What is Nosology?

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  • Written By: Mary McMahon
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  • Last Modified Date: 07 October 2019
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Nosology is the scientific classification of diseases. Essentially, it is like taxonomy for the medical world, categorizing diseases with the use of a variety of criteria so that they can be more easily understood. Like taxonomy, nosology is intended to clearly define the topic under discussion, so that people do not have to explain what they are talking about. Just as biologists know that a Sequoia sempervirens is a redwood tree, nosologists and doctors know that “diabetes mellitus” is a specific disease which can be defined with a clear set of symptoms.

Just as with regular taxonomy, nosology has evolved considerably over the ages. One of the earliest attempts at classifying disease took place in the Arab world around the 10th century, and Linnaeus tried his hand at classifying diseases when he wasn't busy developing the system of biological taxonomy used today. One of the complications for nosology has been that diseases often present in very different ways, and unlike something like a plant, which presents all of its information at once, a disease may be coy about revealing its nature.


There are a number of ways to classify diseases. Some nosologists focus on the etiology or cause of disease, using this as a key characteristic when defining diseases. Others look at the pathogenesis, the physical development of a disease, and some focus on the symptoms of disease. Nosology also usually includes a discussion of which organ system or systems are involved in the disease, allowing people to break things up into categories like “renal disorders” or “mental illness.”

Study in this field involves laboratory work to prove into the hidden nature of disease, along with fieldwork observing and interacting with patients. Many doctors practice nosology on a daily basis, as they interview patients and put their symptoms together like the pieces of a puzzle to determine which condition the patient has. Along the way, the clues to the identity of the problem may also be keys to resolving it, which is why many diseases are classified by etiology. Viral infections, for example, can be treated with anti-viral drugs.

Some branches of medicine have published extensive volumes on nosology. In psychiatry, for example, the Diagnostic and Statistical Manual of Mental Disorders contains a complex listing of psychiatric disorders along with symptoms and criteria for diagnosis. The DSM, as it is known, is constantly being revised to reflect new information in the field of psychiatry.


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Post 6

@JessicaLynn - That's an interesting example. I'm glad people in the medical field aren't so stuck on their ways they don't reclassify things!

I think this discipline makes so much sense from a scientific standpoint also. Imagine a bunch of doctors trying to talk about a patients condition without similar points of reference! Also I'm sure comparing diseases to one another probably results in some breakthroughs on treatment.

Post 5

Nosology sounds like it's a fairly fluid discipline. Doctors and scientists are always making new discoveries, so it stands to reason that the classification of a disease might change.

I read somewhere that homosexuality used to be classified in the DSM as a mental illness. That classification has since been revised, and now there is no mention of homosexuality as a mental disorder in the DSM. I imaging this happens with other diseases and disorders fairly often.

Post 4

@orangey03 - I agree with you. I recently had some time to kill at a library, and I picked up a book of nosology. By the time I left there, I was afraid I had several of the disorders listed!

I spent a lot of time reading about renal disorders. I do actually have kidney problems, but I know my specific condition. However, I seem to have so many of the symptoms associated with other renal disorders that I wonder if I could have multiple conditions simultaneously.

Like you said, though, so many disorders share characteristics. I’m fairly certain that my doctor is able to distinguish between mine and others similar to it. I have to trust in his extensive knowledge of nosology.

Post 3

Though my mother is by no means a doctor, she practices nosology at home on herself, me, and my dad. She has several medical books that she purchased years ago, and sometimes, she reads them just for fun.

Anytime any one of us gets sick, she runs and grabs one of her books. The diseases are listed in alphabetical order, and much like an encyclopedia, the book gives a detailed description of each one that usually takes up two pages. It lists symptoms, risk factors, treatments, and outlooks for all of them.

The thing about nosology is that so many conditions have similar symptoms, so you really need a medical background to distinguish between them. If you were to sit and read that book, you would probably suspect that you have several of the conditions listed based on the symptoms alone.

Post 2

I have a great respect for medical school students who stick with their studies. With all of the diseases that exist in the world, they have to memorize and retain a massive amount of nosology information.

Though studying and reading books helps, the practice they get as interns is way more beneficial. There’s nothing as good as putting what you have learned to use in a real life situation to help you remember it.

I don’t see how they ever learn all there is to know about nosology. They probably just have to consult manuals and books of symptoms to supplement their knowledge.

Post 1

I’m sure that more often than not, having studied and practiced nosology for years leads to successful diagnoses. However, sometimes cases mystify even the most knowledgeable doctors.

When my mother started having episodes of very high blood pressure, she went to the hospital for a stay. The doctors tried everything that their use of nosology allowed, but they could not figure out what was causing it.

They even did a heart catheterization, which gave them a lot of information, but none of it was useful. She went through a period of several months of taking everything from antidepressants to a myriad of blood pressure pills, but nothing worked. One day, it simply went away, and the doctors remain mystified.

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