Background Measurement of the C-reactive protein (CRP) is widely available and is often part of the diagnostic evaluation of patients with suspected Tuberculosis (TB) yet there are little published data regarding its use in this context. We sought to determine CRP’s relationship with TB disease and whether normal CRP levels were seen in those without active TB.

Method We undertook a retrospective review of electronic records from the London TB service between 2004–2013, obtained blood results from hospital computer systems and reviewed case-notes where there was uncertainty regarding sites of disease or availability of culture results.

Results Using data from 533 subjects with active TB, 23% of all cases and 16.5% of culture-confirmed cases had a CRP of ≤5 mg/L. Individuals with HIV co-infection had significantly higher median CRP (see Table).

There was a significantly higher CRP in smear-positive pulmonary disease and those with a positive culture. Sites of disease that could be expected to have a high mycobacterial load (e.g. pulmonary disease and disseminated disease) had a significantly higher CRP than those such as skin, lymph node or CNS disease, where the mycobacterial load is typically low in HIV negative subjects. HIV status, site of disease and culture status remained significantly associated with CRP in a multivariable linear regression analysis.

Conclusions These data suggest that CRP cannot be used as a rule-out test for active TB but does reflect mycobacterial load. This has clinical implications given that individuals with a high mycobacterial load may be more infectious (and hence require enhanced contact tracing), be at greater risk of developing drug resistance if non-adherent, and may require prolonged duration of treatment.