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Why are TB skin tests (TSTs) no longer recommended for residents age 65 years and over?

The Canadian TB Standards advises that residents of Long-Term Care Institutions undergo baseline posterior-anterior and lateral chest x-rays. If the resident has documented results of a prior TST, these should be transcribed into their record.

Routine TSTs upon admission are no longer recommended for clients 65 years of age and older. As people reach old age, the TST may become increasingly unreliable and difficult to interpret. In this population, the TST may not become positive even after a significant TB exposure. As well, unless there is a documented 2-step TST on record, testing after exposure may result in the "boosting effect" being misinterpreted as a true conversion.

Most critically, even for elderly individuals who do convert to a positive TST following a TB exposure, prophylaxis is often not possible, due to their decreased ability to tolerate the hepatotoxicity of Isoniazid (INH).

Why the CXR?

For an elderly person exposed to infectious TB, the most important follow-up is ruling out active TB via careful evaluation of symptoms, CXR, and where indicated, three sputum samples taken at least one hour apart.

Clients under 65 years of age who have a positive TST are more likely to be candidates for TB prophylaxis. In addition to the symptom review for active pulmonary TB disease and chest x-rays, a 2-step TST is required for those less than 65 years of age, unless a previous TST is known to be positive.

What is a 2-step skin test for TB?

This consists of 2 TSTs usually performed within one to four weeks of each other. A 2-step TST, rather than a single TST, is generally only indicated at the initial assessment of someone who will have repeat TSTs at regular intervals. For example, a 2-step TST is recommended for health care workers at the start of employment, to help reduce the chance of a newly-positive TST in the future being misinterpreted as conversion when the TST is repeated. The 2-step TST needs to be performed only once if properly done and documented.

Over decades, the immune response (i.e., a positive TST) related to a remote TB or BCG exposure can go "dormant". A single TST may elicit a negative result; however, re-stimulates the immune recognition so that a second TST at a later time will elicit a much greater response. The reason for a 2-step TST is to detect this "booster effect" at the beginning of TST monitoring (using a 2-step TST), as otherwise it could be confused later with a true TST conversion.

What is recommended for residents being transferred from another facility?

Prior to transfer, the resident should be carefully reassessed for signs and symptoms of active TB, including failure to thrive. This should also include a review of the chest x-ray previously done upon admission to the facility or any more recent radiology. You may wish to use the active TB screening checklist for clinicians to guide the symptom and chest x-ray review. If there are any indications of possible active TB, a repeat chest x-ray, sputum testing, and any other necessary investigations should be done to rule out active pulmonary TB disease before the resident is transferred.

What if a new employee or volunteer had a 2-step TST done, but the first and second steps were done more than four weeks apart?

According to the Canadian TB Standards, the first and second step of a 2-step TST should be done one to four weeks apart. Less than 1 week does not allow enough time to elicit the phenomenon, more than 4 weeks allows the possibility of a true TST conversion to occur if the person had an exposure to infectious TB in the interim. However, the second test can be accepted up to one year later, as long as no exposure to active TB occurred within that year.

What if an employee or volunteer has never had a 2-step TST done, but had a 1-step TST done within this past year?

If the previous TST result was positive (≥ 10 mm), no further skin testing should be done. The person should proceed with a physical exam and a chest x-ray to rule out active TB disease. If the previous TST was negative, another 1-step can now be done and accepted as the second step of a 2-step TST as long as it is within a one year period from the time of the first step. It is important to assess the likelihood that the employee was exposed to active TB since the last TST. If an exposure is suspected, the second TST should be done at least eight weeks after the TB exposure, in order to provide a reliable baseline for future assessments.

A resident had a CXR done two months ago but now has symptoms that could be due to active pulmonary TB. Should a repeat CXR be done prior to admission to our facility?

Yes. If the resident has symptoms suggestive of active TB (i.e., cough lasting longer than two weeks, unexplained weight loss, fever, chills, night sweats, fatigue), a current chest x-ray should be done to rule out active pulmonary TB disease. In addition, three sputum samples should be collected at least one hour apart and submitted to the Public Health Laboratory for testing (Acid Fast Bacilli and Culture). Before admitting the resident, all sputum results should be negative and active pulmonary TB disease ruled out. If the resident has already been admitted to the facility, refer to the Recommendations for TB Screening in Long-Term Care and Retirement Homes, specifically the section regarding "Management of Residents with Suspected Active TB Disease".

If a staff person has received the BCG vaccine in the past do they still need a TST?

Yes. TB skin testing is required for staff who received BCG vaccines in the past. People vaccinated with BCG may have a positive TB skin test if the BCG was given after infancy. However, it is also possible for the positive TST was caused by TB infection, especially if the person was born in or travelled to a country with high rates of TB. It is worth remembering that countries with much higher rates of TB than Canada also use BCG routinely. Thus, adults with a positive skin test who had a BCG vaccination should still be carefully evaluated for possible latent TB infection (LTBI), and offered treatment for LTBI if appropriate.

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Last modified: 
Thursday, October 3, 2019 - 3:30pm