All the following data is from a task a professor gave me and is not connected to a true patient.
Patient's bilirubin level is first 28 micromole/litre and direct bilirubin level is 8 micromole/litre. Three weeks after the first blood test, the patient takes another blood test. This time, total bilirubin level is 48 micromole/litre and direct bilirubin is 9 micromole/litre, meaning that indirect bilirubin level increased by 19 micromole/litre during these three weeks. Liver enzymes (AST, ALT, ALP, GGT) were within normal range both times. Iron wasn't measured the first time, it was normal the second time. What are the likeliest causes for this?
FIRST EDIT: The patient took the first blood test on full stomach and the second one on empty stomach. It's most likely not Gilbert's syndrome because direct bilirubin is also high, although the fact that the increase of bilirubin is mild and that it's higher after not eating may point to Gilbert's syndrome.
SECOND EDIT: I have done some research on the effect of fasting on bilirubin level in patients with Gilbert's syndrome. I have found that after eating 400 kcal per day for two days, indirect bilirubin should be twice as high as the baseline level in a GS patient (The inverse starving test is not a suitable provocation test for Gilbert's syndrome). This patients indirect bilirubin level is twice as high after just about 8 hours of not eating if we assume that the bilirubin level from the first blood test is the baseline level. Considering this, taking the second blood test on an empty stomach is most likely not the cause of the elevation of indirect bilirubin and so the elevation is most likely not a sign of Gilbert's syndrome.