A method of detecting surrogate markers for active tuberculosis in a serum sample, using a lateral flow immunoassay device, the surrogate markers being selected from serum mycolic acid antigen arising from tuberculous mycobacterial infection, serum anti-mycolic acid antibodies arising from tuberculous mycobacterial infection or both, the method including the steps of: combining said serum sample with a labelled monoclonal immunoglobulin antibody or fragment thereof specific to mycobacterial mycolic acid to produce a combined serum sample, said monoclonal antibody or said fragment thereof not cross-reacting with cholesterol and the label being selected so that binding of the labelled antibody to immobilized mycolic acid antigen of mycobacterial origin produces a detectable signal; combining a blank sample with the labelled monoclonal immunoglobulin antibody or fragment thereof which is specific to mycobacterial mycolic acid and which does not cross-react with cholesterol to produce a combined blank sample; and then exposing both the combined serum sample and the combined blank sample to immobilised mycolic acid antigen of mycobacterial origin so that said labelled immunoglobulin antibodies or said fragments thereof in each sample bind to the immobilised antigen to produce detectable signals, the signal produced by the combined blank sample being stronger than that produced by the combined serum sample because of inhibition of binding of the labelled antibody or said fragment thereof in the exposed serum sample arising from prior binding of the labelled monoclonal antibody or said fragment thereof with the mycolic acid antigen in the serum sample or by competitive binding of serum anti-mycolic acid antibodies in the serum sample to the immobilised mycolic acid antigen or both. combining said serum sample with a labelled monoclonal immunoglobulin antibody or fragment thereof specific to mycobacterial mycolic acid to produce a combined serum sample, said monoclonal antibody or said fragment thereof not cross-reacting with cholesterol and the label being selected so that binding of the labelled antibody to immobilized mycolic acid antigen of mycobacterial origin produces a detectable signal; combining a blank sample with the labelled monoclonal immunoglobulin antibody or fragment thereof which is specific to mycobacterial mycolic acid and which does not cross-react with cholesterol to produce a combined blank sample; and then exposing both the combined serum sample and the combined blank sample to immobilised mycolic acid antigen of mycobacterial origin so that said labelled immunoglobulin antibodies or said fragments thereof in each sample bind to the immobilised antigen to produce detectable signals, the signal produced by the combined blank sample being stronger than that produced by the combined serum sample because of inhibition of binding of the labelled antibody or said fragment thereof in the exposed serum sample arising from prior binding of the labelled monoclonal antibody or said fragment thereof with the mycolic acid antigen in the serum sample or by competitive binding of serum anti-mycolic acid antibodies in the serum sample to the immobilised mycolic acid antigen or both.