The 2019 Tuberculin Skin Test Shortage: What CDC Advised

In 2019, CDC warned of an expected 3- to 10-month nationwide shortage of Aplisol, a purified-protein derivative (PPD) tuberculin antigen made by Par Pharmaceuticals. Aplisol was one of only two PPD antigens licensed by the Food and Drug Administration for tuberculin skin tests (TSTs); the other was Tubersol, made by Sanofi Pasteur.

The manufacturer told CDC it expected supplies of the 5 mL vials (50 doses) to be interrupted from June 2019, and the 1 mL vials (10 doses) from November 2019 — or sooner, if demand rose. The time frame was the manufacturer’s own estimate and could change; the FDA’s biologics shortage list tracked the situation.

How TB infection is tested

Two kinds of immunological test detect infection with Mycobacterium tuberculosis:

• the tuberculin skin test; and
• the interferon-gamma release assay (IGRA), a blood test.

Both are used to diagnose latent TB infection and can help diagnose TB disease, but further evaluation is needed to tell the two apart and choose the right treatment. When chest X-rays and mycobacterial cultures are enough to confirm or rule out TB, a skin or blood test may not be needed; still, most U.S. TB cases are diagnosed from a combination of findings that includes one of these tests. When TB disease is strongly suspected, treatment should begin whatever the test results.

CDC’s three recommendations

To keep TB testing going through the shortage, CDC recommended:

1. Use IGRA blood tests instead of skin tests. Clinicians switching should know that IGRA results are interpreted by different criteria from skin tests.
2. Use Tubersol instead of Aplisol. In controlled studies the two products agree closely, giving similar results for most patients.
3. Prioritize the skin tests available, working with state and local health authorities — which may mean deferring testing for some people. CDC recommends testing only people at risk for TB. Those at high risk of infection include:
4. • recent contacts of people with TB disease;
   • people born in, or who travel often to, countries where TB disease is common;
   • people who live or have lived in large group settings, such as homeless shelters or correctional facilities;
   • people whose immune systems are weakened by illness or medication; and
   • children, especially those under 5, who are in any of these groups.

Cautions

Even though the products agree well overall, switching between skin test products, or between skin and blood tests, during repeated testing can produce results that seem to flip — negative to positive, or back — because of differences between the tests themselves rather than any real change in infection. Clinicians should judge results against each person’s likelihood of infection and risk of progressing to TB disease.

Where exposure to TB is unlikely, routine repeat testing could be deferred, in consultation with public health and occupational health authorities. CDC does not recommend annual TB testing of health care workers unless there is a known exposure or ongoing transmission.

Sources

Based on "Nationwide Shortage of Tuberculin Skin Test Antigens: CDC Recommendations for Patient Care and Public Health Practice," Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.