Final answer:
A blood sample is the best specimen for testing levels of iron in the blood due to its direct correlation with hemoglobin synthesis, whereas urine, saliva, and stool samples do not reliably reflect the body's iron status.
Step-by-step explanation:
The best specimen to test for levels of iron in the blood is a blood sample. This is due to iron's critical role in the production of hemoglobin, the protein in red blood cells responsible for oxygen transportation. Blood tests measure serum iron concentration using various assays, such as the one involving Ferrozine, which indicates the iron levels in the bloodstream. Urine, saliva, and stool samples are not as directly indicative of the body's iron status, since physiological excretion of iron is minimal, and these specimens do not reflect the active iron pool being utilized in hemoglobin synthesis within the red blood cells, as accurately as a blood sample does.