title: The pyrazinamide susceptibility breakpoint above which combination therapy fails.
abstract: OBJECTIVE
  To identify the pyrazinamide MIC above which standard combination therapy fails.
  
  
  METHODS
  MICs of pyrazinamide were determined for Mycobacterium tuberculosis isolates, cultured from 58 patients in a previous randomized clinical trial in Cape Town, South Africa. The MICs were determined using BACTEC MGIT 960 for isolates that were collected before standard treatment with isoniazid, rifampicin, pyrazinamide and ethambutol commenced. Weekly sputum collections were subsequently made for 8 weeks in order to culture M. tuberculosis in Middlebrook broth medium. Classification and regression tree (CART) analysis was utilized to identify the pyrazinamide MIC predictive of sputum culture results at the end of pyrazinamide therapy. The machine learning-derived susceptibility breakpoints were then confirmed using standard association statistics that took into account confounders of 2 month sputum conversion.
  
  
  RESULTS
  The pyrazinamide MIC range was 12.5 to >100 mg/L for the isolates prior to therapy. The epidemiological 95% cut-off value was >100 mg/L. The 2 month sputum conversion rate in liquid cultures was 26% by stringent criteria and 48% by less stringent criteria. CART analysis identified an MIC breakpoint of 50 mg/L, above which patients had poor sputum conversion rates. The relative risk of poor sputum conversion was 1.5 (95% CI: 1.2-1.8) for an MIC >50 mg/L compared with an MIC ≤ 50 mg/L.
  
  
  CONCLUSIONS
  We propose a pyrazinamide susceptibility breakpoint of 50 mg/L for clinical decision making and for development of rapid susceptibility assays. This breakpoint is identical to that identified using computer-aided simulations of hollow fibre system output.
authors.affiliation: Office of Global Health, University of Texas Southwestern Medical Center, Dallas, TX, USA Department of Medicine, University of Texas Southwestern Medical Center, Dallas, TX, USA.
authors.forename: Tawanda
authors.initials: T
authors.lastname: Gumbo
authors.affiliation: Division of Clinical Pharmacology, Department of Medicine, University of Cape Town, Observatory, Cape Town, South Africa.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Chigutsa
authors.affiliation: Office of Global Health, University of Texas Southwestern Medical Center, Dallas, TX, USA.
authors.forename: Jotam
authors.initials: J
authors.lastname: Pasipanodya
authors.affiliation: School of Public Health, University of the Western Cape, Cape Town, South Africa.
authors.forename: Marianne
authors.initials: M
authors.lastname: Visser
authors.affiliation: Division of Molecular Biology and Human Genetics, Stellenbosch University, Tygerberg, South Africa.
authors.forename: Paul D
authors.initials: PD
authors.lastname: van Helden
authors.affiliation: Division of Molecular Biology and Human Genetics, Stellenbosch University, Tygerberg, South Africa.
authors.forename: Frederick A
authors.initials: FA
authors.lastname: Sirgel
authors.affiliation: Division of Clinical Pharmacology, Department of Medicine, University of Cape Town, Observatory, Cape Town, South Africa helen.mcilleron@uct.ac.za.
authors.forename: Helen
authors.initials: H
authors.lastname: McIlleron
mesh_terms: D000293:Adolescent; D000328:Adult; D000995:Antitubercular Agents / Q000494:pharmacology* / Q000627:therapeutic use*; D024881:Drug Resistance, Bacterial; D004359:Drug Therapy, Combination / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000187:drug effects*; D011718:Pyrazinamide / Q000494:pharmacology* / Q000627:therapeutic use*; D013019:South Africa; D013183:Sputum / Q000382:microbiology; D016896:Treatment Outcome; D014397:Tuberculosis, Pulmonary / Q000188:drug therapy / Q000382:microbiology*; D055815:Young Adult
chemical_list: D000995:Antitubercular Agents; D011718:Pyrazinamide
keywords: MICs; anti-tuberculosis drugs; drug susceptibility; pharmacokinetics; sputum culture