Whether positive or negative, what effects does iron deficiency have on esr?

ESR to Eval Anemia: You can use the ESR to help differentiate iron deficiency anemia (low esr) from inflammatory anemias (high esr). Both categories of anemias can present with the similar indices, so the ESR can help differentiate the two.

Not much: If ESR is low, is reassuring is not inflammatory process, but high ESR doesnt mean inflammatory anemia or any diagnosis in particular. You can have high or low ESR with or without iron deficiency, have to put in context with other labs and clinical scenario.

No effect by Fe def: The ESR may be useful in differentiating iron deficiency from anemia of chronic disease in patients with a background chronic inflammatory condition such as rheumatoid arthritis. Unfortunately, the ESR is neither sensitive nor specific when used in isolation. An elevated ESR can occur in so many conditions. That's why history taking, physical exam, and collection of other lab data are important.