Final answer:
The best laboratory test indicator of a client in need of TPN is serum albumin, as it reflects nutritional status.
Step-by-step explanation:
The laboratory test that is the best indicator of a client in need of Total Parenteral Nutrition (TPN) is serum albumin.
Serum albumin is the most abundant plasma protein and is an important indicator of nutritional status. Low levels of serum albumin indicate protein/calorie malnutrition, which may necessitate TPN.
Hemoglobin and hematocrit measure the levels of red blood cells and are not specific indicators of TPN needs. Creatinine is a waste product that indicates kidney function and is not directly related to the need for TPN.