Final answer:
A sudden drop in hemoglobin levels is the lab finding that should prompt an investigation for a postpartum hematoma, as it indicates potential internal bleeding which could be due to a hematoma.
Step-by-step explanation:
The lab finding that should prompt investigation for a postpartum hematoma is a drop in hemoglobin. A postpartum hematoma is a collection of blood that forms in the tissue due to blood vessel damage during delivery. Symptoms may include pain and swelling, but a significant indicator in a lab test is a sudden drop in hemoglobin levels, which could indicate bleeding. While elevated serum creatinine could signal kidney issues and elevated white blood cell count could indicate infection, it's the decrease in hemoglobin that specifically suggests a loss of blood internally, potentially from a hematoma. Decreased platelet count could also suggest other bleeding disorders but is not as direct an indicator of a hematoma as a decrease in hemoglobin.