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Module 6: Contact Investigations for Tuberculosis
Reading Material

Decision About Whether to Expand Testing

Evidence of Recent Transmission

After the highest-priority group has been evaluated for TB infection and disease, the contact investigation staff should evaluate the results of testing for evidence of recent transmission. Evidence of recent transmission is provided by any of the following factors:

  • A high infection rate among contacts
  • Infection in a young child
  • A skin test conversion in a contact
  • A secondary case of TB disease

An evaluation of this evidence will help determine whether testing should expand.

Infection Rate

The percentage of contacts with a similar amount of exposure (e.g., close, other-than-close) who have a newly identified positive skin test reaction (5 or more millimeters of induration) is called the infection rate for that group of contacts. (Contacts who had a previously documented positive skin test reaction before being exposed to the TB patient should be excluded from this percentage.) 

To calculate the infection rate among a given group of contacts, the health care worker should follow these steps:

  1. Determine the number of contacts with newly identified positive skin tests.
    • Subtract the number of contacts with a documented previous positive skin test from the total number of contacts with a positive skin test (new or previously documented)
  2. Next, determine the total number of contacts without a documented previous positive skin test.
    • Subtract the number of contacts with a documented previous positive skin test from the total number contacts
  3. Finally, determine the infection rate.
    • Divide the number of contacts with a new positive skin test by the total number of contacts without a documented previous positive skin test
    • Multiply by 100; the resulting percentage is the infection rate for the group of contacts

For an example of how to determine the infection rate, see Figure 6.11.

Figure 6.11 This is an example of how to determine the infection rate.

For the purpose of a contact investigation, a contact's local community is the geographic area where he or she lives and spends time. This may be a residential area or an ethnic community (e.g., groups of people who emigrated from the same geographic area). Recent TB transmission is probable when the infection rate in a group of contacts is greater than the level of skin test positivity in the local community (based on health department data or estimates, if available).

Infection in Young Children

When TB infection or disease occurs in young children, given their age, there is reason to suspect recent transmission. Infected children younger than 4 years of age and children with certain medical conditions are at increased risk of progression to TB disease. A positive tuberculin skin test reaction in a child always warrants careful assessment. A positive tuberculin reaction in a child with recent BCG vaccination may be difficult to interpret. However, if recent exposure has occurred, it is likely that the reaction is due to true TB infection (see Module 3, Diagnosis of Tuberculosis Infection and Disease).

Skin Test Conversions for Contacts

A skin test conversion for a contact is defined differently from a standard skin test conversion. The American Thoracic Society (ATS) defines a standard skin test conversion as a previous negative skin test reaction increasing 10 mm or more within a 2-year period. This definition of a standard skin test conversion typically applies to periodic surveillance of tuberculin-negative persons likely to be exposed to tuberculosis. For example, the definition for a standard skin test conversion is used for persons who undergo regular (e.g., yearly) skin testing as part of a skin testing program at a health care facility or other setting.

A skin test conversion for contacts is defined as a change from less than 5 mm on the initial skin test to a reaction of greater than or equal to 5 mm on the second test, 10 to 12 weeks after exposure. Any contact who has a skin test reaction of 5 mm or more on either the initial test or the follow-up test (10 to 12 weeks after exposure) should be evaluated for treatment for LTBI.

Secondary TB Cases

When a contact has developed TB disease as a result of transmission from an index patient, this is called a secondary case of TB. Contacts of infectious cases who have new positive skin test reactions are at high risk of developing TB disease because they have been recently infected.

It is also possible that the index patient developed TB disease as a result of exposure to a person who still has infectious TB disease. The index patient may identify a contact who was the initial source of his or her TB disease. In this instance, the contact is considered the source of transmission for the index patient; if the source case has not been reported to the health department, this should be done. A contact investigation should be conducted immediately around any source case or secondary case or cases discovered during another investigation.

Making the Decision to Expand Testing

Evidence of transmission may be provided by one or more of the factors discussed above. When there is evidence of transmission in the first group of close contacts tested, the likelihood increases that M. tuberculosis has also been transmitted to contacts with less exposure than the close contacts. Therefore, the testing should be expanded to these contacts. In the example (Figure 6.11), if any of the seven contacts with a newly identified positive skin test reaction is a young child or has a negative reaction on the first skin test and a positive reaction on the test 10 to 12 weeks after exposure (i.e., skin test conversion for contacts), there is evidence that M. tuberculosis was transmitted. In addition, if any of the contacts had TB disease, it is likely to be the result of recent transmission.

The interpretation of the infection rate can be more difficult. In our example (Figure 6.11) the infection rate in the first group of contacts was 70%. This is much higher than the 5%-10% rate estimated for most populations without risk factors, and also higher than the 25% rate that may be seen in some populations with risk factors (e.g., correctional inmates). Frequently, the health department may not have data on the expected infection rates from different communities and populations. In addition, decisions about expanding contact investigations when the close contacts are from countries with a high incidence of TB may be even more difficult because of a high expected rate of previous infection from exposure in their country of birth. Decisions about expanding contact investigations to the next group of contacts should be made by clinical and supervisory staff based on an assessment of all available information.

If there is NO evidence of recent M. tuberculosis transmission among close contacts, that is,

  • If the infection rate is lower than or similar to the level of infection in the community
  • No young children have a positive skin test reaction
  • No contact skin test conversions have occurred
  • No contacts have TB disease

then testing should NOT be expanded to the next group of contacts. Decisions about expanding contact investigations should be made by supervisory clinical and management staff.

If there IS evidence of recent transmission, the next-highest-priority group should be evaluated. The investigation should expand to the next group each time there is evidence of transmission in the group being tested. This should be done as soon as it becomes clear that transmission has probably occurred (e.g., a strong suspicion of TB disease in a contact or several skin test conversions among the last group tested). Once the infection rate in the group being tested is about the same as the infection rate in the local community and there are no other factors indicating recent transmission, testing should be stopped (Figure 6.12).

Figure 6.12  This is a flow chart depicting the decisional analysis for expanding contact investigation testing.

The evaluation of data collected from contact investigations is a complicated process requiring careful interpretation and consideration of available evidence. The particular circumstances of each case (e.g., number of contacts involved, their age, their susceptibility to TB disease) need to be carefully considered in order to expand testing to include all those likely to be at risk.

Concentric Circle Approach

The concentric circle approach (Figure 6.13) is a method of testing contacts in order of their exposure time (close vs. other-than-close) and risk (high priority vs. low priority), with the close contacts and other contacts at high risk of developing TB disease tested first. In this approach, the original TB patient (the index case) is at the center. The circle is divided into three concentric rings to represent the three levels of risk: close (high risk), other-than-close (medium risk), and other-than-close (low risk). The circle is also divided, like a pie, into segments that represent the three types of environment where the contact may have taken place:

  • Household or residential
  • Work or school
  • Leisure or recreation environments

Figure 6.13  This is a graphic of the concentric circle approach to testing contacts in order of their exposure time (close vs. other-than-close) and risk (high priority vs. low priority), with the close contacts and other contacts at high risk of developing TB disease tested first.  Adapted from Etkind SC. Contact tracing in tuberculosis. In: Reichman L, Hershfield E, eds. Tuberculosis: A Comprehensive International Approach. New York: Marcel Dekkar; 1993:283.

The highest-priority group, consisting of close contacts and of people at high risk of developing TB disease, is the circle closest to the index circle. This means that this group is tested first. Close contacts can be found in each segment of the concentric circle (i.e., household or residential, work or school, and leisure or recreation environments). It is essential to test close contacts in all segments of the concentric circle, not just the household segment. Each of the circles represents groups of contacts, with the highest-priority groups nearest to the center and the lowest-priority groups farthest from the center. If there is evidence of transmission in one group, then the next outer circle of contacts should be tested, until there is no longer evidence of transmission.

Determining the level of the exposure of contacts or what "circle" or priority a given contact is for a contact investigation should be determined by supervisory clinical and management staff. 

Sometimes people who were not identified as close contacts come to the health department for evaluation or are present during field investigation and testing because they think they might have been exposed to the TB patient.

Likewise, when contact investigations are conducted in an institutional setting (school or worksite), decisions about expanding a contact investigation may be guided by principles other than those discussed previously, which are largely based on observing the number of documented new infections. For example, health departments may be requested to test all the employees or students in a specific setting, even if data show transmission did not occur with close contacts and thus the other contacts are not considered at risk for TB infection. Requests to expand contact investigations in institutional settings where data show transmission did not occur with close contacts are often driven by fear and misunderstanding of the risk of M. tuberculosis transmission. Decisions about expanding testing in these situations should be made by supervisory clinical and management staff.

As the contact investigation progresses, the health care worker should make sure that all contacts who were scheduled for testing received initial tests and attended the follow-up appointments (that is, skin test reading and, if needed, chest x-ray or sputum exam). The adherence of the index case and of any contact with TB disease should be monitored to ensure completion of adequate therapy. In addition, it is important to monitor the adherence of contacts who begin a regimen of treatment for LTBI.
 

Study Questions 6.35-6.37

6.35. What factors show evidence of recent TB transmission?

6.36. How is the infection rate calculated for a group of contacts?

6.37. What is the concentric circle approach?

Answers 


 
Case Study 6.8
The contacts in Case Study 6.7 were retested 12 weeks after their last exposure to Hector while he was infectious. Luis and Henry were given window period prophylaxis during the window period. The results of the repeat skin testing of contacts with an initial negative reaction are as follows:
Contact conversions: Negative reactions: Initial Positive Reactions
Coworker A (11 mm) Alma (4 mm) Mimi (11 mm)
The bartender (10 mm) Joe (2 mm) Javier (13 mm)
Luis (8 mm) Henry (0 mm)  
  Coworker B (3 mm)  
  Coworker C (0 mm)  
  Coworker D (4 mm)  
  • What was the infection rate in this group of contacts? Don't forget to include contacts with an initial positive reaction.
  • The expected infection rate in Hector's community is about 12%. Is there any evidence of TB transmission in the first group of contacts?
  • Should testing be expanded to the next group of contacts?

Answers 

Evaluation of Contact Investigation Activities

The Evaluation

To complete the investigation, an evaluation of the contact investigation activities should be conducted with or by a supervisor to determine such things as

  • Were an appropriate number of contacts identified?
  • Were the highest-priority contacts located and tested?
  • Was the contact investigation performed in all settings: household or residence, work or school, and leisure or recreational environments?
  • Was the contact investigation expanded appropriately?
  • Were contacts completely evaluated (including second skin test if needed) and given appropriate therapy if they had TB infection or disease?
  • How many infected contacts completed a regimen of treatment for LTBI?
  • Did all identified cases complete an adequate treatment regimen?

The answers to these questions will help determine how successful the contact investigation has been. 

Information from individual contact investigations will be compiled and evaluated by management staff as part of ongoing program evaluation activities. The results of these program evaluations are used to

  • Determine effectiveness
  • Identify areas in need of improvement
  • Prioritize program activities and resources

Program evaluation is a critical component of any program. Evaluation of program performance is important to ensure that program resources and priorities are being used effectively on the highest priority activities.

TB prevention and control efforts should be targeted to the groups at highest risk for TB infection, as well as to the groups at highest risk for progression from TB infection to TB disease. Contacts of infectious cases of TB are one such high-risk group. Effective and successful contact investigations can help prevent additional cases of TB infection and disease and reduce further transmission of M. tuberculosis.
 

Study Questions 6.38-6.39

6.38. List seven questions that should be answered in an evaluation of a contact

6.39. As part of program evaluation activities, what will the results of a contact investigation help management staff determine?

Answers 


 
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