Intradermal injections and tuberculin testing

Intradermal tuberculin testing

Intradermal (ID) injections are given just under the epidermis. Because the drug is dispersed in an area where many nerves are present, it causes momentary burning or stinging. Small amounts of medication are injected. In ambulatory care, intradermal injections may be given for the Mantoux tuberculin skin test (TST) and allergy testing.

Two types of tuberculin tests have been approved by the FDA: the skin test and the blood test. The TST is used to determine whether a patient is infected with Mycobacterium tuberculosis. It is important for the medical assistant to be knowledgeable about and skilled at administering and reading the test.

Tuberculin skin testing

The TST can be given to most patients, including infants, pregnant women, patients infected with the human immunodeficiency virus (HIV), and those vaccinated with bacillus Calmette-Guérin (BCG). Patients with a history of BCG vaccination may have a positive reaction to the TST. If a patient mentions a history of receiving BCG vaccine, it is important for the medical assistant to inform the provider before performing the skin test. The provider may opt to have the patient undergo the blood test instead.

The TST is contraindicated in patients who had the following:

• A severe reaction to a past TST (e.g., necrosis, ulcers, blisters, or anaphylaxis)
• A history of a positive TST result
• A live-virus vaccine less than 4 to 6 weeks earlier (A TST and a live-virus vaccine can be given on the same day, or the TST can be given 4 to 6 weeks after the vaccine.)

Tuberculin skin test procedure and reading

To perform the TST, tuberculin purified protein derivative (PPD) 0.1 mL is given ID into the inner surface of the forearm. A tuberculin syringe and needle, with the bevel facing upward, are used to slowly inject the PPD, creating a tense, pale wheal . The wheal must measure 6 to 10 mm in diameter, or the test must be repeated.

The patient returns within 48 to 72 hours to have the test read. Reading before or after this time invalidates the results. When reading the test, palpate the site to check for a raised, hardened area, called an induration. If an induration is felt, measure the raised area across the forearm (perpendicular to the bone). The erythema (redness) is not measured. The diameter of the induration is read in millimeters (mm) and must be noted in the health record. The medical assistant can never state whether the test result is positive or negative. The provider uses the test results and the patient history to determine if the patient has tuberculosis (TB). A TST reading between 5 and 15 mm can be positive for different populations.

Summary of intradermal injections

Syringe used	1mL (tuberculin syringe)
Needle size	1/4 to 1⁄2 inch; 25−27 gauge
Angle of entry	5 to 15 degrees with bevel facing upward
Maximum volume	0.1mL
Common sites	Forearm, upper arm, and middle of the back
Additional sites	Separate by at least 2 inches
Patient position	Sitting with arm extended if using forearm
Administration	Pull skin taut at injection site

Incorrect tuberculin skin test readings

At times, the TST reading may be incorrect. A false-positive reaction means the person reacted to the test even though no M. tuberculosis is present. Reasons for false-positive reactions with TST include the following:

• Lung infection with nontuberculous mycobacteria (NTM). This organism is found in the water and soil and is inhaled.
• Previous vaccination with BCG vaccine. Many foreign-born patients from countries with a high risk of TB may have received BCG abroad. The BCG vaccine has been approved by the FDA, but it is used in very limited situations.
• Incorrect administration or reading of the TST.

A false-negative reaction means the person may not have reacted to the test, even though the patient is infected with M. tuberculosis. Reasons for false-negative results with TST include the following:

• Weakened immune system
• Exposure to TB infection within the previous 8 to 10 weeks
• Very old TB infection
• Patient is younger than 6 months old
• Recently received a live-virus vaccine (e.g., measles, yellow fever, chickenpox), or had a viral infection (e.g., influenza), or received corticosteroids or immunosuppressive medications (a false reaction may occur up to 5 to 6 weeks afterward)
• Incorrect administration or reading of TST

Two-step testing

In some patients who have had a TB infection, the body forgets to react to the TST. This can occur if the infection was many years before. The initial TST may be negative, and the person may have a false-negative reaction. Receiving a second TST can help the body remember the infection, thus causing a more accurate (positive) reading. This is called the booster effect or booster phenomenon. The second TST can be done 1 to 3 weeks after the initial test was read.

New residents in long-term care facilities (e.g., nursing homes) usually have a two-step TST done. Healthcare students and professionals also need to have a two-step TST. Once the two-step TST has been completed, they need yearly TSTs. If the time since the last TST exceeds 1 year, they may need to complete another two-step TST.

Tuberculin blood tests

The FDA has approved two tuberculin blood tests: the the QuantiFERON-TB Gold Plus (QFT-Plus) test and the T-SPOT.TB test. If a patient has had the BCG vaccine or is unable to return for the skin test reading, the blood test is preferred. The blood test replaces the skin test. A healthcare professional or student only needs one blood test initially, unlike the two-step skin test. After the initial test, a yearly skin or tuberculin blood test is required.

After the medical assistant draws a patient’s blood, it is sent to the laboratory for analysis. The error rate with a blood test is much lower than with the palpated, measured reading of the TST. The next step will depend on whether the TB blood test result is positive or negative:

• Positive: The person has been infected with TB. The provider will order additional tests to determine if the person has a latent TB infection or TB disease. (See Chapter 26 for more information on these two diseases.)
• Negative: It is unlikely the person has TB.