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A Quick Guide to Disinfection, Sterilization, and Antisepsis

  • by Allison Chae
  • Jan 02, 2018
  • Biological Processes, PCAT Blog

Microbiology is one of the main components of the Biological Processes section of the PCAT, for good reason—prescribing medications against microbial diseases is a major part of the everyday practice of pharmacy, and you have to know your enemy (pathogenic microbes) in order to be able to defeat them! Part of succeeding in microbiology on the PCAT is to use terms very precisely, according to their technical definitions. In this blog post, we’ll review a set of terms that are often used interchangeably in everyday life, but in the scientific and medical world, have very distinct meanings: disinfection, sterilization, and antisepsis. These terms are especially important now (in January 2018), as a nasty flu season is kicking into gear throughout the United States!


The first distinction we will want to make is between disinfection and sterilization. In a nutshell, this corresponds to a decision about how thoroughly we plan on removing the microbes. Sterilization refers to completely removing all living microorganisms and viruses from the surface of an inanimate object. In short, it’s the “kill ‘em all” option. One of the most common methods of sterilization is autoclaving, which you may have encountered in lab research. Autoclaving involves subjecting a surface to saturated steam under high pressure at a temperature well over the boiling point of water. When correctly done, autoclaving will inactivate all bacteria, spores, fungi, and viruses, although prions (infectious misshaped proteins) may remain present. Other sterilization techniques include UV radiation, heat, extremely noxious chemicals, and filtration.


Disinfection, instead, refers to the removal of most microorganisms, especially pathogenic microorganisms. In short, it’s the “good enough” approach. In particular, spores may remain present after disinfection. Many disinfecting agents interfere with the cell walls of bacteria, although there are many different disinfecting agents with a wide range of mechanisms (which go beyond the scope of the PCAT), so some disinfecting agents will be more effective against specific types of microorganisms than others. Bleach is one of the most common disinfectants you may have used, and 70% ethanol is another common option. Interestingly, 70% ethanol is more effective than 90+% ethanol because the presence of water in the more dilute ethanol mixture promotes entrance of the ethanol into the cell, meaning that its effects will be more widely distributed.

Since highly effective disinfecting agents may be very noxious or pose health risks, a tradeoff must often be made between effectiveness and usability. High-level disinfectants, such as hydrogen peroxide, formaldehyde, and peracetic acid, will remove or inactivate the vast majority of microorganisms, whereas low-level disinfectants, like the phenolic compounds found in Lysol®, tend to be effective against some classes of bacteria and viruses, but not all. In medical contexts, the level of required disinfection depends on how an instrument is used. High-level disinfection is a must for anything that will come into contact with mucous membranes or broken skin, whereas intermediate- or low-level disinfection may be enough for instruments that will only come into contact with intact skin.


Antisepsis is like disinfection, but specifically applied to living tissue and skin. The crucial point with antiseptics is that when you’re applying a compound to a living being, it’s especially important to strike a balance between the desired effect of killing off microorganisms and the need to avoid unpleasant/unwanted effects caused by the application of a noxious, irritating chemical. The alcohol swabs that are used to clean a patient’s skin before administering an injection are a commonly encountered example. Iodine is also used as an antiseptic to disinfect wounds.

There’s a lot more to learn about microbiology than these terms, but hopefully this discussion has given a useful example of how to precisely and carefully approach scientific terminology, as well as some useful suggestions for this flu season! If you’d like to put this knowledge to the test with some realistic PCAT practice, try our PCAT practice tests, or our free PCAT practice bundle. Need more guidance or looking for more in-depth PCAT prep? Check out our online PCAT course or our private, one-on-one PCAT tutoring.

We wish you the best of luck in your journey towards pharmacy school!


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