Author’s response to reviews Title: Diagnostic utility of zinc protoporphyrin to detect iron deficiency in Kenyan preschool children: a community-based survey Authors:

no context or background is provided as to why this is an important study question. This is now better explained . it is likely unnecessary to include all inclusion and exclusion criteria in the abstract this listing takes up a large portion of the word allowance. We believe that a listing of eligibility criteria is essential to define the study population. Background page 4, last paragraph the authors indicate that ZPP has "little diagnostic utility as a screening marker to manage iron deficiency in pregnant women". Please explain why you would then decide to study this test again in the current population Findings from pregnant women do not necessarily apply to preschool children. Apart from physiological differences due to age and pregnancy (e.g. haemodilution), pregnant women may have less inflammation because of lower exposure to infections, or because they have acquired higher levels of protective immunity against such infections, resulting in a lower inflammatory response to infections. Methods exclusion criteria why were children with twin siblings and those with Hb <70g/l excluded? We excluded twins for practical reasons (in many settings in developing countries, parents find it unacceptable that their children are allocated to different interventions; in addition, this would also likely result in cross-overs, i.e. mixing of interventions). We now explained that we excluded children with haemoglobin concentration <70 g/L for ethical reasons, because the trial had a placebo arm (LN. 160-161). earlier in the manuscript it is noted that this was an analysis of data collected in an rct. It would be helpful to the reader if you briefly describe the nature and purpose of the RCT To provide context, we have now given more information about the trial (LN. 105-108 ). were the children tested for thalassemia? No. Results As noted above, the lower rates of iron deficiency identified in children with inflammation/infection vs. children who were well make it clear that ferritin alone was not an appropriate test in this population for case identification See our reply to a similar question raised by reviewer 1 above. The authors have not explained the reasons for testing both whole blood ZPP and erythrocyte ZPP. Please explain the differences between these tests, and what the anticipated differences would be in test results. This was explained in the Methods section (LN.132-135).