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Difference between Sterile and Pyrogen Free

Difference between Sterile and Pyrogen Free

Depyrognation and sterilization are different concepts. All sterile things are not pyrogen free. Pyrogens are endoroxins those require high temperature to denature.

Introduction

It is often believed that being sterile means being pyrogen-free but this is not correct. First of all, you need to understand what pyrogen is. Pyrogen is any substance that causes fever in animals and humans when it gets into their body. A very common symptom of fever is increasing in temperature so pyrogen is sometimes seen as any substance that causes an increase in temperature in humans and animals.

Endotoxins are very common pyrogens and they are contained in the outer membrane of the cell of some Gram-negative bacteria. When these bacteria are killed by the immune system of their hosts (usually animals and humans), antibiotics or sterilization, they release dangerous endotoxins during their cell lysis.


That being said, you should easily understand the difference between being sterile and being pyrogen-free as explained here. The process of sterilization is the killing, removal or deactivation of living microorganisms from equipment, solutions or pharmaceutical vials. So any solution that is free from living microorganisms is sterile.

However, it is possible that a solution was contaminated with some Gram-negative bacteria before sterilization and it is also possible that these bacteria released some endotoxins while being killed off during the process of sterilization. Now, this is what you must understand. Sterilization only kills off living microorganisms but it does not have an effect on their endotoxins. So, a solution can be sterile but still have dangerous endotoxins.

Being sterile means being free from microorganisms and being pyrogen-free means being free from fever-causing substances. The difference can be likened to insects and their eggs. Since some insecticides can only kill insects but not their eggs, it is possible for an apartment to be free from insects but not free from their eggs.

Detection of Pyrogens

The 2 common tests to detect the presence of pyrogens in a solution are the Rabbit Pyrogen Test and the Limulus Amebocyte Lysate (LAL). Using the former, some rabbits are injected with the solutions being tested and if the temperature of the rabbits increases, the presence of pyrogens is confirmed. This test has several drawbacks.

1.  It is tedious because the temperature of the rabbits will be taken at 30-minute intervals for about three hours.

2. Some substances like steroids have the ability to increase animal body temperature when injected into their bodies. This could lead to a false positive result.

3. It takes some endotoxins several days to react in the body and not just hours. This can lead to a false negative result.

4. The rabbit test only detects the presence of pyrogens. It cannot be used to determine the quantity.

The LAL test has almost phased out the rabbit test. It is done with the hemolymph of the horseshoe crab. When it comes into contact with pyrogens, the hemolymph begins to clot. There are different versions of the test for the quantitative and the qualitative determination of endotoxins in a solution. It is cheap and it takes care of the shortcomings of the rabbit pyrogen test.

Pyrogen Removal

Once the presence of pyrogen is confirmed in solution, pyrogen removal becomes the next step. The common pyrogen removal methods are Ion Exchange Chromatography, Ultra Filtration, and Distillation. Since the total removal of pyrogens can be very difficult, it may be a better idea to deactivate or destroy the lipopolysaccharide (LPS) molecules in the endotoxins.

Some common methods to destroy LPS molecules are Acid-Base Hydrolysis, Oxidation, Heating, and Sodium Hydroxide methods.

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