The Sputum acid-fast bacilli (AFB) smear is most specific for diagnosing active pulmonary tuberculosis because it directly detects the presence of Mycobacterium tuberculosis in respiratory secretions. This method confirms active infection as it identifies live bacteria rather than just an immune response. Unlike the Mantoux test, which indicates exposure but not active disease, and Widal or blood cultures, which are unrelated or less specific, AFB smear provides rapid and definitive evidence crucial for timely treatment. Its specificity lies in detecting the mycobacteria’s characteristic acid-fast cell wall, making it the gold standard initial diagnostic tool.

Reference: Harrison's Principles of Internal Medicine, Fauci et al., 20th Edition.