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Volume: 22S1 • April 1996
Guidelines for Preventing the Transmission of
Tuberculosis in Canadian Health Care Facilities and
Other Institutional Settings
Appendix B
Tuberculin Skin Testing
The Canadian Tuberculosis Standards (43) and the
article by the Canadian Thoracic Society (9) provide
further information concerning tuberculin skin
testing.
The TST should be performed by injecting 5 tuberculin
units (TU) of purified protein derivative (PPD)
intradermally in the anterior aspect of the forearm
(Mantoux test). Only 5 TU PPD should be used since
there is no standardized interpretations of other
doses of PPD. In Canada, the multi-puncture
tuberculin tests are no longer considered to be
appropriate screening tests.
In order to minimize inter-observer variation, the
technique of injecting PPD and interpreting reactions
must be standardized. Health care facilities may
choose to train a cadre of professionals to read TSTs
so that the test result may be read in the immediate
work environment of the HCW.
TST tests must be read 48 to 72 hours after the TST
is applied and, as stated above, must be read by
trained staff members. The designation of a
"significant" reaction is based on the size of
induration present (measurement, in millimeters, of
the induration present at right angles to the long
axis of the forearm) and the person's medical
history. No single cut-off size can be established.
Organizations, such as the American Thoracic Society
and the Canadian Thoracic Society, have published
schemes to interpret TST results. These schemes are
slightly different. This document uses the
interpretations recommended by the Canadian Thoracic
Society (2).
A converter is defined as someone who has a positive
TST within 2 years of having a documented negative
TST.
Administration of a TST
- Follow universal precautions.
- Seat your client comfortably, resting his/her
exposed arm on a firm, well lighted surface.
- Clean the injection site with an alcohol pad and
allow it to dry completely.
- Use a single-dose, disposable tuberculin syringe
and a 1 ¤2-inch, 26 or 27 gauge needle with a
short bevel.
- Use PPD tuberculin as the injectable because it
provides a testing material of standardized strength.
Confirm the dosage required to administer 5 TU of PPD
tuberculin (usually 0.1 mL).
- Draw up a little more than 0.1 mL of PPD solution
in the TB syringe. Hold the syringe upright and tap
it lightly to remove air, then expel one drop. Check
that a full 0.1 mL remains in the syringe.
- Avoid areas on the skin that are red or swollen.
Avoid visible veins.
- The usual injection site is on the anterior
surface of the forearm, about four inches below the
elbow. Avoid placing a TST on an area of skin that
has anything which would interfere with reading the
TST.
- Stretch the skin taut with your non-dominant
hand. While holding the syringe parallel to (almost
resting on) the surface, insert the needle, bevel up,
so that the tip of the needle is visible just below
the surface of the skin.
- As you slowly inject the contents of the syringe,
you will feel a slight resistance. A firm, white
wheal about 6-10 mm in diameter should appear at the
injection site immediately.
- If the injectable (PPD) leaks out onto the skin
and no wheal appears, it means you did not place the
needle deeply enough. If the wheal is shallow and
diffuse, you have given the injection too deeply. In
either case, administer a second injection at least
two inches from the first site and circle the second
injection site.
- You may see a drop of blood when you withdraw the
needle. This is normal. Offer the participant a 2x2
gauze to remove the blood. Advise the participant not
to press the gauze over the injection site but to
just dab gently to remove the blood. This will avoid
squeezing out the tuberculin thereby disrupting the
test.
- Do not recap needle. Place the syringe in a
puncture-resistant container.
Instructions to Patient
- Do not rub or scratch the site of the TST test.
- Keep area clean.
- No restrictions with respect to diet, type of
activities.
- Return in 48 to 72 hours for reading.
Reading a TST
- The TST is read 48 to 72 hours after being
applied. However, if a person presents more than 72
hours after the TST was applied and the test result
is greater than 10 mm, this test should be considered
positive. If a person presents after 72 hours and the
result is less than 10 mm, the test is invalid and
must be repeated.
- The TST must be read by a trained physician or
nurse.
- The reading should be made in good light, with
the person's forearm slightly flexed at the elbow.
- The presence or absence of induration should be
documented. Erythema or redness is not measured. The
development of erythema does not indicate infection.
- Induration is determined by inspecting the arm
from a side view against the light as well as by
direct light and by palpating the arm with a gentle
stroke of the finger.
- If induration is present, the diameter is
measured across the width of the forearm, (e.g.,
measure the width at right angles to the long axis of
the forearm). Sometimes the precise edge of
induration is difficult to palpate. Use a pen to help
mark the beginning and end points of induration. Use
a flexible ruler to measure the size of induration
between the pen points.
- Record the size of induration in millimetres. If
the measurement falls between demarcations on the
ruler, record the smaller of the two numbers. If the
participant has no induration, record the result as 0
mm.
Interpreting a TST (Canadian Tuberculosis Standards
(43) )
After recording the size of the TST, it is
necessary to interpret if the test is positive or
negative to ensure that the appropriate follow-up
actions are taken.
If none of the risk factors listed above is
present, a tuberculin reaction of 5 to 9 mm is
classified as negative.
- A tuberculin reaction of 10 mm or more is
classified as positive for all individuals (i.e., the
test result for a participant who has a history of a
BCG will be interpreted as positive when a reaction
of 10 mm or greater is present).
False Negatives
A person with a negative TST may be infected with
M. tuberculosis but, because of a number of
factors, the immune system can not respond to the TST.
This person has a false-negative TST. This reinforces
the need for obtaining an adequate medical history
prior to testing. If individuals have a temporary
condition likely to yield a false-negative test at the
time of screening, their TST should be postponed.
Reasons for a false-negative test include giving the
TST:
- within 1 month of administering a live virus
vaccine (polio, measles, rubella, mumps or yellow
fever); if the live vaccine has already been given,
ideally WAIT 30 days before administering the
TST;
N.B. A TST may be given PRIOR to
administration of a live virus vaccine or at the same
time as the live vaccine.
Individuals with the above conditions are not
necessarily precluded from having a TST performed. If a
positive response occurs, it must be read and
interpreted as such. Medical personnel must be alert to
the possibility of a false-negative response if
the conditions listed above are present and ensure that
a thorough evaluation of the risk of the individual
being infected with M. tuberculosis is
performed.
False-negative results also occur when
- incorrect injection technique is used;
- the TST is read by an inexperienced person; and
- improper storage, exposure to light or heat of
the tuberculin has occurred.
A significant TST reaction almost always
represents previous M. tuberculosis infection.
Other causes of reactivity include cross-reactivity to
non-tuberculous mycobacterial infections or reception
of BCG vaccine (live attenuated mycobacterial strain
derived from M. bovis). The prevalence of atypic
mycobacterial infection varies geographically and thus
varies in its importance.
Vials of PPD should be stored at 2° C to 8° C
under conditions of minimal air contact. A small
decrease in PPD potency has occurred within 24 hours
of contact with air, thus open vials should be used
within 1 month.
For a discussion of two-step testing, see Section
IV.C.
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