Discussion

The guidelines recommend the use of 2 or more GeneXpert MTB/RIF to investigate and diagnose TB in low-burden countries, especially when there is a strong clinical TB suspicion.^{Reference Lippincott, Miller and Popowitch9-Reference Sulis, Agliati and Pinsi11} Brazil is a high-burden country, and considering its high cost (US$10 per cartridge), national guidelines have limited GeneXpert MTB/RIF use to a single test per patient.^{12,Reference Cowan, Chandler and Kracen13}

Recent studies have demonstrated the possibility of using only 1 GeneXpert MTB/RIF to determine the continuation of respiratory isolation because performing >1 molecular test did not produce a superior result.^{Reference Chaisson, Roemer and Cantu8,Reference Poonawala, Leekha and Medina-Moreno14,Reference Al-Darraji, AbdRazak and Ng15}

The economic importance of early patient removal from isolation beds has also been emphasized.^{Reference Chaisson, Duong and Cattamanchi16} Cowan et al^{Reference Cowan, Chandler and Kracen13} estimated a shorter median of hospitalization in airborne isolation with the single GeneXpert MTB/RIF strategy, which would result in a cost savings of up to US$11,466 per patient compared to AFB smear. Thus, using a single GeneXpert MTB/RIF to replace 3 AFB smears to determine the need for isolation in hospitals could reduce costs.^{Reference Chaisson, Roemer and Cantu8}

In countries with limited financial resources, adopting a single GeneXpert MTB/RIF would lead to additional benefits. Discontinuation of respiratory isolation based on the molecular test would reduce hospital expenditures and facilitate the reallocation of resources in deficient areas without compromising occupational risk.^{Reference Chaisson, Duong and Cattamanchi16}

Although theoretical, our model represents real scenarios of clinical practice and demonstrates, as have other studies such as Chaisson et al^{Reference Chaisson, Roemer and Cantu8} and Poonwala et al,^{Reference Poonawala, Leekha and Medina-Moreno14} a high single GeneXpert MTB/RIF NPV (ie, 99% in both studies) that is even higher than the NPV of sequential AFB smears. In our study, in group 3, a single GeneXpert MTB/RIF showed performance similar to that of 3 AFB smears, but in a shorter period. As far as we know, no other Brazilian study has evaluated GeneXpert MTB/RIF NPV in the airborne isolation room management. In cases in which clinical or radiological suspicion of tuberculosis is strong, even in the presence of a negative GeneXpert MTB/RIF, each service should define more conservative decisions.

Regarding the mean time to remove patients from isolation beds, Chaisson et al^{Reference Chaisson, Roemer and Cantu8} reported a mean of 66 hours and, after the use of GeneXpert MTB/RIF, this time was reduced to 26 hours on average. In a study by Lippincott et al,^{Reference Lippincott, Miller and Popowitch9} this mean was 68 hours, which decreased to 20.8 hours with the implementation of the molecular test. The same improvement was described by Millman et al,^{Reference Millman, Dowdy and Miller17} whose results demonstrated a mean of 65 hours using AFB and 33.6 hours in the GeneXpert MTB/RIF scenario.

Poonwala et al^{Reference Poonawala, Leekha and Medina-Moreno14} applied an active model of improvement in communication and logistics from hospital admission to the release of the molecular test results, and they obtained a time of only 12 hours for patient removal from airborne isolation. In the traditional model, these researchers also obtained values of ~65 hours. This study highlights a relevant point associated with the introduction of new technology: improvements in hospital process and logistics management must be undertaken to achieve a significant reduction in the time required to discharge a patient from an airborne isolation room.

In this study, the detection of positive AFB smear and negative GeneXpert MTB/RIF strongly suggests NTM infection. No official data of NTM prevalence in Brazil are available because it is not a reportable disease. The main limitation of the GeneXpert MTB/RIF compared with the AFB smears is that it allows only the diagnosis of TB, whereas AFB smears can also indicate NTM infection.^{Reference Aubry and Veziris18}

Our study has several limitations. It was performed in a tertiary-care, university hospital, and other studies on this topic have been conducted in different settings. Only 35% of the patients in our study had had the 3 AFB smears usually requested in the respiratory isolation model. Although interesting, analyzing the causes of this low number of samples goes beyond the scope of this study. However, our data analysis suggests that most of these cases might have <3 AFB smears due to low suspicion of TB (acute disease or other confirmed pulmonary diseases) or due to the positivity of an AFB smear in the first collected sputum, which reinforces TB probability and formally indicates respiratory isolation for the patient. Finally, patient records were not systematically analyzed, and they were used only if there was any question about TB diagnostic confirmation.

On the other hand, some strengths of our study should be mentioned. The hospital’s mycobacterial laboratory is staffed by specialized technicians in all stages of the process, with 1 professional performing AFB smears, which demonstrates higher sensitivity values than other studies, such as Lee et al,^{Reference Lee, Kee and Shin19} who demonstrated AFB smear sensitivity of 38%. Also, the clinical-laboratory relationship in our hospital favors the implementation of protocols and process improvements.

In conclusion, a single GeneXpert MTB/RIF is a fast and strong predictor for TB absence in a high-complexity hospital, and these results are quite similar to those obtained in recent studies in low-burden settings. The use of a single GeneXpert MTB/RIF may also increase patient rotation through isolation rooms, with a potential positive impact in the emergency room and infectious diseases wards.