The cases, at least four of which were fatal, were reported to the Food and Drug Administration between 1998 and May of this year. Researchers who reviewed the cases say that the immune-system protein infliximab suppresses -- called tumor necrosis factor alpha (TNF-alpha) -- appears to be key in controlling TB infection.

Around 150,000 people worldwide have received infliximab infusions since the drug's approval in 1998. In August, Malvern, Pennsylvania-based Centocor, which markets infliximab as Remicade, announced it was changing the drug's prescribing information to address the TB concerns. The revised labeling states that patients should be tested and treated for inactive, or latent, TB prior to infliximab therapy.

Latent TB refers to a chronic, but symptomless and noncontagious, infection. Such TB infections are prevalent throughout the world because in most people, the immune system is able to suppress TB bacteria.

Up to 15 million Americans are estimated to have latent TB infections.

When the immune system is suppressed -- as it is in patients on infliximab -- latent TB can become active. Active TB usually attacks the lungs, causing symptoms such as a severe cough, chest pain and weight loss. It is spread through the air from person to person.

The New England Journal of Medicine October 11, 2001;345:1098-1104