NURSES MAY BE required, depending on facility policy and state requirements, to undergo periodic screening for tuberculosis (TB) to prevent the potential transmission of the disease to patients and colleagues. This article will review how TB is transmitted, the screening tests available, what the test results mean, and treatment recommendations for nurses with TB.
What is TB?
An infection with the bacterium Mycobacterium tuberculosis, TB is spread through respiratory pathways. Airborne particles, called droplet nuclei, are generated by those with pulmonary TB while coughing, sneezing, speaking, or even singing. The bacteria must then be inhaled to produce an infection.1 Skin-to-skin contact, fomites, and saliva don't transmit the disease.2 (See Risk factors for TB exposure and infection for more information.)
Once a person is infected, the TB bacteria are in the body and an immune response occurs. If this immune response is adequate to prevent bacteria from replicating, the bacterial infection lies dormant in the infected individual's system (latent TB). Individuals with latent TB have M. tuberculosis in their bodies but aren't symptomatic and can't infect anyone else with the disease. If the immune system can't contain the bacteria, active disease occurs.2 Active TB is most often found in the lungs, but can also be found in other parts of the body, such as the brain, spine, or kidneys.3
Active disease develops in 5% to 10% of those infected with the bacteria, usually 6 to 18 months after the initial infection.4 Several risk factors make some individuals more susceptible to developing active TB disease (see Risk factors for progression of TB infection to active disease). If not treated properly, TB can be fatal, and an untreated person can spread the disease to others.2
Screening for TB
The most common screening test used is the tuberculin skin test (TST), also called the purified protein derivative (PPD) skin test or Mantoux TST. The protein contained in the test solution is extracted from the bacteria and doesn't contain any live or active components.2
The TST is performed using a tuberculin syringe. With the needle bevel up, the practitioner injects 0.1 mL of solution intradermally into the inner surface of the forearm. This immediately creates a small wheal or “bubble” 6 to 10 mm wide, which is later absorbed. If no wheal appears, the injection was given too deeply and test results won't be accurate.
The practitioner shouldn't apply an adhesive bandage after administering the injection because it could cause irritation and interfere with interpretation of the results. A 2 × 2 gauze pad can be used to gently dab any blood that appears.
When an individual is infected with M. tuberculosis, the immune system recognizes the TST solution and deploys T cells that release lymphokines. The lymphokines cause vasodilation and a delayed cutaneous hypersensitivity reaction, resulting in an area of induration (hardening of tissue).5 After 48 to 72 hours, a trained reader will palpate the area and measure the indurated area (not erythema) perpendicular to the long axis of the forearm in millimeters.6
It's important to note that erythema, which is a common reaction to the solution, doesn't indicate a positive result. It isn't possible to visually read a TST accurately; the area must be palpated for an accurate reading. An incubation period of 2 to 8 weeks should be anticipated after an exposure before the TST will be positive.1
Some individuals with latent TB, particularly older adults and those who are immunocompromised, have a limited immune response and may test negative on an initial TST. One way to address this concern is using a two-step testing procedure. If the first TST is negative, a second TST is done 1 to 3 weeks later. If the person has latent TB, the initial TST may stimulate the immune system, causing a positive (or “boosted”) result on the second TST. If the second test is negative, the individual isn't likely to be infected with TB. If the second TST is positive, the person was likely infected a long time ago. Following a two-step testing procedure can reduce the chance of missing an actual TB infection (false negative) and also reduces the risk that the accurate positive result seen after a false negative isn't misinterpreted as recent TB infection.7
Vaccines and TST
Some individuals may react to the TST even if they're not infected with TB. Some cross-reactivity exists between the live-virus bacillus Calmette-Guerin (BCG) vaccine and the TST. Although not given routinely in the United States, the BCG vaccine is administered in some parts of the world to prevent disseminated TB in children.8 This vaccination can contribute to a false-positive TST.
A blood test for TB called an interferon-gamma release assay (IGRA), which isn't affected by the BCG vaccine, may be appropriate for those who've recently had a BCG vaccine.5 The IGRA is used instead of, but not in addition to, the TST for these patients. IGRAs are expensive and aren't available everywhere. They may be used for individuals with a higher likelihood of inaccurate screening results (see False-positive and false-negative results on TST for more information).2
Most adults who have a positive TST and a childhood history of the BCG vaccine have a positive TST not because of the vaccine but because they have latent TB. The CDC recommends interpreting TST results the same way regardless of whether a patient has received the BCG vaccine. If available, the IGRA can help avoid this issue.5
Other live-virus vaccines may interfere with TST reactions. Anyone who's scheduled to receive a TST can have it done either on the same day as most live-virus vaccines or 4 to 6 weeks after the administration of the live-virus vaccine.7
Determining TB infection
The clinician should obtain a thorough medical history including signs or symptoms of TB. A cough of 3 weeks or longer, anorexia, hemoptysis, chest pain, weight loss, night sweats, and fever are all signs of possible active TB disease.1 Patients should also be asked if they've ever been exposed to TB and/or had a positive TST in the past. Responses would then help direct an appropriate physical exam.
Because a TST is only a screening mechanism, additional follow-up is necessary to diagnose TB. A positive TST is followed by a posterior-anterior chest X-ray to detect chest abnormalities that may suggest TB and, if appropriate, sputum cultures for mycobacteria.9 Those with positive chest X-rays and sputum cultures may need hospitalization and isolation until treatment renders them noninfectious.
Nurses who have a positive TST but are asymptomatic should be evaluated further and can continue to work while tests are being done to rule out TB.10 Nurses with a positive TST but no known risk factors and negative chest X-rays have latent TB. They're considered not infectious and can continue to work.
Once infected with latent TB, the immune system must remain strong to prevent active disease. Changes in immune status related to medication (such as corticosteroids and chemotherapy), disease processes, pregnancy, and aging raise the risk of active disease.11
Conversion from latent disease to active disease is one of the major contributing factors to the presence of TB in low endemic countries such as the United States.2 It's estimated that more than 10 million people in the United States are infected with latent TB but only 300,000 are treated each year.12 Studies have found that in general populations, approximately 20% of those with latent TB decline treatment and less than half of those who begin treatment complete the full course of therapy, increasing their chance of later developing active TB with a drug-resistant strain of the bacteria.12 Healthcare workers are more likely to decline treatment completely and healthcare workers who do accept treatment are more likely to fail to complete treatment regimens. One possible explanation for this lack of treatment compliance among healthcare workers is feeling pressured to start treatment initially but not being concerned enough about latent TB status to comply with the full course of therapy.1
Individuals with positive TSTs, regardless of disease status, shouldn't have additional TSTs. Subsequent TSTs would likely produce a progressively larger immune response and yield no additional information.5
Several treatment options are available for nurses with latent TB infection. Isoniazid can be taken daily for 9 months (or twice weekly with directly observed therapy).1 A higher dose may be taken for 6 months. A small number (<1%) of those who take isoniazid can develop hepatotoxicity. Liver function should be monitored closely and those taking the drug should avoid alcohol.11
No detectable differences are seen in compliance or completion rates between the 6-month and 9-month treatment options, so patient preference and tolerance are the greatest deciding factors when selecting a treatment plan.12
For those who may have been exposed to isoniazid-resistant TB or who can't tolerate isoniazid, the recommended course of treatment is rifampin daily for 4 months.4 In 2011, the CDC added an additional option that involves taking isoniazid and rifapentine once a week in directly observed therapy for 12 weeks.4 The shortest course of treatment currently available, this may be a preferable option for healthy adults who aren't pregnant.13 Increased completion rates have been seen with both the 4-month rifampin treatment regimen as well as the 12-week combination treatment.12 Completing the full course of therapy prevents later activation of TB disease.
Besides considering treatment for latent TB, nurses should share their latent TB status with their primary care providers. This helps to ensure appropriate monitoring should conversion to active TB disease status occur in the future.11
Periodic TB testing may be part of a facility policy or state requirement for nurses in your area. Keeping up to date on the latest guidelines for testing and treatment options available can help prevent transmission of TB to coworkers, family, and vulnerable patients.
Risk factors for TB exposure and infection2
- close contact with people with infectious TB disease
- immigration from areas of the world with high rates of TB
- working or residing with people who are at high risk for TB
- working or residing in crowded settings, such as healthcare facilities, homeless shelters, correctional facilities, and long-term-care facilities.
Risk factors for progression of TB infection to active disease2
Risk factors that can diminish immune response and increase the risk progressing to active TB disease after untreated latent infection include:
- infants and young children
- older adults
- HIV infection
- substance abuse
- lung disease
- diabetes mellitus
- severe kidney disease
- low body weight
- organ transplants
- prolonged corticosteroid therapy
- specialized treatment for rheumatoid arthritis or Crohn disease.
False-positive and false-negative results on TST7
Some individuals may react to the TST even though they're not infected with M. tuberculosis. Causes of false-positive reactions include:
- infection with nontuberculosis mycobacteria
- previous BCG vaccination
- incorrect method of TST administration
- incorrect interpretation of reaction
- wrong bottle of antigen used.
Some individuals may not react to the TST even though they are infected with M. tuberculosis. Reasons for false-negative reactions include:
- cutaneous anergy (the inability to react to skin tests because of a weakened immune system)
- recent TB infection (incubation period before the test is positive is 8 to 10 weeks after exposure)
- long-standing TB infection in older patients
- infants often show false-negative results
- recent live-virus vaccination (measles, smallpox)
- overwhelming TB disease
- viral illnesses (measles, chickenpox)
- incorrect method of TST administration
- incorrect interpretation of reaction.