TB Skin Testing
Tuberculosis is a serious, progressive, communicable disease caused by the bacteria Mycobacterium tuberculosis.
The bacteria can affect a number of internal organs but lung involvement is common. There may be no symptoms, particularly in the early stages of the disease.
The Mantoux tuberculin skin test (or TST) is the standard method of determining whether a person is infected with this bacteria.
The TST is performed by injecting 0.1 ml of tuberculin purified protein derivative (PPD) into the inner surface of the forearm. The extent of the skin reaction to this injection determines the presence of either current or prior infection.
Begin by washing your hands and putting on protective gloves.
Inspect the box, looking for damage, the lot number, and the expiration date.
Open the box and remove the vial. Confirm that the vial contains tuberculin (5 TU per 0.1 ml). Confirm the lot number and expiration date and make sure they match those on the box. If the vial has been previously used, confirm the date of first use. It is not recommended that the vial be used if it has been entered and in use for more than 30 days.
The box also contains detailed directions. You should read the directions carefully if you are not familiar with this procedure.
Inspect the contents of the vial. It should be clear, colorless, and without sediment. The rubber stopper should be smooth, without defects or leaks.
A tuberculin syringe is used for this procedure. It holds a maximum of 1.0 ml, although the standard test dose is 0.1 ml.
The needle is short, (one-quarter to one-half inch), and narrow (26 or 27 gauge).
Cleanse the rubber stopper with an antiseptic such as alcohol.
Draw 0.1 ml of air into the assembled needle and syringe.
Insert the needle straight down into the vial.
Inject the vial with 0.1 ml of air. Don’t over-inject or the air pressure may create leaks in the vial.
Invert the vial so air won’t get into your syringe.
Clear any air from the syringe and adjust the dose to exactly 0.1 ml.
Withdraw the needle and if you are not going to immediately administer the test, safely recap the needle.
Cleanse the skin with a disinfectant such as alcohol.
Use your non-dominant hand to stabilize the skin while you slip the needle, bevel up, just beneath the surface of the skin at a very narrow angle.
Once underneath the skin, there is no need to aspirate, simply inject the 0.1 ml of tuberculin solution into the skin, raising a pale bleb or wheal. The bleb should be 6-10 mm in diameter. The bleb will disappear within minutes.
Withdraw the needle. There is no need for a bandaid or dressing of the injection site. Some people draw a wide circle around the bleb to identify its location, but this is not necessary.
The skin test reaction is read any time between 48 and 72 hours after injection. The results are recorded in millimeters of induration, or hardness surrounding the injection site.
The reaction should be measured in millimeters of the induration (palpable, raised, hardened area or swelling). Don’t measure erythema (redness). The diameter of the indurated area should be measured across the forearm (perpendicular to the long axis).
An induration of 15 millimeters or more is considered positive in any person, including persons with no known risk factors for TB.
An induration of 10 millimeters or more is considered positive in
– Recent immigrants from high-prevalence countries
– Injection drug users
– Residents and employees of high risk congregate settings such as prisons and hospitals
– Persons with clinical conditions that place them at high risk
– Children < 4 years of age
– Infants, children, and adolescents exposed to adults in high-risk categories
An induration of 5 millimeters or more is considered positive in
– HIV-infected persons
– A recent contact of a person with TB disease
– Persons with fibrotic changes on chest radiograph consistent with prior TB
– Patients with organ transplants
– Persons who are immunosuppressed