Why do they test the fluid samples from a thoracentesis for amylase and glucose?

Evaluate cause: A thoracentesis involves removing fluid from the pleural cavity. Laboratory values obtained from the fluid may enable a doctor to determine the cause of the fluid accumulation. Amylase present in the fluid could mean either pancreatitis or, more rarely, esophageal perforation. Glucose<40mg/dl, possible infection, >40mg/dl probably not infectious.

Fluid: Amylase is to test to see if it is related to the pancreas. The glucose can help determine if it is related to infection.