[Table of Contents]

Volume: 22S1 • April 1996

Guidelines for Preventing the Transmission of Tuberculosis in Canadian Health Care Facilities and Other Institutional Settings

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) )

• A tuberculin reaction of 0 to 4 mm is classified as negative. Be aware of conditions that cause a false-negative TST (see below).
• A tuberculin reaction of 5 to 9 mm or more is classified as positive when any of the following risk factors are present:

  a. The individual was a recent close contact of an individual with infectious TB.
  b. The individual has a chest x-ray with apical fibrotic lesions suggestive of old healed TB.
  c. The individual is known or suspected of having HIV infection or is immunosuppressed because of other conditions.

• 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

• 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;

• within 1 month of the following illnesses: measles, mumps, chicken pox, infectious mononucleosis; typhoid, brucellosis, influenza or whooping cough (pertussis);
• during a severe or febrile illness [A small percentage (5% to 10%) of people with active TB will not have a positive TST];
• to an individual who has Hodgkin's disease, sarcoidosis or who is infected with HIV;
• to an individual who is taking corticosteroids or immunosuppressive drugs;
• to an individual with severe metabolic disturbances, such as chronic renal failure, severe protein deficiency or burns; and
• to individuals at the extremes of age

  - newborns < 6 weeks of age cannot respond to a TST

  - children aged 6 weeks to 6 months and the elderly may not respond to a TST.

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.

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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