pyrogen

A pyrogen is any substance that, when it enters the bloodstream, causes a fever. The name comes from the Greek for fire-producing. In a parenteral product a pyrogen is an invisible contaminant: it does not make a solution look cloudy or spoiled, yet injecting it can trigger chills, a sharp rise in temperature, and in severe cases dangerous shock.

Most pyrogens of concern in manufacturing are biological. The classic and most important are endotoxins — fragments of the outer membrane of dead Gram-negative bacteria — but other microbial components, and even some chemical and material-derived substances, can be pyrogenic too. The body reacts to them as a danger signal: immune cells release messengers that reset the brain's temperature control upward, producing fever even though no live infection is present.

Because pyrogens can survive after the microbes that produced them are killed, sterilization alone is not enough. A product can be perfectly sterile yet still pyrogenic if it was contaminated earlier in the process. Manufacturers therefore prevent pyrogen entry at the source — using pyrogen-free water for injection, depyrogenated glass, and clean processing — and then test the finished product to confirm pyrogens are within strict limits.

A useful clarification: all endotoxins are pyrogens, but not all pyrogens are endotoxins. Testing has shifted over the years from the historical rabbit fever test toward in-vitro methods, but the underlying requirement is unchanged — a parenteral must not carry enough pyrogen to harm the patient.