Three complementary measures
Serum iron varies through the day. Ferritin reflects the body's iron stores. Transferrin and its saturation show how iron is carried.
Preparation
The sample is best taken in the morning, fasting, because iron varies with the time of day and with meals. Mention any iron treatment you are taking.
Reading it
Ferritin can also rise with inflammation: that is why the doctor reads the iron panel alongside the full blood count, sometimes the CRP, and your history.