IFNG and tuberculosis: Description of 42 sputum smear-negative or sputum non-producing subjects undergoing sputum induction for the investigation of possible tuberculosis (subjects are categorised by final diagnoses of active TB; quiescent or latent TB; and not TB, and percentage value of PPD-specific CD4+ IFN-γ+ lymphocytes [%PPD-specific CD4+IFN-γ+])