Determinants of PCR performance (Xpert MTB/RIF), including bacterial load and inhibition, for TB diagnosis using specimens from different body compartments

Sci Rep. 2014 Jul 11;4:5658. doi: 10.1038/srep05658.

Abstract

The determinants of Xpert MTB/RIF sensitivity, a widely used PCR test for the diagnosis of tuberculosis (TB) are poorly understood. We compared culture time-to-positivity (TTP; a surrogate of bacterial load), MTB/RIF TB-specific and internal positive control (IPC)-specific C(T) values, and clinical characteristics in patients with suspected TB who provided expectorated (n = 438) or induced sputum (n = 128), tracheal aspirates (n = 71), bronchoalveolar lavage fluid (n = 152), pleural fluid (n = 76), cerebral spinal fluid (CSF; n = 152), pericardial fluid (n = 131), or urine (n = 173) specimens. Median bacterial load (TTP in days) was the strongest associate of MTB/RIF positivity in each fluid. TTP correlated with C(T) values in pulmonary specimens but not extrapulmonary specimens (Spearman's coefficient 0.5043 versus 0.1437; p = 0.030). Inhibition affected a greater proportion of pulmonary specimens than extrapulmonary specimens (IPC C(T) > 34: 6% (47/731) versus 1% (4/381; p < 0.0001). Pulmonary specimens had greater load than extrapulmonary specimens [TTPs (interquartile range) of 11 (7-16) versus 22 (18-33.5) days; p < 0.0001]. HIV-infection was associated with a decreased likelihood of MTB/RIF-positivity in pulmonary specimens but an increased likelihood in extrapulmonary specimens. Mycobacterial load, which displays significant variation across different body compartments, is the main determinant of MTB/RIF-positivity rather than PCR inhibition. MTB/RIF C(T) is a poor surrogate of load in extrapulmonary specimens.

MeSH terms

  • Adult
  • Aged
  • Antibiotics, Antitubercular / pharmacology*
  • Bacterial Load / drug effects*
  • Bronchoalveolar Lavage Fluid / chemistry
  • Bronchoalveolar Lavage Fluid / microbiology
  • Drug Resistance, Bacterial
  • Female
  • HIV Infections / diagnosis*
  • HIV Infections / epidemiology
  • Humans
  • Lung / metabolism
  • Lung / microbiology
  • Male
  • Middle Aged
  • Mycobacterium tuberculosis / drug effects*
  • Mycobacterium tuberculosis / isolation & purification
  • Predictive Value of Tests
  • Prevalence
  • Real-Time Polymerase Chain Reaction / standards*
  • South Africa / epidemiology
  • Sputum / microbiology
  • Tuberculosis / diagnosis*
  • Tuberculosis / epidemiology

Substances

  • Antibiotics, Antitubercular