Pulmonary Tuberculosis

Chest radiography is the primary screening tool in identification of undiagnosed active cases of infectious pulmonary TB in order to treat & render them noninfectious.

The tuberculin skin test is used to detect the presence of lung TB infection. It is recommended for asymptomatic high–risk persons and students (universities demands).

High-risk groups for screening include:

• Close contacts with known or suspected active TB case.
• Persons with HIV infection
• Persons with chest radiographic findings suggestive of past TB who have not received adequate therapy.
• occupational exposure to TB especially health care workers

Tuberculin Skin Testing (TST)

• The TST is performed by injecting 5 tuberculin units of purified protein derivative (PPD) intradermally in the anterior aspect of the forearm (Mantoux test). Only 5 TU PPD should be used since there is no standardized doses of PPD.
• In order to minimize inter-observer variation, the technique of injecting PPD and interpreting reactions must be standardized. Health care facilities may choose to train a cadre of professionals to read TSTs so that the test result may be read in the immediate work environment of the HC.
• TST must be read 48 to 72 hours after performing the TST by trained staff members. The classification of a "significant" reaction is based on the size of induration present (measurement, in millimeters, of the induration present at right angles to the long axis of the forearm) and the person's medical history.
• A converter is defined as someone who has a positive TST within 2 years of having a documented negative TST.

Administration of TST

• Seat your client comfortably, resting his/her exposed arm on a firm, well lighted surface.
• Clean the injection site with an alcohol pad and allow it to dry completely if necessary.
• Use a single-dose, disposable tuberculin syringe and a 1/2 inch, 26 or 27 gauge needle with a short bevel.
• Confirm the dosage required to administer 5 TU of PPD tuberculin (usually 0.1 mL).
• Draw up a little more than 0.1 mL of PPD solution in the TB syringe. Hold the syringe upright and tap it lightly to remove air, then expel one drop. Check that a full 0.1 mL remains in the syringe.
• The usual injection site is on the anterior surface of the forearm, about four inches below the elbow.
• Avoid placing a TST on an area of skin that has anything which would interfere with reading the TST such as area red, swollen or visible veins.
• Stretch the skin tightly with your non-dominant hand. Insert the needle, bevel up, at a degree of 15 so that the tip of the needle is visible just below the surface of the skin.
• Inject the solution slowly in the anterior surface of the forearm, about four inches below the elbow.   
• As you slowly inject the contents of the syringe, you will feel a slight resistance
• A firm white wheal about 6-10 mm in diameter should appear at the injection site immediately.
• If the injectable (PPD) leaks out onto the skin and no wheal appears, it means you did not place the needle deeply enough.
• If the wheal is shallow and diffuse, you have given the injection too deeply.
• In either case, administer a second injection at least two inches from the first site and circle the second injection site.
• You may see a drop of blood when you withdraw the needle. This is normal. Offer the participant 2x2 gauze to remove the blood. Advise the participant not to press the gauze over the injection site but to just dab gently to remove the blood. This will avoid squeezing out the tuberculin disrupting the test.
• Do not recap needle. Place the syringe in a puncture-resistant container.
• Circle the injection site and write the date and time of injection.

Post Test Instructions

• Do not rub or scratch the site of the tuberculin test.
• Keep area clean.
• Return in 48 to 72 hours for reading.

NB.  Tuberculin Skin Testing (TST) may be given in the following situations:

• Prior or at the same time to administration of a live virus vaccine, i.e., in the same day or 4-6 weeks after the administration of the live virus vaccine. 
• 1 month of the following illnesses: measles, mumps, chicken pox, infectious mononucleosis; typhoid, brucellosis, influenza or whooping cough (pertussis).
• During a severe or febrile illness [A small percentage (5% to 10%) of people with active TB will not have a positive TST].
• To an individual with Hodgkin's disease, sarcoidosis or who is infected with HIV.
• To an individual who is taking corticosteroids or immunosuppressive drugs.
• To an individual with severe metabolic disturbances, such as chronic renal failure, severe protein deficiency or burns; and
• To an individual at the extremes of age
• newborns < 6 weeks of age cannot respond to a TST
• Children aged 6 weeks to 6 months and the elderly may not respond to a TST.
• Individuals with the above conditions are not necessarily precluded from having a TST. If a positive response occurs, it must be read and interpreted as such.
• Medical personnel must be alert to the possibility of a false-negative response if the conditions listed above are present and ensure that a thorough evaluation of the risk of the individual being infected with M. tuberculosis is performed.

Reading Tuberculin Skin Testing (TST)

• The TST is read 48 to 72 hours after being injected. However, if a person presents more than 72 hours after the TST was injected and the test result is greater than 10 mm, this test should be considered positive. If a person presents after 72 hours and the result is less than 10 mm, the test is invalid and must be repeated.
• The TST must be read by a trained physician or nurse in good light, with the person's forearm slightly flexed at the elbow.
• The presence or absence of induration should be documented. Erythema or redness is not measured. The development of erythema does not indicate infection.
• Induration is determined by inspecting the arm from a side view against the light as well as by direct light and by palpating the arm with a gentle stroke of the finger.
• If induration is present, the diameter is measured across the width of the forearm, (e.g., measure the width at right angles to the long axis of the forearm). Sometimes the precise edge of induration is difficult to palpate. Use a pen to mark the beginning and end points of induration. Use a flexible ruler to measure the size of induration between the pen points.
• Record the size of induration in millimeters. If the measurement falls between demarcations on the ruler, record the smaller of the two numbers. If the participant has no induration, record the result as 0 mm.

Repeating TST

In general, there is no risk associated with repeated tuberculin skin test placements. If a person does not return within 48-72 hours for a tuberculin skin test reading, a second test can be placed as soon as possible. There is no contraindication to repeating the TST, unless a previous TST was associated with a severe reaction.

Interpreting Tuberculin Skin Testing (TST)

• After recording the size of the induration, it is necessary to interpret if the test is positive or negative to ensure that the appropriate follow-up actions are taken.
• A tuberculin reaction of 0 to 4 mm is classified as negative, taking in consideration of some conditions that cause a false-negative TST result (see below).
• A tuberculin reaction of 5 to 9 mm or more is classified as positive when any of the following risk factors are present otherwise is considered as negative:
• The individual was a recent close contact with an individual with infectious TB.
• The individual has a chest x-ray with apical fibrotic lesions suggestive of old healed TB.
• The individual is known or suspected of having HIV infection or is immunosuppressed because of other conditions.
• A tuberculin reaction of 10 mm or more is classified as positive for all individuals. (i.e., the test result for a participant who has a history of a BCG will be interpreted as positive when a reaction of 10 mm or greater is present).

False Negative TST

• A person with a negative TST may be infected with M. tuberculosis but, because of a number of factors, the immune system can not respond to the TST. This person has a false-negative TST. This reinforces the need for obtaining an adequate medical history prior to testing. If individuals have a temporary condition likely to yield a false-negative test at the time of screening, their TST should be postponed.

Reasons for a false-negative

• Within 1 month of administering a live virus vaccine (polio, measles, rubella, mumps or yellow fever); if the live vaccine has already been given, ideally WAIT 30 days before administering the TST.
• Use of incorrect injection technique.
• TST is read by an inexperienced person, and
• Improper storage, exposure to light or heat of the tuberculin solution.
• (Vials of PPD should be stored at 2° C to 8° C under conditions of minimal air contact. A small decrease in PPD potency has occurred within 24 hours of contact with air, thus open vials should be used within one day)

A significant TST reaction almost always represents previous M. tuberculosis infection. Other causes of reactivity include cross-reactivity to non-tuberculous mycobacterial infections or reception of BCG vaccine (live attenuated mycobacterial strain derived from M. bovis). The prevalence of atypical mycobacterial infection varies geographically and thus varies in its importance.

Created by: Dr Farouq Al-Zurba