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Education Materials > Publications > Self-Study Modules on TB > Module 6 > Field Investigation

Self-Study Modules on Tuberculosis

Module 6: Contact Investigations for Tuberculosis

Field Investigation

What Is a Field Investigation?

The next step is to conduct a field investigation. This means visiting the patient's home or shelter, workplace (if any), and the other places where the patient said he or she spent time while infectious. The field investigation is important and should be done even if the patient interview has already been conducted. The purpose of the field investigation is to identify contacts and evaluate the environmental characteristics of the place in which exposure occurred. The field investigation may provide additional information for the risk assessment and identify additional contacts. During field visits, the health care worker should

  • Observe environmental characteristics such as room size, crowding, and ventilation, to estimate the risk of TB transmission
  • Identify additional contacts (especially children) and their locating information, such as phone numbers and addresses
  • Look for evidence of other contacts who may not be present at the time of the visit (Figure 6.5) (for example, pictures of others who may live in or visit the house, shoes of others who may live in the house, or toys left by children)
  • Interview and skin test close contacts who are present and arrange for reading of the results
  • Educate the contacts about the purpose of a contact investigation, the basics of transmission, the risk of transmitting M. tuberculosis to others, and the importance of testing, treatment, and follow-up for TB infection and disease
  • Refer contacts who have TB symptoms to the health department for a medical evaluation, including sputum collection

Figure 6.5  This is a picture of a health care worker conducting a field investigation in a patient's home. She is looking for evidence of other contacts (for example, pictures of others who may live or visit the house, shoes of others who may live in the house, or toys left by children).

Health care workers should remember to follow infection control precautions while visiting a potentially infectious TB patient at home or in any other location. These precautions may include wearing a personal respirator (see Module 5, Infectiousness and Infection Control).

Another critical consideration during field investigations is safety. Health care workers should become familiar with policies and recommendations of local law enforcement agencies and health department administration regarding personal safety. Current information on local high-risk areas for crime can be very valuable in planning and conducting safe field visits. General safety precautions that are recommended for the health care worker include

  • Wearing an identity badge with a current photo
  • Working in pairs when visiting a potentially dangerous area
  • Informing someone of your itinerary and expected time of return, especially if you anticipate problems
Study Questions 6.19-6.21

6.19. What is a field investigation?

6.20. List six tasks a health care worker should perform during a field investigation.

6.21. List three general safety precautions that are recommended for the health care workers who conduct field investigations.


Case Study 6.4
Carmen is a 24-year-old TB patient. She was recently diagnosed with TB by the medical director of the college health center on campus. Carmen is not from the United States. She lives with her aunt and is taking classes at the local college. Susan, the health care worker, is conducting a field investigation. Susan has already conducted a patient interview with Carmen while she was in the hospital. In the interview, Carmen only identified her aunt, as well as three friends from college as contacts. Refer back to Figure 6.5 on page 50 and use the photo to assist in answering the following question.
  • Based on what is visible in the photo on page 50, is there evidence of any possible contacts in Carmen's house other than her aunt?


Risk Assessment for M. tuberculosis Transmission

Risk of Transmission

Based on the information obtained during the medical record review, the patient interview, and the field investigation, the risk that M. tuberculosis was transmitted in each place and the risk to the individual contacts in that place must be assessed. Assessing this risk is crucial because it helps determine which contacts should be given high priority for testing and evaluation. The risk of transmission depends on three main factors:

  • The infectiousness of the TB patient
  • The environmental characteristics of each place
  • The characteristics of the contact's exposure

Infectiousness of the TB Patient

The infectiousness of the TB patient as a factor for M. tuberculosis transmission is dependent upon the duration of time over which the patient was infectious, as well as the estimated degree of infectiousness. The degree of infectiousness is estimated from information on the patient's symptoms, sputum smear results, and other conditions affecting infectiousness that should have been collected during the medical record review and the patient interview. The greater the degree of infectiousness, the more likely transmission will occur. Estimating the period of infectiousness and determining the degree of infectiousness were covered earlier in the Medical Record Review Section.

Environmental Characteristics

The risk of transmission in a particular place depends on the concentration of infectious droplet nuclei in the air -- that is, the number of droplet nuclei in a certain amount of air. The greater the concentration of droplet nuclei, the more likely that TB organisms have been transmitted. The patient's infectiousness affects the number of droplet nuclei generated. In addition, the concentration of droplet nuclei in a room depends on three environmental characteristics (Figure 6.6):

  • Size of the room
  • Amount of ventilation
  • Presence of air cleaning systems

Size of the room. The risk of transmission is high when an infectious TB patient spends time in a small, enclosed space. It is also high when many people are crowded together in close physical proximity to each other. The risk of transmission is lower when an infectious patient is in a very large room that is not very crowded.

Amount of ventilation. In a room that is well ventilated (for example, a room with an open window or an air ventilation system), fresh air comes into the room, diluting the concentration of droplet nuclei. Also, some of the droplet nuclei in the room may be carried outside. Therefore, in rooms with good ventilation the concentration of droplet nuclei, and therefore the risk of transmission, is lower. In rooms that receive no ventilation, the risk of transmission is increased.

Figure 6.6 This is a series of images showing the environmental factors affecting the concentration of droplet nuclei, including room size, ventilation, and air cleaning systems

Air cleaning systems. Infectious droplet nuclei can be removed from the air if the air is filtered through high-efficiency particulate air (HEPA) filters. Alternatively, the tubercle bacilli contained in the droplet nuclei can be killed by ultraviolet lights (see Module 5, Infectiousness and Infection Control). These features, which are used in many hospitals and in some shelters, clinics, correctional facilities, and drug treatment centers, may lower the risk of M. tuberculosis transmission.

Characteristics of the Contact's Exposure

The length and closeness of exposure between the patient and a particular contact are the key factors in assessing the contact's risk of becoming infected. Contacts at higher risk are those who

  • Frequently spend a lot of time with the patient
  • Have been physically close to the patient

Close contacts. Close contacts are persons who had prolonged, frequent, or intense contact with a person with TB while he or she was infectious. Close contacts are more likely to become infected with M. tuberculosis than contacts who see the patient less often, especially if time is spent together in small or crowded, poorly ventilated places. Close contacts usually include anyone who has been physically close to the patient, such as anyone who has shared a house or room with the patient or has frequently spent time with the patient (i.e., recreation, leisure, work, or school). Close contacts are also found in places other than the household of the patient.  

The factors affecting the risk of transmission are summarized in Table 6.5. When it is unclear whether a contact is at higher risk according to these factors (for example, if the exposure may or may not have occurred before the period of infectiousness), it is better to be cautious and consider the contact at higher risk.

Table 6.5
Factors Affecting the Risk of TB Transmission
Factor Contacts at Higher Risk
Infectiousness of the TB patient Contacts exposed to patients with a high degree of infectiousness based on the following factors
  • Laryngeal or pulmonary TB
  • AFB sputum smear-positive
  • Cavitary disease on chest x-ray
  • Cough
  • Positive culture for Mycobacterium tuberculosis
Environmental characteristics Contacts exposed to the patient in
  • Small or crowded rooms
  • Areas that are poorly ventilated
  • Areas without air-cleaning systems
Characteristics of the contact's exposure Contacts who
  • Frequently spend a lot of time with the patient
  • Have been physically close to the patient

Study Questions 6.22-6.25

6.22. What three main factors should be considered in the risk assessment for TB transmission?

6.23. Why is it important to know the period of infectiousness?

6.24. Name three environmental characteristics that would put contacts at higher risk of infection.

6.25. Which contacts are at higher risk of becoming infected?


Case Study 6.5
A health care worker has just interviewed a 47-year-old TB patient, Derrick Jones. Derrick has had a cough for about 2 months and started treatment for TB disease 3 days ago. He lives alone in a small apartment on 41st Street. He is currently unemployed, but in the past 2 months he has worked the night shift with two other employees at the convenience store on 39th Street. During the day, Derrick goes to friends' apartments nearby or stays in his apartment to watch TV. He often goes to the local bar in his neighborhood with his friends Reggie and Melvin. He usually eats at one of two restaurants—the 39th Street Diner or Susie's Kitchen.

His girlfriend, Tonya, is present at the interview and is aware that Derrick is being treated for TB. Derrick says she spends the night several times a week and often brings her 2-year-old son. When Tonya leaves, Derrick says that another girlfriend, Kelly, has stayed over about 10 times in the past 2 months. Last month, Derrick spent several days at Kelly's house, where she lives with her mother.

  • Based on this information, who are the potential contacts? Which contacts are at higher risk for infection and why?
  • While the health care worker is at Derrick's apartment for the interview, what else should be done?



Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination -

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