Transmission of Mycobacterium tuberculosis is a recognized risk in health care facilities. The magnitude of the risk to HCWs varies considerably by the type of health care facility, the prevalence of TB in the community, the patient population served, the HCW’s occupational group, the area of the health care facility in which the HCW works and the effectiveness of TB infection-control interventions. The risk may be higher in areas where patients with TB are provided care before diagnosis and initiation of TB treatment and isolation precautions (e.g., in clinic waiting areas and emergency departments) or where diagnostic or treatment procedures that stimulate coughing are performed. Nosocomial transmission of M. tuberculosis has been associated with close contact with persons who have infectious TB and with the performance of certain procedures (e.g., bronchoscopy, endotracheal intubation and suctioning, open abscess irrigation and autopsy). Sputum induction and aerosol treatments that induce coughing may also increase the potential for transmission of M. tuberculosis. Personnel in health care facilities should be particularly alert to the need for preventing transmission of M. tuberculosis in those facilities in which immunocompromised persons (e.g., HIV-infected persons) work or receive care—especially if cough-inducing procedures, such as sputum induction and aerosolized pentamidine treatments, are being performed.
Transmission and Pathogenesis
M. tuberculosis is carried in airborne particles, or droplet nuclei, that can be generated when persons who have pulmonary or laryngeal TB sneeze, cough, speak or sing. The particles are an estimated 1 to 5 μm in size and normal air currents can keep them airborne for prolonged time periods and spread them throughout a room or building. Infection occurs when a susceptible person inhales droplet nuclei containing M. tuberculosis and these droplet nuclei traverse the mouth or nasal passages, upper respiratory tract and bronchi to reach the alveoli of the lungs. Once in the alveoli, the organisms are taken up by alveolar macrophages and spread throughout the body. Usually within two to ten weeks after initial infection with M. tuberculosis, the immune response limits further multiplication and spread of the tubercle bacilli; however, some of the bacilli remain dormant and viable for many years. This condition is referred to as latent TB infection. Persons with latent TB infection usually have positive purified protein derivative (PPD)-tuberculin skin-test results, but they do not have symptoms of active TB, and they are not infectious.
In general, persons who become infected with M. tuberculosis have approximately a 10% risk for developing active TB during their lifetimes. This risk is greatest during the first two years after infection. Immunocompromised persons have a greater risk for the progression of latent TB infection to active TB disease; HIV infection is the strongest known risk factor for this progression. Persons with latent TB infection who become co-infected with HIV have approximately an 8 to 10% risk per year for developing active TB. HIV-infected persons who are already severely immunosuppressed and who become newly infected with M. tuberculosis have an even greater risk for developing active TB.
The probability that a person who is exposed to M. tuberculosis will become infected depends primarily on the concentration of infectious droplet nuclei in the air and the duration of exposure. Characteristics of the TB patient that enhance transmission include:
- disease in the lungs, airways or larynx
- presence of cough or other forceful expiratory measures
- presence of acid-fast bacilli (AFB) in the sputum
- failure of the patient to cover the mouth and nose when coughing or sneezing
- presence of cavitation on chest radiograph
- inappropriate or short duration of chemotherapy
- administration of procedures that can induce coughing or cause aerosolization of M. tuberculosis (e.g., sputum induction).
Environmental factors that enhance the likelihood of transmission include:
- exposure in relatively small, enclosed spaces
- inadequate local or general ventilation that results in insufficient dilution and/or removal of infectious droplet nuclei
- recirculation of air containing infectious droplet nuclei.
Characteristics of the persons exposed to M. tuberculosis that may affect the risk for becoming infected are not as well defined. In general, persons who have been infected previously with M. tuberculosis may be less susceptible to subsequent infection. However, reinfection can occur among previously infected persons, especially if they are severely immunocompromised. Vaccination with Bacille of Calmette and Guérin (BCG) probably does not affect the risk for infection; rather, it decreases the risk for progressing from latent TB infection to active TB. Finally, although it is well established that HIV infection increases the likelihood of progressing from latent TB infection to active TB, it is unknown whether HIV infection increases the risk for becoming infected if exposed to M. tuberculosis.
Several TB outbreaks among persons in health care facilities have been reported recently in the United States. Many of these outbreaks involved transmission of multidrug-resistant strains of M. tuberculosis to both patients and HCWs. Most of the patients and some of the HCWs were HIV-infected persons in whom new infection progressed rapidly to active disease. Mortality associated with those outbreaks was high (with a range of 43 to 93%). Furthermore, the interval between diagnosis and death was brief (with a range of median intervals of 4 to 16 weeks). Factors contributing to these outbreaks included delayed diagnosis of TB, delayed recognition of drug resistance and delayed initiation of effective therapy, all of which resulted in prolonged infectiousness, delayed initiation and inadequate duration of TB isolation, inadequate ventilation in TB isolation rooms, lapses in TB isolation practices and inadequate precautions for cough-inducing procedures and lack of adequate respiratory protection.
Fundamentals of TB infection control
An effective TB infection-control programme requires early identification, isolation and effective treatment of persons who have active TB. The primary emphasis of the TB infection-control plan should be on achieving these three goals. In all health care facilities, particularly those in which persons who are at high risk for TB work or receive care, policies and procedures for TB control should be developed, reviewed periodically and evaluated for effectiveness to determine the actions necessary to minimize the risk for transmission of M. tuberculosis.
The TB infection-control programme should be based on a hierarchy of control measures. The first level of the hierarchy, and that which affects the largest number of persons, is using administrative measures intended primarily to reduce the risk for exposing uninfected persons to persons who have infectious TB. These measures include:
- developing and implementing effective written policies and protocols to ensure the rapid identification, isolation, diagnostic evaluation and treatment of persons likely to have TB
- implementing effective work practices among HCWs in the health care facility (e.g., correctly wearing respiratory protection and keeping doors to isolation rooms closed)
- educating, training and counselling HCWs about TB
- screening HCWs for TB infection and disease.
The second level of the hierarchy is the use of engineering controls to prevent the spread and reduce the concentration of infectious droplet nuclei. These controls include:
- direct source control using local exhaust ventilation
- controlling direction of airflow to prevent contamination of air in areas adjacent to the infectious source
- diluting and removing contaminated air via general ventilation
- air cleaning via air filtration or ultraviolet germicidal irradiation (UVGI).
The first two levels of the hierarchy minimize the number of areas in the health care facility where exposure to infectious TB may occur, and they reduce, but do not eliminate, the risk in those few areas where exposure to M. tuberculosis can still occur (e.g., rooms in which patients with known or suspected infectious TB are being isolated and treatment rooms in which cough-inducing or aerosol-generating procedures are performed on such patients). Because persons entering such rooms may be exposed to M. tuberculosis, the third level of the hierarchy is the use of personal respiratory protective equipment in these and certain other situations in which the risk for infection with M. tuberculosis may be relatively higher.
Specific measures to reduce the risk for transmission of M. tuberculosis include the following:
1. Assigning to specific persons in the health care facility the supervisory responsibility for designing, implementing, evaluating and maintaining the TB infection-control programme.
2. Conducting a risk assessment to evaluate the risk for transmission of M. tuberculosis in all areas of the health care facility, developing a written TB infection-control programme based on the risk assessment and periodically repeating the risk assessment to evaluate the effectiveness of the TB infection-control programme. TB infection-control measures for each health care facility should be based on a careful assessment of the risk for transmission of M. tuberculosis in that particular setting. The first step in developing the TB infection-control programme should be to conduct a baseline risk assessment to evaluate the risk for transmission of M. tuberculosis in each area and occupational group in the facility. Appropriate infection-control interventions can then be developed on the basis of actual risk. Risk assessments should be performed for all inpatient and outpatient settings (e.g., medical and dental offices). Classification of risk for a facility, for a specific area and for a specific occupational group should be based on the profile of TB in the community, the number of infectious TB patients admitted to the area or ward, or the estimated number of infectious TB patients to whom HCWs in an occupational group may be exposed and the results of analysis of HCW PPD test conversions (where applicable) and possible person-to-person transmission of M. tuberculosis. Regardless of risk level, the management of patients with known or suspected infectious TB should not vary. However, the index of suspicion for infectious TB among patients, the frequency of HCW PPD skin testing, the number of TB isolation rooms and other factors will depend on the level of risk for transmission of M. tuberculosis in the facility, area or occupational group.
3. Developing, implementing and enforcing policies and protocols to ensure early identification, diagnostic evaluation and effective treatment of patients who may have infectious TB. A diagnosis of TB may be considered for any patient who has a persistent cough (i.e., a cough lasting for longer than 3 weeks) or other signs or symptoms compatible with active TB (e.g., bloody sputum, night sweats, weight loss, anorexia or fever). However, the index of suspicion for TB will vary in different geographic areas and will depend on the prevalence of TB and other characteristics of the population served by the facility. The index of suspicion for TB should be very high in geographic areas or among groups of patients in which the prevalence of TB is high. Appropriate diagnostic measures should be conducted and TB precautions implemented for patients in whom active TB is suspected.
4. Providing prompt triage for and appropriate management of patients in the outpatient setting who may have infectious TB. Triage of patients in ambulatory-care settings and emergency departments should include vigorous efforts to identify promptly patients who have active TB. HCWs who are the first points of contact in facilities that serve populations at risk for TB should be trained to ask questions that will facilitate identification of patients with signs and symptoms suggestive of TB. Patients with signs or symptoms suggestive of TB should be evaluated promptly to minimize the amount of time they are in ambulatory-care areas. TB precautions should be followed while the diagnostic evaluation is being conducted for these patients. TB precautions in the ambulatory-care setting should include placing these patients in a separate area apart from other patients and not in open waiting areas (ideally, in a room or enclosure meeting TB isolation requirements), giving these patients surgical masks to wear and instructing them to keep their masks on and giving these patients tissues and instructing them to cover their mouths and noses with the tissues when coughing or sneezing. Surgical masks are designed to prevent the respiratory secretions of the person wearing the mask from entering the air. When not in a TB isolation room, patients suspected of having TB should wear surgical masks to reduce the expulsion of droplet nuclei into the air. These patients do not need to wear particulate respirators, which are designed to filter the air before it is inhaled by the person wearing the mask. Patients suspected of having or known to have TB should never wear a respirator that has an exhalation valve, because the device would provide no barrier to the expulsion of droplet nuclei into the air.
5. Promptly initiating and maintaining TB isolation for persons who may have infectious TB and who are admitted to the inpatient setting. In hospitals and other inpatient facilities, any patient suspected of having or known to have infectious TB should be placed in a TB isolation room that has currently recommended ventilation characteristics (see below). Written policies for initiating isolation should specify the indications for isolation, the person(s) authorized to initiate and discontinue isolation, the isolation practices to follow, the monitoring of isolation, the management of patients who do not adhere to isolation practices and the criteria for discontinuing isolation.
6. Effectively planning arrangements for discharge. Before a TB patient is discharged from the health care facility, the facility’s staff and public health authorities should collaborate to ensure continuation of therapy. Discharge planning in the health care facility should include, at a minimum, a confirmed outpatient appointment with the provider who will manage the patient until the patient is cured, sufficient medication to take until the outpatient appointment and placement into case management (e.g., directly observed therapy (DOT)) or outreach programmes of the public health department. These plans should be initiated and in place before the patient’s discharge.
7. Developing, installing, maintaining and evaluating ventilation and other engineering controls to reduce the potential for airborne exposure to M. tuberculosis. Local exhaust ventilation is a preferred source control technique, and it is often the most efficient way to contain airborne contaminants because it captures these contaminants near their source before they can disperse. Therefore, the technique should be used, if feasible, wherever aerosol-generating procedures are performed. Two basic types of local exhaust devices use hoods: the enclosing type, in which the hood either partially or fully encloses the infectious source, and the exterior type, in which the infectious source is near but outside the hood. Fully enclosed hoods, booths or tents are always preferable to exterior types because of their superior ability to prevent contaminants from escaping into the HCW’s breathing zone. General ventilation can be used for several purposes, including diluting and removing contaminated air, controlling airflow patterns within rooms and controlling the direction of airflow throughout a facility. General ventilation maintains air quality by two processes: dilution and removal of airborne contaminants. Uncontaminated supply air mixes with the contaminated room air (i.e., dilution), which is subsequently removed from the room by the exhaust system. These processes reduce the concentration of droplet nuclei in the room air. Recommended general ventilation rates for health care facilities are usually expressed in number of air changes per hour (ACH).
This number is the ratio of the volume of air entering the room per hour to the room volume and is equal to the exhaust airflow (Q, in cubic feet per minute) divided by the room volume (V, in cubic feet) multiplied by 60 (i.e., ACH = Q / V x 60). For the purposes of reducing the concentration of droplet nuclei, TB isolation and treatment rooms in existing health care facilities should have an airflow of greater than 6 ACH. Where feasible, this airflow rate should be increased to at least 12 ACH by adjusting or modifying the ventilation system or by using auxiliary means (e.g., recirculation of air through fixed HEPA filtration systems or portable air cleaners). New construction or renovation of existing health care facilities should be designed so that TB isolation rooms achieve an airflow of at least 12 ACH. The general ventilation system should be designed and balanced so that air flows from less contaminated (i.e., more clean) to more contaminated (less clean) areas. For example, air should flow from corridors into TB isolation rooms to prevent spread of contaminants to other areas. In some special treatment rooms in which operative and invasive procedures are performed, the direction of airflow is from the room to the hallway to provide cleaner air during these procedures. Cough-inducing or aerosol-generating procedures (e.g., bronchoscopy and irrigation of tuberculous abscesses) should not be performed in rooms with this type of airflow on patients who may have infectious TB. HEPA filters may be used in a number of ways to reduce or eliminate infectious droplet nuclei from room air or exhaust. These methods include placement of HEPA filters in exhaust ducts discharging air from booths or enclosures into the surrounding room, in ducts or in ceiling- or wall-mounted units, for recirculation of air within an individual room (fixed recirculation systems), in portable air cleaners, in exhaust ducts to remove droplet nuclei from air being discharged to the outside, either directly or through ventilation equipment, and in ducts discharging air from the TB isolation room into the general ventilation system. In any application, HEPA filters should be installed carefully and maintained meticulously to ensure adequate functioning. For general use areas in which the risk for transmission of M. tuberculosis is relatively high, ultraviolet lamps (UVGI) may be used as an adjunct to ventilation for reducing the concentration of infectious droplet nuclei, although the effectiveness of such units has not been evaluated adequately. Ultraviolet (UV) units can be installed in a room or corridor to irradiate the air in the upper portion of the room, or they can be installed in ducts to irradiate air passing through the ducts.
8. Developing, implementing, maintaining and evaluating a respiratory protection programme. Personal respiratory protection (i.e., respirators) should be used by persons entering rooms in which patients with known or suspected infectious TB are being isolated, persons present during cough-inducing or aerosol-generating procedures performed on such patients and persons in other settings where administrative and engineering controls are not likely to protect them from inhaling infectious airborne droplet nuclei. These other settings include transporting patients who may have infectious TB in emergency transport vehicles and providing urgent surgical or dental care to patients who may have infectious TB before a determination has been made that the patient is non-infectious.
9. Educating and training HCWs about TB, effective methods for preventing transmission of M. tuberculosis and the benefits of medical screening programmes. All HCWs, including physicians, should receive education regarding TB that is relevant to persons in their particular occupational group. Ideally, training should be conducted before initial assignment and the need for additional training should be re-evaluated periodically (e.g., once a year). The level and detail of this education will vary according to the HCW’s work responsibilities and the level of risk in the facility (or area of the facility) in which the HCW works. However, the programme may include the following elements:
- the basic concepts of M. tuberculosis transmission, pathogenesis and diagnosis,
including information concerning the difference between latent TB infection and active
TB disease, the signs and symptoms of TB and the possibility of reinfection
- the potential for occupational exposure to persons who have infectious TB in the
health care facility, including information concerning the prevalence of TB in the
community and facility, the ability of the facility to properly isolate patients who have
active TB, and situations with increased risk for exposure to M. tuberculosis
- the principles and practices of infection control that reduce the risk for transmission of
M. tuberculosis, including information concerning the hierarchy of TB infection-control
measures and the written policies and procedures of the facility. Site-specific control
measures should be provided to HCWs working in areas that require control
measures in addition to those of the basic TB infection-control programme.
- the importance of proper maintenance for engineering controls (e.g., cleaning UVGI lamps and ensuring negative pressure in TB isolation rooms)
- the purpose of PPD skin testing, the significance of a positive PPD test result and the importance of participating in the skin-test programme
- the principles of preventive therapy for latent TB infection; these principles include the indications, use, effectiveness and the potential adverse effects of the drugs
- the HCW’s responsibility to seek prompt medical evaluation if a PPD test conversion
occurs or if symptoms develop that could be caused by TB. Medical evaluation will
enable HCWs who have TB to receive appropriate therapy and will help to prevent
transmission of M. tuberculosis to patients and other HCWs.
- the principles of drug therapy for active TB
- the importance of notifying the facility if the HCW is diagnosed with active TB so that contact investigation procedures can be initiated
- the responsibilities of the facility to maintain the confidentiality of the HCW while
ensuring that the HCW who has TB receives appropriate therapy and is non-
infectious before returning to duty
- the higher risks associated with TB infection in persons who have HIV infection or
other causes of severely impaired cell-mediated immunity, including (a) the more
frequent and rapid development of clinical TB after infection with M. tuberculosis, (b)
the differences in the clinical presentation of disease and (c) the high mortality rate associated with multiple drug resistant-TB in such persons
- the potential development of cutaneous anergy as immune function (as measured by CD4+ T-lymphocyte counts) declines
- information regarding the efficacy and safety of BCG vaccination and the principles of PPD screening among BCG recipients
- the facility’s policy on voluntary work reassignment options for immunocompromised HCWs.
10. Developing and implementing a programme for routine periodic counselling and screening of HCWs for active TB and latent TB infection. A TB counselling, screening and prevention programme for HCWs should be established to protect both HCWs and patients. HCWs who have positive PPD test results, PPD test conversions or symptoms suggestive of TB should be identified, evaluated to rule out a diagnosis of active TB and started on therapy or preventive therapy if indicated. In addition, the results of the HCW PPD screening programme will contribute to evaluation of the effectiveness of current infection-control practices. Because of the increased risk for rapid progression from latent TB infection to active TB in human immunodeficiency virus, HIV-infected or otherwise severely immunocompromised persons, all HCWs should know if they have a medical condition or are receiving a medical treatment that may lead to severely impaired cell-mediated immunity. HCWs who may be at risk for HIV infection should know their HIV status (i.e., they should be encouraged to voluntarily seek counselling and testing for HIV antibody status). Existing guidelines for counselling and testing should be followed routinely. Knowledge of these conditions allows the HCW to seek the appropriate preventive measures and to consider voluntary work reassignments.
11. ll HCWs should be informed about the need to follow existing recommendations for infection control to minimize the risk for exposure to infectious agents; implementation of these recommendations will greatly reduce the risk for occupational infections among HCWs. All HCWs should also be informed about the potential risks to severely immunocompromised persons associated with caring for patients who have some infectious diseases, including TB. It should be emphasized that limiting exposure to TB patients is the most protective measure that severely immunosuppressed HCWs can take to avoid becoming infected with M. tuberculosis. HCWs who have severely impaired cell-mediated immunity and who may be exposed to M. tuberculosis may consider a change in job-setting to avoid such exposure. HCWs should be advised of the legal option in many jurisdictions that severely immunocompromised HCWs can choose to transfer voluntarily to areas and work activities in which there is the lowest possible risk for exposure to M. tuberculosis. This choice should be a personal decision for HCWs after they have been informed of the risks to their health.
12. Employers should make reasonable accommodations (e.g., alternative job assignments) for employees who have a health condition that compromises cell-mediated immunity and who work in settings where they may be exposed to M. tuberculosis. HCWs who are known to be immunocompromised should be referred to employee health professionals who can individually counsel the employees regarding their risk for TB. Upon the request of the immunocompromised HCW, employers should offer, but not compel, a work setting in which the HCW would have the lowest possible risk for occupational exposure to M. tuberculosis.
13. All HCWs should be informed that immunosuppressed HCWs should have appropriate follow-up and screening for infectious diseases, including TB, provided by their medical practitioner. HCWs who are known to be HIV-infected or otherwise severely immunosuppressed should be tested for cutaneous anergy at the time of PPD testing. Consideration should be given to retesting, at least every 6 months, those immunocompromised HCWs who are potentially exposed to M. tuberculosis because of the high risk for rapid progression to active TB if they become infected.
14. Information provided by HCWs regarding their immune status should be treated confidentially. If the HCW requests voluntary job reassignment, the privacy of the HCW should be maintained. Facilities should have written procedures on confidential handling of such information.
15. Promptly evaluating possible episodes of M. tuberculosis transmission in health care facilities, including PPD skin-test conversions among HCWs, epidemiologically associated cases among HCWs or patients and contacts of patients or HCWs who have TB and who were not promptly identified and isolated. Epidemiological investigations may be indicated for several situations. These include, but are not limited to, the occurrence of PPD test conversions or active TB in HCWs, the occurrence of possible person-to-person transmission of M. tuberculosis and situations in which patients or HCWs with active TB are not promptly identified and isolated, thus exposing other persons in the facility to M. tuberculosis. The general objectives of the epidemiological investigations in these situations are as follows:
- to determine the likelihood that transmission of and infection with M. tuberculosis has occurred in the facility
- to determine the extent to which M. tuberculosis has been transmitted
- to identify those persons who have been exposed and infected, enabling them to receive appropriate clinical management
- to identify factors that could have contributed to transmission and infection and to implement appropriate interventions
- to evaluate the effectiveness of any interventions that are implemented and to ensure that exposure to and transmission of M. tuberculosis have been terminated.
16. Coordinating activities with the local public health department, emphasizing reporting and ensuring adequate discharge follow-up and the continuation and completion of therapy. As soon as a patient or HCW is known or suspected to have active TB, the patient or HCW should be reported to the public health department so that appropriate follow-up can be arranged and a community contact investigation can be performed. The health department should be notified well before patient discharge to facilitate follow-up and continuation of therapy. A discharge plan coordinated with the patient or HCW, the health department and the inpatient facility should be implemented.