Rapid Drug Susceptibility Testing with a Molecular Beacon Assay Is Associated with Earlier Diagnosis and Treatment of Multidrug-Resistant Tuberculosis in California

Abstract

To assess the clinical impact of a molecular beacon (MB) assay that detects multidrug-resistant tuberculosis (MDR TB), we retrospectively reviewed records of 127 MDR TB patients with and without MB testing between 2004 and 2007. Use of the MB assay reduced the time to detection and treatment of MDR TB.The emergence and spread of multidrug-resistant tuberculosis (MDR TB) threatens TB control worldwide. MDR TB is resistant to the most effective first-line agents, isoniazid (INH) and rifampin (RIF). The cornerstone of MDR TB diagnosis is culture and drug susceptibility testing (DST) of Mycobacterium tuberculosis isolates. Conventional methods for culture and first-line DST require weeks, causing delays in MDR TB diagnosis and treatment, which in turn are associated with advanced disease (6, 8), treatment failure (4), and transmission. Rapid molecular methods have been developed for the detection of mutations that confer drug resistance (3, 5, 7), and several public health organizations have endorsed molecular DST for TB (2, 9).The California Department of Public Health has been performing a molecular beacon (MB) assay in which real-time PCR is performed on acid-fast bacilli (AFB) smear-positive sputa or M. tuberculosis cultures to detect mutations conferring resistance to INH and RIF. TB providers in California are encouraged to submit specimens for MB testing from patients in whom drug-resistant TB is suspected (i.e., patients with a history of prior TB treatment, patients born in nations with high rates of resistant TB, patients failing TB treatment, and those who are known contacts to MDR TB cases). The assay is not FDA approved but is highly sensitive and specific (5), detects silent mutations infrequently, and shows Ͼ95% agreement with phenotypic DST results after testing on nearly 200 unique samples (unpublished). We performed a retrospective cohort study among MDR TB patients to assess the clinical impact of the MB assay in a public health setting in California.This study used deidentified demographic and clinical data that were collected by the California Department of Public Health for TB surveillance. Standard TB treatment was defined as any regimen containing at least 3 drugs, including INH and RIF. MDR treatment was defined as any regimen containing at least 4 drugs and including at least 2 second-line anti-TB medications (e.g., a fluoroquinolone, injectable agent [amikacin, kanamycin, capreomycin], para-aminosalicylic acid, cycloserine, ethionamide, or linezolid) (1, 10). The time to sputum culture conversion was defined as the interval between the collection dates of the first positive sputum culture and the first consistently negative sputum culture. Median times were compared using Wilcoxon's two-sample test. Proportions were tested using chi-square testing or Fisher's exact test. The MB assay was performed as previously described (5).Of 139 culture-positive MDR TB cases reported in California from 2004 through 2007, 12 cases were excluded because the MDR TB treatment start date could not be determi...

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