There are two important clinical questions for GPs; firstly, when should they use inflammatory marker tests, and secondly, how should they interpret results? The answer to the first question relates to the sensitivity and specificity of the tests, which determine whether they are useful to rule-out or rule-in respectively. We state that 'inflammatory markers should not be used as a rule-out test' because, with an overall sensitivity of <50%, they are normal, and would therefore ‘miss’, around half of those patients with relevant pathology. The reason for highlighting this message is that it is in direct contradiction to our previous qualitative research which suggesting GPs tend to use inflammatory markers as a non-specific 'rule-out' test.1 The second question, about how to interpret results, depends upon the positive (and negative) predictive value of the tests; overall 15% of those with a positive test were found to have some relevant pathology, and this figure rises, unsurprisingly, with higher test results. We agree that this is also an important finding and we hope that figures 3-5 in the online version of the article will be useful for clinicians trying to interpret a raised inflammatory marker, allowing them to determine the likelihood of disease in relation to inflammatory marker levels. References 1. Watson J, de Salis I, Hamilton W, Salisbury C. ‘I’m fishing really’ — inflammatory marker testing in primary care: a qualitative study. Br J Gen Pract 2016; 66 (644): e200-e206. DOI: 10.3399/bjgp16X683857.