Self-Study Modules on Tuberculosis
Module 6: Contact Investigations for Tuberculosis
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
- 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
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:
- Determine the number of contacts with newly identified positive
- 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)
- 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
- 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
Figure 6.11 This is an example of how to determine the infection
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,
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;
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
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
factors show evidence of recent TB transmission?
6.36. How is the infection rate calculated for a group
6.37. What is the concentric circle approach?
|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:
||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
- 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?
Evaluation of Contact Investigation Activities
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
- How many infected contacts completed a regimen of treatment
- 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
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.
|Study Questions 6.38-6.39
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