Salivary Gland Pathology: The most likely cause of isolated elevated amylase levels is salivary gland pathology, such as salivary gland inflammation, infection, or tumor. This is because amylase is also produced in the salivary glands, and conditions affecting these glands can lead to elevated amylase levels without affecting lipase.

Pancreatic Pathology with Early or Mild Disease: Although lipase is often more specific for pancreatic disease, early or mild pancreatic conditions (like pancreatitis or a small pancreatic tumor) might initially present with elevated amylase and normal lipase levels.
Macroamylasemia: A condition where amylase is bound to immunoglobulins, leading to elevated serum amylase levels without any actual pancreatic or salivary gland disease.
Renal Failure: Decreased renal clearance can lead to elevated serum amylase levels, as the kidneys are responsible for clearing amylase from the blood.

Pancreatic Cancer: Although less common, pancreatic cancer can present with elevated amylase levels, especially if the tumor involves the pancreatic ducts. Missing this diagnosis could be catastrophic due to the poor prognosis of pancreatic cancer.
Severe Pancreatitis: In some cases of severe pancreatitis, especially if there's significant pancreatic necrosis, lipase levels might not be elevated, or the elevation might be transient. It's crucial not to miss this diagnosis due to its high morbidity and mortality.

Pregnancy-Related Conditions: Certain conditions in pregnancy, such as hyperemesis gravidarum, can lead to elevated amylase levels, although this is rare.
Drug-Induced Hyperamylasemia: Some medications can cause elevated amylase levels without any underlying pancreatic or salivary gland disease.
Familial Hyperamylasemia: A rare genetic condition leading to chronically elevated amylase levels without any symptoms or disease.