Gilbert's Syndrome: A benign condition characterized by a mild increase in unconjugated bilirubin due to reduced activity of the enzyme uridine diphosphate glucuronyltransferase (UGT). It is often asymptomatic and may be triggered by stress, fasting, or illness. The normal AST level supports this diagnosis, as liver enzymes are typically not elevated in Gilbert's Syndrome.

Hemolysis: An increase in red blood cell breakdown can lead to elevated bilirubin levels. Conditions such as autoimmune hemolytic anemia or hereditary spherocytosis could be responsible. Normal AST levels do not rule out hemolysis, as liver function may remain unaffected.
Vitamin Deficiency: Deficiencies in vitamins like B12 or folate can lead to increased bilirubin due to ineffective erythropoiesis. These deficiencies might not directly affect AST levels.
Medication-Induced Hyperbilirubinemia: Certain medications can cause an increase in bilirubin levels, either by inducing hemolysis or affecting liver function. The normal AST suggests that liver damage is minimal or not present.

Hemoglobinopathies: Conditions like sickle cell disease or thalassemia can cause hemolysis and subsequent hyperbilirubinemia. Although less common, missing these diagnoses could have significant implications for patient management and outcome.
Liver Congestion: Right heart failure can lead to liver congestion, causing an increase in bilirubin levels. AST might remain normal if the liver cells are not significantly damaged.
Budd-Chiari Syndrome: A rare condition caused by thrombosis of the hepatic veins, leading to liver dysfunction and potentially elevated bilirubin. Early diagnosis is crucial for effective management.

Crigler-Najjar Syndrome: A rare genetic disorder affecting the UGT enzyme, leading to severe unconjugated hyperbilirubinemia. It is less likely given the relatively mild elevation of bilirubin in this case.
Dubin-Johnson Syndrome: A rare genetic disorder characterized by chronic jaundice due to an inability to secrete conjugated bilirubin into the bile. It typically presents with a more significant elevation of conjugated bilirubin.
Rotor Syndrome: Similar to Dubin-Johnson Syndrome, it is a benign condition with a defect in the hepatic storage or excretion of bilirubin, leading to a mixed increase in conjugated and unconjugated bilirubin.