Pyrogens: Definition, Active Locations, Pyrogen Testing and Origin

They are agents or molecules that cause fever in the human body.

These agents are present in some bacteria, which are called:

  • Endotoxins : which are found right in the cells of bacteria that are gram-negative.
  • Exotoxins: they are characterized by being the molecules that certain bacteria produce internally and expel them or secrete them to the outside.

Pyrogens endotoxins and exotoxins are released by bacteria but endotoxins are externalized in the form of lipopolysaccharides.

Active sites in pyrogens

These molecules, because they originate or belong to certain bacteria, can be obtained in areas or surfaces in constant contact or contaminated by them.

It is important to know that poor cleaning or improper use of materials that maintain constant use can be factors or places prone to acquiring the bacteria that develop pyrogens.

In the case of composite objects or made with glass, they tend to be the ideal place to store endotoxins and they do not usually come off only with soap and water, so washing them only with this product does not guarantee their extermination.

An alternative can be the autoclave although this molecule in some cases can be resistant to heat and remain on the surface of glass materials.

To be completely eliminated, they must be exposed to high temperatures, approximately 250 degrees Celsius and for at least a period of at least 30 minutes.

Some specialists suggest that the depyrogenation process is often more difficult to carry out than sterilization because they are not completely destroyed despite efficient boiling or autoclaving.

On the other hand, there is a hydrothermal method or process created for the complete elimination of endotoxins, which consists of a temperature of 130ºC for an estimated time of at least 60 minutes or if possible at 140ºC for 30 minutes to generate a high vapor and saturated with a flow system.

This process also serves to purify water under the same temperature and time indicated, without having to resort to ultrafiltration or the anion exchanger.

This method is functional for medical instruments or devices that have the ability to withstand high temperature levels.

Pyrogen test

To determine if a component, compound, product or any physical object generates fever, a pyrogen test must be performed through an injection in a living model such as laboratory rabbits or in vitro models.

That is why there are quality tests focused on parenteral products that are categorized as follows:

  • Qualitative fever test.
  • Bacterial endotoxin test.
  • Limulus amebocyte lysate test.

Among the descriptive of each of the tests it can be mentioned that:

The qualitative tests of fever in rabbits are carried out by injecting substances with pyrogens and then measuring the body temperature for established periods and then measuring the behavior of the animal and thus distinguishing the reactions it has.

On the other hand, bacterial endotoxin tests are carried out through the application of a gel that changes color when lipids A are present.

Finally, the limulus amoebocyte lysate tests consist of the rupture of these cells when pyrogens are present.


Pyrogens can be derived from biological and chemical nature which in turn are divided into endogenous and exogenous.

Endogenous pyrogens include:

  • Thyroid hormones.
  • Cytokines
  • Adrenalin.

On the other hand, pyrogens from exogenous chemical substances have been modified as active principles, being as follows:

  • Amphotericin B.
  • Vancomycin.
  • Atropine.
  • Methylene blue.
  • Silica particles.
  • Adjuvants

It is also found in extracts of microorganisms, such as:

  • Yeasts
  • Gram (-) bacteria.
  • Gram (+) bacteria.
  • Virus.

In general, pyrogens act under small doses, just like for example a microgram, but not all humans or animals have the same level of tolerance or sensitivity for the appearance of fever peaks or not.

For example, a rabbit can last less time than a human, in this case a baby, if it is found ingesting a problem or contaminated nursing solution before fever symptoms begin.