Which action should the nurse take when administering and reading the tuberculosis (tb) skin test

This annex gives information on administering, reading and interpreting a tuberculin skin test (TST).

A TST is the intradermal injection of a combination of mycobacterial antigens that elicit an immune response (delayed-type hypersensitivity), represented by induration, which can be measured in millimetres.

The standard method of identifying people infected with M. tuberculosis is the TST using the Mantoux method. Multiple puncture tests should not be used as these tests are unreliable (because the amount of tuberculin injected intradermally cannot be precisely controlled).

This annex describes how to administer, read and interpret a TST using 5 tuberculin units (TU) of tuberculin PPD-S. An alternative to 5 TU of tuberculin PPD-S is 2 TU of tuberculin PPD RT 23.

1. Locate and clean injection site 5–10 cm (2–4 inches) below elbow joint

   • Place forearm palm-up on a firm, well-lit surface.

   • Select an area free of barriers (e.g. scars, sores, veins) to placing and reading.

   • Clean the area with an alcohol swab.

2. Prepare syringe

   • Check expiry date on vial and ensure vial contains tuberculin PPD-S (5 TU/0.1 ml).

   • Use a single-dose tuberculin syringe with a short (¼- to ½-inch) 27-gauge needle with a short bevel.

   • Clean the top of the vial with a sterile swab.

   • Fill the syringe with 0.1 ml tuberculin.

3. Inject tuberculin (see Figure A3.1)

   • Insert the needle slowly, bevel up, at an angle of 5–15°.

   • Needle bevel should be visible just below skin surface.

4. Check injection site

   • After injection, a flat intradermal wheal of 8–10 mm diameter should appear. If not, repeat the injection at a site at least 5 cm (2 inches) away from the original site.

5. Record information

   • Record all the information required by your institution for documentation (e.g. date and time of test administration, injection site location, lot number of tuberculin).

The results should be read between 48 and 72 hours after administration. A patient who does not return within 72 hours will probably need to be rescheduled for another TST.

1. Inspect site

   • Visually inspect injection site under good light, and measure induration (thickening of the skin), not erythema (reddening of the skin).

2. Palpate induration

   • Use fingertips to find margins of induration.

3. Mark induration

   • Use fingertips as a guide for marking widest edges of induration across the forearm.

4. Measure diameter of induration using a clear flexible ruler

   • Place “0” of ruler line on the inside left edge of the induration.

   • Read ruler line on the inside right edge of the induration (use lower measurement if between two gradations on mm scale).

5. Record diameter of induration

   • Do not record as “positive” or “negative”.

   • Only record measurement in millimetres.

   • If no induration, record as 0 mm.

Interpretation of TST depends on two factors:

diameter of the induration;

person's risk of being infected with TB and of progression to disease if infected.

Induration of diameter ≥5 mm is considered positive in:

HIV-positive children;

severely malnourished children (with clinical evidence of marasmus or kwashiorkor).

Induration of diameter ≥10 mm is considered positive in:

all other children (whether or not they have received BCG vaccination).

Causes of false-negative and false-positive TSTs are listed in Table A3.1.

1.

Guidance for national tuberculosis programmes on the management of tuberculosis in children. Geneva: World Health Organization; 2006. (WHO/HTM/TB/2006.371) [PubMed: 17044200]