Toxic metabolites (N-acetylisoniazid, pyrazinoic acid, and desacetylrifampicin) of first-line anti-tuberculosis drugs (ethambutol, pyrazinamide, rifampicin, and isoniazid) are identified by liquid chromatography–mass spectrometry, which is important in selecting the required dosages of these drugs for the maximum therapeutic effects and the reduction of side effects. Characteristic chromatographic profiles of blood plasma samples from donors with pulmonary tuberculosis (“pathology”) and a clinically healthy group (“norm”) are obtained and processed by principal component analysis and the k-nearest neighbor method. A perspective of this approach for obtaining of additional diagnostic criteria for pulmonary tuberculosis is shown.