What is tuberculin skin testing (139)?
Tuberculin skin test (TST), first introduced in 1908, remains the most commonly used test for TB infection in the world due to its low cost and feasibility to implement in low resource settings. There is no need for complex laboratory equipment or procedures or trained technicians, although training is required for proper intradermal administration of the tuberculin purified protein derivative (PPD). Intradermal injection of tuberculin causes delayed hypersensitivity reaction if a person has TB infection. The T-cells in such individuals are sensitized due to prior M. tuberculosis infection and the tuberculin causes recruitment of these immune cells to the skin test site, causing a local inflammatory reaction. Nurses and other health care workers with the right training and skills can administer and read a TST. They should also be able to explain to the persons tested what the result of the test implies in their specific case.
How is TST administered?
TST administration should be standardized for each country using either 5 tuberculin units (TU) of purified protein derivative (PPD-S) or 2 TU of tuberculin PPD RT23, which give similar reactions in individuals infected with M. tuberculosis. For standardization of readings and interpretation of results, 5 TU of tuberculin PPD RT23 is used more widely.
Tuberculin should be injected strictly intradermally on the inner surface of the forearm. A discrete wheal (6–10 mm) should be produced when the injection is given correctly. If improperly administered, another test dose can be given immediately at a site at least 5 cm (2 inch), away from the original injection. A note in the record should indicate the site chosen for the second test. After the test, it is extremely important to make sure that the individual returns for the test reaction read within 48–72 hours. The person is instructed to keep the test site clean, uncovered and not to scratch or rub the area.
What are the steps in tuberculin skin testing?
1. Preparations and test administration
– Counsel the person about the test and explain the need to come back within 48–72 hours to read the test result.
– Ideal injection site is the palmar side of the left forearm, 5–10 cm from the elbow joint, free from muscle margins, hair, veins, tattoos, sores or scars. The area should be cleaned with soap and water, if visibly dirty. Other sites can also be used (such as the right forearm, both sides of chest and back) if there is injury, wound, big scars, bandage, surgical casts and complete tattoo of left forearm or amputation.
– Tuberculin vials are multidose vials (10 doses or 50 doses). The vials should be stored at 2–8°C and not exposed to direct sunlight. The vial can be used up to one month after opening and should be discarded if the fluid colour changes or after 30 days.
– Use 1 ml graduated syringe/tuberculin syringe which can dispense 0.1 ml solution accurately using a one quarter to one half inch, 27-gauge needle. Draw 0.1 ml (5 Units) of tuberculin or as per manufacturer instructions, expel air and excess drops.
– Tuberculin should be injected within 20 minutes of loading into the syringe.
– After gentle cleaning of the site with an alcohol swab, stretch the selected area of the skin using the thumb and forefinger, insert the needle slowly with the bevel pointing upwards at an angle of 5 to 15 degrees, advance the needle through the epidermis approximately 3 mm so that the entire bevel is covered and visible just under the skin. Release the stretched skin and slowly inject tuberculin and check for leakage. If there is no leakage, continue to inject slowly until the complete 0.1 ml solution has been administered and remove the needle quickly. Gently blot the injection site if a drop of blood appears with alcohol-based disinfectant without squeezing out tuberculin.
– Proper injection will result in a pale wheal 6–10 mm in diameter. If the wheal is less than 6 mm in diameter the test needs to be repeated.
2. Reading TST reaction
– The test should be read between 48–72 hours (NOT before 48 hours or after 72 hours).
– Reading should be performed in a good light, with the forearm slightly flexed at the elbow. Gauge presence of induration (palpable, raised, hardened area or swelling), starting with inspection and then palpation with light, gentle motions. Sweep fingertips over the surface of forearm in all four directions to locate margins or edges of induration. Using the fingertip as a guide, mark the widest edges of the induration across the forearm lightly with a fine line or dot. If the margins of induration are irregular, mark and measure the longest diameter.
– The diameter of induration is measured across the forearm, from the thumb side of the arm to the little finger side. Using a plastic scale, place the zero-ruler line inside the edge of marked fine line or dot and measure the ruler line inside the right dot or edge of fine line. If the measurement falls between two divisions on the millimetre scale, record the lower division.
– Do not measure the redness or bruise.
– Alternatively, use the ballpoint pen method for reading (140). A ball point pen line may be drawn on the transverse axis of the forearm, starting 1–2 cm away from the visible skin test reaction and moving slowly towards its centre, exerting moderate pressure against the skin. The point where resistance to pen displacement occurs determines the outer limit of the induration. Mark lightly with a fine line or dot at the widest edges of the induration across the forearm and use a ruler to measure the diameter as above.
3. Recording variables
– Note exact location of TST administration.
– If there is no induration, record as “Zero”. Otherwise, record the exact size of the induration in millimetres. Do not record as positive or negative.
– Record adverse events at the test site if any, such as formation of vesicles, bullae, lymphangitis, ulceration or necrosis.
How are TST reactions interpreted?
TST does not measure immunity to TB but the degree of hypersensitivity to tuberculin. A skin test result is interpreted considering the person’s risk of being infected with TB and progression to disease when infected as well as size of the induration in millimetres. However, there is no correlation between the size of induration and likelihood of current TB disease (poor positive predictive value) or future risk of developing TB disease (141). There is no correlation between the size of TST reactions post-BCG vaccination and protection against TB disease. Overall, results of TST must be interpreted carefully considering individual clinical risk factors before determining the size of the induration that is positive (5 mm, 10 mm or 15 mm). Formation of vesicles, bullae, lymphangitis, ulceration and necrosis at the test site should also be noted as they may indicate a high degree of tuberculin sensitivity and hence the presence of TB infection (142).
A negative test may indicate lack of infection with M. tuberculosis or that the person has acquired infection recently and not enough time has elapsed for the body to react to the skin test. From the time of infection to the development of cell-mediated immunity there is a window period of up to 12 weeks, when TST would be negative. Most children with a negative result may not be infected with M. tuberculosis. Immunologically compromised individuals, especially those with HIV and low CD4 T-cell counts or severe malnutrition, frequently show negative results from the PPD test. The absence of cell-mediated immunity to tuberculin may be due to lack of previous sensitization or due to anergy because of immune suppression. The following table classifies conditions where different induration sizes are considered to indicate TB infection.
What are false-positive reactions?
Some persons may react to TST even though they are not infected with M. tuberculosis. The causes may include
• Infection with non-tuberculosis mycobacteria
• Previous BCG vaccination (prior vaccination may cause false-positive result for many years) (143)
• Low risk of TB exposure (due to low specificity of TST, most positive reactions in low-risk individuals should be considered false-positives) (144)
• Errors in TST administration or interpretation of reaction.
What are false-negative reactions?
A negative TST result usually signifies that the individual has never been exposed to M. tuberculosis. However, the following factors may cause a false-negative result or diminished ability to respond to tuberculin.
• Cutaneous anergy (i.e. inability to react due to weakened immune system or immune suppressive medication)
• Recent TB infection (within 12 weeks of exposure) or very old TB infection (many years)
• All children below five years of age; the younger the child, the higher the probability
• Recent live-virus vaccination (measles and smallpox) or viral illnesses (measles and chicken pox)
• Overwhelming TB disease (miliary TB, TB meningitis)
• Error in TST administration (subcutaneous injection, insufficient dose) or interpretation of reaction.
What are the adverse reactions of TST?
• Within 12 hours: fever, injection site pain, pruritus, discomfort, erythema, rash.
• Up to 3 days of TST: injection site haemorrhage or haematoma, vesicles, ulcers and necrosis (highly sensitive individuals). Injection site scar may also occur due to strongly positive reactions.
Who can receive a TST?
Most persons can receive a TST. It is contraindicated only for persons who have had a severe reaction (such as necrosis, blistering, anaphylactic shock or ulcerations) to previous TST. It is not contraindicated even among infants, children, pregnant women, persons who are HIV-infected, or persons who have been vaccinated with BCG.
How often can TSTs be repeated?
In general, there is no risk associated with repeated TST placements. If a person does not return within 48–72 hours for a TST reading, a second test can be placed as soon as possible.
When is TST contraindicated?
• Known hypersensitivity to any of the contents of TST
• Severe reactions to previous tuberculin tests
• Situations where repeat test will not provide any new information (such as past TST reactions ≥ 15 mm, diagnosed TB disease or known previous TB disease)
• Infants under 12 weeks old.
What is a boosted reaction?
When sensitization to mycobacteria has occurred many years earlier, an initial TST may produce negative or weakly positive response, there being too few sensitized lymphocytes in the circulation to produce a significant local response. If the test however is repeated, a larger reading may be obtained due to the immune response being ‘recalled ‘or ‘boosted’ by the first test. Administering such a second TST after an initial negative reaction is called two-step testing. The second boosted reading is considered for decision-making or future comparison. Boosting is maximum within one to five weeks of the initial test and may continue for up to two years (145).
Why is two-step testing conducted?
Two-step testing is useful for initial skin testing among adults who are going to be retested periodically, such as health care workers. This two-step approach can reduce the likelihood that a boosted reaction to a subsequent TST will be misinterpreted as a recent infection (considered in low TB burden settings).
What is TST conversion?
While boosting is recall of hypersensitivity response in the absence of new infection, conversion is the development of new or enhanced hypersensitivity due to infection with tuberculous or nontuberculous mycobacteria, including BCG vaccination. A change in TST reactivity within two years of a negative test (146) is called conversion, negative to a positive reaction and increased induration ≥ 10 mm compared to the previous reading.
Conversion has been associated with an annual TB incidence of 4% among adolescents (147) and up to 6% among contacts of smear-positive TB patients (148). If it is decided to retest a contact for conversion, the second tuberculin test should be done after eight weeks of last contact with the source patient.
What is TST reversion?
TST reversion is defined as change of TST result from positive to negative. This is uncommon among healthy individuals however it is noted among the elderly during two-step testing.
Can TST be given to persons receiving vaccinations?
Vaccination with live viruses may interfere with TST reactions. For persons scheduled to receive a TST, testing should be done:
• either on the same day as the vaccination with live-virus vaccine, or
• 4–6 weeks after the administration of the live-virus vaccine (such as for pertussis, measles, rubella, polio, yellow fever).
How does BCG vaccination affect TST results?
BCG vaccination may give false-positive result among children. When BCG is given at birth, as done in most parts of the world, it has a variable, limited impact on TST. History of BCG vaccination however has limited effect on interpretation of TST results later in life.