We would like to thank all the correspondents for their comments, which are most helpful. The ESR calculator is useful, though our local laboratory has now ceased analysing ESRs (preferring viscosity), for the reasons given in the article. It was useful to hear of differential rises in inflammatory markers in lupus, and that this may help to differentiate between this and other rheumatological disorders.

In reply to the other comments, it is worth starting from the position that no test is perfect.

For childhood appendicitis, we gave details of a systematic review,1 which showed an association between a high ESR/CRP and the chance of appendicitis. The Stefanutti paper was published after this review, though Stefanutti concluded ‘White blood cell count or CRP values alone do not appear to provide any useful additional information to the surgeon.’2 Again this seems to be a case of associations being real but insufficiently strong to override other clinical features.

Two correspondents were concerned about using a negative inflammatory marker as a rule out test: for myeloma and for polymyalgia / temporal arteritis. We made it clear that negative tests can occur in polymyalgia: the Ellis paper, studying a hospital population, had the highest proportion at 22%, 3 though the recent BMJ review quoted 4% overall.4 Similarly, we agree negative tests can rarely occur in myeloma. These rare occurrences emphasise the importance for all clinicians of being prepared to revisit their diagnosis (or non-diagnosis) if the patient does not improve. That is the point at which additional testing or referral is often warranted.

References
1. Bundy DG, Byerley JS, Liles EA, Perrin EM, Katznelson J, Rice HE. Does this child have appendicitis? Jama 2007;298(4):438-51.
2. Stefanutti G, Ghirardo V, Gamba P. Inflammatory markers for acute appendicitis in children: are they helpful? Journal of Pediatric Surgery 2007;42(5):773-76.
3. Ellis M, Ralston S. The ESR in the diagnosis and management of the polymyalgia rheumatica/giant cell arteritis syndrome. Annals of the Rheumatic Diseases, 1983;42:168-72.
4. Hassan N, Dasgupta B, Barraclough K. Giant cell arteritis. BMJ 2011;342:d3019.

Competing interests: None declared