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Tuberculin Skin Test (TST)

Preparing for a TST

Prepare for TB skin testing by gathering a 26 or 27 mm needle, a 1.0 ml syringe, alcohol, PPD 5TU solution and ball point pen. Epinephrine Hydrochloride Solution (1:1000) and other appropriate agents should be available for immediate use in case an anaphylactic or acute reaction occurs.

The label on the vial should indicate the expiration date. If it has been open more than 30 days or the expiration date has passed, the vial should be discarded and a new vial used.  When you open a new vial, write the date and your initials on the label to indicate when the vial was opened and by whom.

To ensure potency and accuracy of tubersol solution:

  • Do not freeze - store in refrigerator between 2º C and 8º C.
  • Store and transport in the dark and avoid exposure to light.

Patient education

  • Discuss why the skin test is given, what is involved in the procedure and when the individual should return for the test to be read (result provided).
  • If the individual cannot return within the 48-72 hour time period, do not administer the test. Instead, schedule another time that allows the individual to come for both the test and the return appointment.

NOTES:

  • Follow appropriate hand washing and hand hygiene during the procedure.
  • To prevent needle stick injuries, do not recap the used needle and dispose in puncture resistant container.
  • If the intradermal implant does not produce a wheal, repeat the test on the opposite arm and make note of the second implant site.
  • It is normal for a drop of blood to appear at the injection site, even when the needle is inserted properly. Should this happen, lightly blot the blood away with a gauze pad or cotton ball. Do not cover with an adhesive bandage as this will interfere with the test.
  • Remember to instruct the individual to avoid scratching the site, keep the site clean and dry, and avoid putting creams, lotions or adhesive bandages on it. Mild itching, swelling or irritation may occur as normal reactions and usually go away within a week.
  • Do not use EMLA or similar local anesthetic cream.

For more information call 1-877-721-7520, ext. 8809 and ask to speak to a TB nurse on the ID Team.

Useful links:

Approaching the skin


Cleanse the skin in the area to be tested with alcohol and allow to air dry.  Prepare the syringe with 0.1 ml of PPD 5TU solution and clear the syringe of any air. With the bevel up, approach the skin at a 5º-15º angle. The injection should be placed on the palm-side up surface of the forearm, about 10 cm below the elbow.

The wheal (front view)


A wheal, which is elevated about 1 mm above the surrounding skin, is formed with an orange-peel like surface.

The wheal (side view)


The wheal - 6·10 mm in diameter will usually disappear within 10-15 minutes.

Marking the site


Using a pen, mark the test site so that in 48-72 hours the site can be readily located for reading.

Reading the TST test


Mark the border of the induration by moving the tip of a pen at a 45º angle laterally toward the site of the injection. The tip will stop at the edge of the induration, if present. Repeat the process on the opposite side of the induration. Read bump, not redness.

Taking the measurement


Measure the induration in millimetres, using the transverse diameter to the long axis of the forearm. Record measurement in the individual's chart or record sheet. Redness with no induration is read as 'O' mm. Recordings of positive, negative, doubtful, significant and non-significant are not recommended.

Necrotic pustular reaction


A necrotic pustular reaction results when the area tested is covered (note the marks left from the bandage) because of itching. As a result, the reaction is exacerbated.

Photos reproduced by permission of Dr. Maro Steben.

Size

The size of the reaction is only one element of interpreting a positive TST. Consideration should be given to the size, positive predictive value and risk of disease if the person is truly infected.

CTS. (2014). 7th Edition, Page 75

Positive predictive value

The positive predictive value is the probability that the positive test result represents the true presence of TB infection. Factors to consider:
  • risk factors for TB (endemic country, close contact of an active case of TB, the elderly, aboriginal Canadians)
  • BCG vaccination
 

BCG Key Information

BCG vaccination can be ignored as a cause of a positive TST if:
  • BCG vaccination was given in Infancy, and the person tested is now aged 10 years or older
  • There is a high probability of TB infection: close contacts of an infectious TB case, Aboriginal Canadians from a high-risk community or immigrant/visitors from a country with high TB incidence
  • There is a high risk of progression from TB infection to disease

BCG should be considered the likely cause of a positive TST if:
  • BCG vaccine was given after 12 months of age AND there has been no known exposure to active TB disease or other risk factors AND the person is either Canadian-born non-Aboriginal OR an immigrant/visitor from a low TB incidence country

Why Do 2-Step TST

  • To assist with establishing a true baseline result for people who will have serial TSTs in future.
  • A positive TST may gradually wane over the years. The initial TST completed as part of the 2-step process may stimulate the immune response, and a more significant reaction may occur when the person is retested 1-4 weeks later. This delayed response is called the ‘booster’ phenomenon, but represents the accurate baseline.
  • If a baseline is not measured through 2-step TST, a more significant reaction on subsequent tests may be interpreted as a recent TB infection when in fact the more significant reaction may represent the ‘booster’ phenomenon and accurate baseline.
 

Who Should Have 2-Step TST

2-step TST is for individuals who have a higher risk of being exposed to TB in their workplace or day-to-day environment. They may then have subsequent TSTs conducted at regular intervals or in the event of an exposure to contagious TB.

For example:
  • health care professionals
  • corrections staff
  • shelter staff
  • residents of long-term care facilities

How to Do 2-Step Testing

The same materials and techniques of administration and reading should be used.
  • If the first test is not positive (see table below for definition of positive), a second TST is administered 1 week to 1 year later. For most low-risk persons, positive is > 10mm. Less than 1 week does not allow enough time to elicit the ‘booster’ phenomenon.
  • If the first test is positive, a second TST should not be administered.
  • If either the first or the second TST is positive:
  • The person should be referred for medical evaluation and chest x-ray.
  • The person does not need a TST in the future, as there is no clinical utility for future tests. The test does not stratify the risk of TB infection by size of reaction when it is positive. They are all just classified as positive.

 

Interferon Gamma Release Assay (IGRA)

Interferon Gamma Release Assay is a blood test that is more specific for M.TB than the TST because they are not shared with any other BCG strains and most strains of non-tuberculous mycobacteria.  This blood test can be ordered but is currently not covered by OHIP.
IGRA testing is done on a MOHLTC general requisitionand test must be performed at laboratory that has the IGRA kit.

 

Risk of developing active TB disease

After primary infection the lifetime cumulative risk for the development of active TB is generally estimated to be 10 per cent, half will occur in the first two years after infection. Certain factors increase the risk of TB reactivation including AIDS, HIV infection, transplantation, silicosis, chronic renal failure requiring hemodialysis, carcinoma of the head and neck, recent infection and abnormal chest x-ray.

 

Management of positive TST result

Medical evaluation after a positive reaction should include assessment for symptoms, risk factors and chest radiography. In the presence of symptoms or abnormal x-ray, sputum for acid-fast bacilli smear and culture should be taken. In subjects without evidence of active TB, a recommendation should be made regarding therapy for LTBI, based on the interpretation of the TST.

Please contact the health unit's ID Program and ask to speak to a TB nurse at 1-877-721-7520 ext. 8809 with any questions or concerns.

The following persons should not undergo tuberculin skin testing:

  • Individuals with severe blistering tuberculin reactions in the past or extensive burns or eczema present over testing site.
  • Individuals with documented active tuberculosis or a well-documented history of adequate treatment for TB infection or disease.
  • Documented positive skin test.
  • Individuals with major viral infections.
  • Those who have received measles immunizations within the past four weeks. No data are available regarding the effect on TST of other live virus immunizations — mumps, rubella, varicella and yellow fever — but it would seem prudent to follow the same four week guideline.
  • TST can be performed the same day as receiving a live virus immunization.

 

There is no contraindication for individuals who:

  • are pregnant or are breastfeeding
  • have a history of BCG
  • or have an undocumented history of past positive TST

 

PRECAUTIONS:

  • Acute allergic reactions, including anaphylaxis, angioedema, urticaria and/or dyspnea have been very rarely reported following skin testing with Tubersol. See “Risk of Serious Allergic Reactions Following Tubersol [Tuberculin Purified Protein Derivative (Mantoux)] Administration” (available from: http://www.healthycanadians.gc.ca/recall-alert-rappel-avis/hc-sc/2005/14373a-eng.php
  • Epinephrine Hydrochloride Solution (1:1000) and other appropriate agents should routinely be available for immediate use in case an anaphylactic or other acute hypersensitivity reaction occurs.
  • Health care providers should be familiar with the current recommendations of the National Advisory Committee on Immunization for monitoring the patient for immediate reactions for a period of at least 15 minutes after inoculation and for the initial management of anaphylaxis in non-hospital settings.

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