What lab tests are recommended before initiating Total Parenteral Nutrition (TPN)?

Laboratory Tests Before Initiating TPN

Before starting TPN, obtain baseline electrolytes (sodium, potassium, calcium, phosphorus, magnesium), triglycerides, serum urea, blood glucose, complete blood count, liver function tests (ALT, AST, bilirubin, GGT, alkaline phosphatase), and coagulation studies. 1

Essential Baseline Laboratory Panel

Core Metabolic Assessment

• Electrolytes: Sodium, potassium, calcium, phosphorus, and magnesium are mandatory to determine nutritional deficiencies and establish baseline values before PN initiation 1
• Glucose: Blood glucose monitoring is critical as hyperglycemia is one of the most common TPN complications, with baseline values needed to guide initial dextrose dosing 1
• Triglycerides: Baseline lipid levels must be obtained since TPN provides 20-30% of calories as lipids, and levels should be maintained below 400 mg/dL (optimally) or at minimum below 700-800 mg/dL 1
• Renal function: Serum urea and creatinine help determine protein requirements and assess kidney function 1

Nutritional Status Markers

• Complete blood count and total lymphocyte count: These visceral protein markers help identify malnutrition severity 1
• Liver function tests: ALT, AST, bilirubin, GGT, and alkaline phosphatase establish baseline hepatic function, as TPN can cause hepatobiliary complications 1
• Coagulation studies: Baseline coagulation parameters are needed to assess vitamin K status and overall synthetic liver function 1

Additional Considerations for Specific Populations

• Pre-albumin or retinol-binding protein: These shorter half-life proteins better reflect acute nutritional status than albumin, though albumin is commonly low due to acute phase response rather than true malnutrition 1
• Serum urea: Helps characterize protein deficit and guides amino acid dosing 1

Critical Monitoring for Refeeding Syndrome Prevention

Patients at high risk for refeeding syndrome require particularly close attention to phosphorus, potassium, and magnesium levels before and during TPN initiation. 2

• Approximately 84% of patients starting TPN develop one or more electrolyte abnormalities, with hypophosphatemia occurring in 30%, hypomagnesemia in 27.5%, and hypokalemia in 27.5% of patients 2
• High-risk patients (severely malnourished, prolonged fasting >7-10 days, significant weight loss) are more likely to develop electrolyte abnormalities despite preventative measures 2
• Thiamine (Vitamin B1) should be administered prior to starting glucose infusion to reduce the risk of Wernicke's encephalopathy in malnourished patients 3

Micronutrient Assessment for Long-Term TPN

While not required before initiating TPN, baseline vitamin and trace element levels should be considered for patients expected to require long-term PN:

• Vitamins: A, D, E, B12 should be evaluated in long-term PN-dependent patients 1
• Trace elements: Ferritin, iron, zinc, copper, selenium, and manganese should be assessed 1
• These are typically measured at 6-12 month intervals once TPN is established, but baseline values can guide initial supplementation 1

Common Pitfalls to Avoid

• Do not delay TPN initiation to obtain extensive micronutrient panels in acutely ill patients; basic metabolic assessment is sufficient to start safely 1
• Do not attribute hypoalbuminemia solely to malnutrition in hospitalized patients, as it commonly reflects acute phase response to inflammation rather than nutritional status 1
• Do not overlook refeeding syndrome risk assessment based on clinical history and baseline electrolytes, as this determines initial caloric provision (typically starting at 12.8 kcal/kg/day in high-risk patients versus 25-30 kcal/kg/day in normally nourished patients) 1, 2
• Ensure adequate monitoring frequency is established from the start: daily glucose monitoring (optimally 4 times daily), daily electrolytes in high-risk patients, and 2-3 times weekly in stable patients 1