Based on clinical findings and after corroborating with the X ray findings, if the doctor finds evidence of tuberculosis, it can be treated as tuberculosis even if Mantoux test and sputum test are negative. At advanced age like that of your father, the immunity automatically comes down and susceptibility to tuberculosis increases. However, for confirmation, TB Gold Quantiferon test is considered as gold standard for diagnosis and you may opt for that if you have a confusion. Please note that it is a costly investigation.