover it all. Another drawback is that TPN requires surgically implanting a central venous line catheter, such as a Groshong catheter. This involves outpatient surgery with rare complications. There are small surgical scars, and the plastic tubing extends out of the chest exit site.