Guidelines for preventing the transmission of tuberculosis in healthcare settings involve airborne isolation (AII) of suspect active cases and require 3 respiratory specimens negative by smear for acid-fast bacilli (AFB) prior to discontinuing isolation.Reference Taylor, Nolan and Blumberg 1 This guideline is based on a demonstrated lower transmission risk due to low organism burden indicated by 3 specimens being AFB negative even if they are culture positive.Reference Behr, Warren and Salamon 2 However, this procedure requires 2 or more days while the patient remains in AII. Early discontinuation of isolation can reduce costs by >40% if it can be done safely without risking transmission of active disease.Reference Kalimuddin, Tan, Tan and Low 3
Detection of Mycobacterium tuberculosis (Mtb) via a nucleic-acid amplification test (NAAT) of respiratory specimens has been incorporated into protocols in an effort to reduce AII time and costs. The XPert MTB/RIF (Cepheid, Sunnyvale, CA) has shown improved sensitivity for NAAT compared to traditional AFB microscopy (Table 1). However, there remains concern that discontinuing isolation based on a single specimen with negative NAAT may miss some 20% of culture-positive cases, prompting recent guidelines calling for 2 specimens with negative NAAT. 4 While prediction of low transmissibility from 3 serial AFB-negative smears is considered acceptable for discontinuing AII,Reference Behr, Warren and Salamon 2 similar data are not yet available for NAAT. A recent large public health contact investigation found that time to detection in culture (TTD) of >9 days was a better surrogate than a negative AFB smear for predicting lower transmissibility.Reference O’Shea, Koh, Munang, Smith, Banerjee and Dedicoat 5 However, TTD has not been previously examined along with AFB stain and NAAT to determine whether it supports discontinuing isolation after 1 specimen is both AFB negative and NAAT negative. The present study tests the hypothesis that patients who have a first respiratory specimen negative by both AFB smear and NAAT but culture positive will have predictably long TTD, with all 3 parameters indicating low organism burden, predicting low risk for transmission.
TABLE 1 Prior Studies Reporting Sensitivity and Specificity of XPert MTB/RIF Assay for Detection of Culture-Positive Pulmonary TuberculosisFootnote a
NOTE. NAAT, nucleic acid amplification test; AFB, acid-fast bacilli; Sens, sensitivity; Spec, specificity.
a Where available, comparison to sensitivity and specificity of serial AFB staining is provided.
METHODS
Specimen Identification
This study was a retrospective observational study of respiratory specimens from 117 patients at the University of California Irvine Medical Center (UCI) from 2013 to 2015 and from 670 patients at Orange County Health Care Agency (OCHCA) in 2014. From this database, the first specimens from 94 patients that were Mtb culture positive (34 at UCI and 60 at OCHCA) were the basis of our analysis. The specimens included satisfactorily induced or expectorated sputa per standard mycobacteriology guidelines.
Specimen Analysis
Standard processing included decontamination, concentration, and microscopy for AFB; the smear was graded from 0 to 4+ based on World Health Organization (WHO) parameters. Specimens obtained from OCHCA (refrigerated) and UCI (frozen) were analyzed using the Cepheid XPert MTB/RIF NAAT. Specimens were cultured in both liquid mycobacterial growth indicator tubes (MGIT) and on solid Lowenstein-Jensen agar.
Data Analysis
The proportion of specimens that were NAAT positive and the distributions of TTD and AFB grade showed close concordance between specimens obtained from OCHCA and UCI (data not shown); therefore, all 94 cases were combined for final analysis. The correlation between AFB grade and TTD was calculated using the Pearson coefficient. Data were stratified into groups by AFB grade and by NAAT result, and group means were compared using the Student t test with Bonferroni correction. The doubling time of Mtb in MGIT liquid culture was estimated by double logarithmic modeling assuming exponential growth. Our study could not measure actual transmission rates, which would require long-term contact investigation of a large database with molecular typing.
RESULTS
Specimens Negative by Both AFB Stain and NAAT Have Long TTD
The results of the AFB smear, NAAT, and TTD for each specimen are displayed in Figure 1. Among the 94 first respiratory specimens that were culture positive for Mtb, 61 (65%) were AFB positive, and all were also NAAT positive. Of the remaining 33 first specimens that were AFB smear negative, an additional 21 specimens (64% of smear negative specimens and 22% of the total) were NAAT positive.
FIGURE 1 Time to detection (TTD), acid-fast bacilli (AFB) smear, and nucleic acid amplification test (NAAT) results for 94 Mtb culture-positive first respiratory specimens. (a) Data table of patient groups stratified by AFB smear, TTD with a risk cutoff of 9 days, and NAAT result. (b) Scatter plot of individual patients showing distribution of TTD grouped by AFB smear and NAAT. The groups with lower risk of transmissibility of tuberculosis are indicated, with relative risk estimates based on prior studies for TTD>9 daysReference O’Shea, Koh, Munang, Smith, Banerjee and Dedicoat 5 and negative AFB smear.Reference Behr, Warren and Salamon 2
The key issue concerning potential transmissibility involves the remaining 12 first respiratory specimens (13% of the total) that were AFB negative and NAAT negative but still grew Mtb (ie, were culture positive). These 12 specimens all had TTD ≥9 days (mean, 23 days; range, 13–40 days). All specimens with TTD <9 days were detected using the XPert MTB/RIF assay. The estimated relative rates of transmission are noted in Figure 1 for prolonged TTDReference O’Shea, Koh, Munang, Smith, Banerjee and Dedicoat 5 and AFB-negative cases.Reference Behr, Warren and Salamon 2
TTD inversely correlates with AFB grade
Among the AFB-negative specimens, NAAT-positive specimens trended toward shorter TTD than NAAT-negative specimens (23 vs 19 days; P=.049). As AFB grade increased from 0 to 4+, TTD shortened from 20 days to 5 days, with P values ranging from .002 to .006. Overall, TTD negatively correlated with AFB grade (ρ=−0.63). Based on this correlation and assuming exponential growth, the calculated doubling time for Mtb in MGIT was 25 hours.
DISCUSSION
The sensitivity of XPert MTB/RIF NAAT was 87%, consistent with a range of 81%–94% in studies of at least 40 culture-positive first specimens (Table 1).Reference Dinnes, Deeks and Kunst 6 The reported sensitivity of 73%–76% for AFB microscopy of 3 specimens is uniformly either equivalent or inferior to a single negative NAAT. In addition, 2 studies (Luetkemeyer et al and Ou et al, see Table 1) reported only a small improvement in sensitivity adding a second NAAT (84% to 90% and 93% to 94%, respectively). Relying on a single NAAT assay for discontinuing AII is marginally cost-effective, with savings of $55 per patientReference Choi, Miele, Dowdy and Shah 7 and up to 36 hours decreased AII.Reference Lippincott, Miller, Popowitch, Hanrahan and Van Rie 8
However, regarding potential transmission and need for continued AII, requiring a second specimen would double the costs, including the second NAAT and prolonged AII. Cost savings are greater in high-risk subgroups.Reference Marks, Cronin and Venkatappa 9 , Reference Van den Wijngaert, Scheen, Vandenberg and Dediste 10 Nevertheless, the small increase in sensitivity of a second NAAT is unlikely to be cost-effective unless it is applied to high-risk patients.
Importantly, transmissibility is related to organism load, which is usually estimated by AFB grade. Single specimens negative by both AFB and NAAT are predominantly culture negative; in our data, only 13% of positive cultures were AFB negative and NAAT negative. These patients, despite low organism load robustly signified by negative AFB and NAAT plus predictably longer TTD, are still of concern due to occasional transmission from AFB smear-negative cases.Reference Behr, Warren and Salamon 2 However, these cases are reported from large household or occupational exposure investigations where other factors favor transmission, namely cumulative exposure time, proximity, air exchange, and absence of protective gear. Conversely, in acute-care hospitals, cumulative exposure time and air-exchange (even without AII) suggest substantially lower transmission risk. Indeed, genotype-confirmed transmission reports from AFB-negative cases do not cite episodes within acute-care hospitals. Therefore, with the added reassurance that first specimens from AFB-negative NAAT-negative cases will have predictably longer TTD, we conclude that such cases have even lower transmission risk in acute-care hospitals than prior reports of transmission from AFB-negative cases.
A limitation of our study is the absence of epidemiological data ruling out transmission from AFB-negative NAAT-negative cases with long TTD, which would require large-scale contact investigation. Strengths of the study are the reasonably large number of cases with complete data for AFB grade, NAAT, and TTD, with the first reported analysis comparing these parameters.
Thus, based on our findings, we implemented a protocol for discontinuing AII for low-risk patients based on a single specimen negative by both AFB and NAAT. However, we maintain exceptions for high-risk patients who have immunocompromise, disseminated TB, or cavitary disease, who merit additional testing.
ACKNOWLEDGMENTS
The authors would like to thank the personnel of the Clinical Microbiology Laboratory at University of California Irvine and the Public Health Laboratory at the Orange County Health Care Agency for their support.
Financial support: No financial support was provided relevant to this article.
Potential conflicts of interest: All authors report no conflicts of interest relevant to this article.
