What Is ICD-10? | Coding & Billing

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What Is This About?

ICD is a coding system for every single diagnosis currently in existence. Every single one of them. There have been 10 iterations of this huge list. And we are currently in the 10th one, therefore the name ICD-10. So, every time you diagnose a patient, there is a code attached to that diagnosis. Hypertension, diabetes, abdominal pain, endometriosis, uterine fibroids. You name it.

Why Should Medical Professionals Care?

The name ICD comes from International Classification of Diseases. This diagnosis system is paramount in health systems worldwide, not only in the United States, because it helps you better convey the exact diagnosis or presumptive diagnosis of your patient. That is from the clinical standpoint. From the financial and billing standpoint, it helps ensure the right diagnosis gets the right billing. It would be extremely difficult to run a billing system without proper classification of a diagnosis. Think about a library. Looking for a book in a sea of other books without a classification system would be a nightmare. Same story here.

But the origins of the ICD classification do not stem from the need for a billing system. In fact, its origins are a little gloomier. Its history goes back more than a century, when statisticians were trying to answer a much more basic question: Why are people dying?

The International Classification of Diseases Started With Death: A Little History of the ICD System

The original motivation to create this system came from governments and physicians who wanted a uniform way to answer a simple public-health question: “What are people dying from?” The roots go back centuries to mortality records such as London’s Bills of Mortality, which tracked deaths from things like plague. By the late 1800s, countries were collecting mortality statistics, but everyone classified causes of death differently. One country might classify a death one way and another country in another way, which made comparisons difficult.

Florence Nightingale, the very famous nurse, famously used mortality statistics during the Crimean War in the mid-1800s to show that many soldiers were dying from disease and poor sanitary conditions, not simply from battlefield wounds. Her work became an important part of the broader 19th-century movement to systematically record and analyze causes of illness and death. The World Health Organization specifically mentions Mrs. Nightingale's Crimean War experience as part of the historical development leading to our modern disease classification.

Then, a French statistician, Jacques Bertillon, developed the Bertillon Classification of Causes of Death, which became the direct ancestor of the ICD. His system was formally adopted by the International Statistical Institute in 1893. The idea was simple but powerful: If countries classified causes of death using the same system, researchers could compare mortality across cities, countries, and populations.

In 1900, the French government organized an international conference in Paris to revise Bertillon's classification. The result was the International List of Causes of Death, which contained about 179 categories. Then, international revisions of this classification followed approximately every decade. But medicine was changing. Eventually, knowing only why people died was no longer enough. Doctors, hospitals, researchers, and governments also wanted to classify the diseases of people who were still alive.

The Second Part of This Story: From Mortality to Medicine

A major transformation occurred in 1948. The World Health Organization had just been established, and it assumed responsibility for the classification. With the Sixth Revision of the classification (ICD-6), the system expanded beyond causes of death to include causes of illness, or morbidity. The system then became a morbidity and mortality classification.The classification kept growing as medicine became more sophisticated. Eventually came ICD-7, ICD-8, ICD-9, and our current ICD-10. The World Health Organization adopted ICD-10 in 1990, and it came into international use beginning in the 1990s. So, about thirty-something years ago.

Now, in the United States, there is an important distinction. ICD-10 is the WHO classification used for international disease and mortality statistics. ICD-10-CM means International Classification of Diseases, Tenth Revision, Clinical Modification. It is the expanded American version used to code diagnoses in clinical care. The United States implemented ICD-10-CM on October 1, 2015. That is the system most American clinicians like you and me, interact with every day.

Explaining the ICD System. Think of ICD as a Tree

The easiest way to understand ICD-10 is to picture a tree. At the bottom is a large trunk. As you move upward, the trunk divides into branches. Those branches divide into smaller branches. And those divide again. Each step gives you more information. Let's use an example from my specialty. Take uterine fibroids. You can begin with the broad category:

D25: Leiomyoma of uterus of uterine fibroid.

Then move farther along the branches:

D25.0: Submucosal fibroid
D25.1: Intramural fibroid
D25.2: Subserosal fibroid
D25.9: Fibroid, unspecified

You are describing the same general disease, but each branch tells us something more specific about it. You start broad, and then you branch toward specificity. That is the logic behind ICD.

Why Are There So Many Codes?

Because there are a growing number of diseases, and in medicine, specificity matters. ICD-10-CM contains nearly five times as many diagnosis codes as ICD-9-CM. The newer system added greater detail, including laterality, expanded injury descriptions, additional characters, and more precise classifications. And it is also worth noticing that as new diseases develop, they are constantly added to this long list. The list is updated at least annually now

Let's use an orthopedics example. Consider an injury. It matters if the injury is located on the right or left side. What was the bone or location? Was it an open or closed fracture? Is this the initial encounter, or a subsequent treatment encounter? Is it healing as expected? Is there a delay in healing or a complication?

Those details may sound excessive when looking at a codebook. But collectively, they create better clinical and administrative data. Specificity allows us to ask much better questions for a condition, like: How many patients have a particular disease? Or how frequently are they hospitalized? Which treatments are being used? Remember the COVID-19 pandemic. Without this classification, it would have been very difficult for countries to properly communicate information.

Eventually, insurance companies also started using the system to answer questions such as: What conditions are becoming more common? How much does caring for a particular population cost? And, increasingly, how should health care services be reimbursed?

So, in summary, a system originally created to count causes of death became part of the infrastructure for epidemiology, research, public health, quality measurement, health policy, reimbursement, and medical billing.

ICD Codes and CPT Codes Are Not the Same Thing: A Common Misperception

This distinction is important because it is a common mistake among clinicians. ICD-10-CM tells us why the patient is receiving care. CPT tells us what you did for that diagnosis.

As a GYN surgeon, if I evaluate a patient with uterine fibroids, the fibroid diagnosis may generate an ICD-10-CM code. The office visit, ultrasound, biopsy, or surgery performed to evaluate or treat that condition would generate separate CPT codes. That is a completely different system. ICD describes the diagnosis. CPT describes the service.

And payers frequently use the relationship between the two to determine if a service was medically necessary and if it should be reimbursed.

The Bottom Line

More than a century ago, statisticians wanted a consistent way to answer a basic question: What are people dying from? Those efforts evolved into something far larger. Today, ICD provides a common language shared between countries to describe diseases. The thousands of codes can make ICD-10 look unnecessarily complicated. But there is an underlying structure. It is like a tree. You start with the trunk, you follow the branches, and you become more specific. Once you understand that structure, ICD-10 begins to look much easier to understand.

Key Takeaways

✓ ICD did not begin as a billing system. Its roots are in international efforts to standardize the classification of causes of death.
✓ Think of ICD-10 as a tree. A broad diagnostic category is the trunk. Each additional character moves farther along the branches and provides greater clinical specificity.
✓ ICD-10-CM tells us what condition the patient has; CPT tells us what was done. Together, these coding systems form much of the language behind modern medical documentation, data, and reimbursement.

The beginning of wisdom is to call things by their proper name.”

Attributed to Confucius

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