The In-Depth Definition of the International Classification of Diseases (ICD)
Every diagnosis, symptom, or medical condition has a story to tell—and the way that story is recorded is through the International Classification of Diseases, commonly known as ICD. Created by the World Health Organization (WHO), this universal coding system gives every health condition a distinct code so that doctors, hospitals, researchers, and insurers around the world can all speak the same language when it comes to describing why care was provided.
If you’ve ever reviewed your medical bill or Explanation of Benefits (EOB) and noticed a combination of letters and numbers listed next to your visit summary, those are ICD codes. They tell your insurer—and your own health record—the underlying reason for your visit. The Current Procedural Terminology (CPT) system, on the other hand, represents the what—the actual service or procedure your healthcare professional performed—while the International Classification of Diseases (ICD) explains the why—the diagnosis or condition that justified that care. Together, CPT and ICD codes create a complete record of both the reason for your visit and the actions taken to treat it.
For healthcare professionals, ensuring that ICD and CPT codes align correctly is essential. Each ICD code must support the medical necessity of the CPT code that follows it—meaning the why has to justify the what. When these two don’t match, insurance claims can be denied, payments delayed, or records flagged for audit. Proper alignment protects the integrity of a patient’s care story—linking every diagnosis to the right treatment or service.
Patients play an active role in this process too. The questions you’re asked during check-in, your medical history updates, or even the symptom descriptions you give during a visit—all of these help your provider document the why accurately. When you clearly explain what brought you in, when your symptoms began, or what’s changed since your last appointment, you’re helping your healthcare team choose the most accurate ICD codes to describe your condition. That, in turn, supports accurate CPT coding, fair billing, and a complete medical record.
Understanding the International Classification of Diseases and Its Purpose
The ICD system organizes diseases, conditions, and related health problems into categories that are universally recognized. This consistency allows for accurate communication between healthcare providers, insurance companies, and public health organizations worldwide.
Every time you receive care—whether for a chronic illness, injury, or preventive screening—your provider assigns at least one ICD code that identifies why that care took place. For example, if you visit your doctor for persistent knee pain, an ICD code such as M25.561 (“Pain in right knee”) documents the reason for your appointment.
This documentation serves multiple purposes:
- It explains why a service was provided, linking the visit to a specific diagnosis.
- It ensures that insurance claims can be processed accurately.
- It helps track disease trends, guiding public health policy and research.
Without ICD codes, the “why” of healthcare would be lost in translation.
ICD Codes: The “Why” Behind Every Service
While the Current Procedural Terminology (CPT) system tells what a licensed healthcare professional did—the exam, test, or treatment—the ICD system explains why it was done. These two systems work hand in hand: CPT captures the action, ICD captures the reason.
Think of it this way:
- ICD codes describe the problem or diagnosis (the “why”).
- CPT codes describe the solution or service (the “what”).
For instance:
- ICD code E11.9 – Type 2 diabetes mellitus without complications
- CPT code 83036 – Hemoglobin A1c test (a lab test ordered to monitor diabetes)
Together, they create a complete, traceable record of your healthcare visit—showing not just what happened, but why it was necessary.
If you’d like to learn more about how the CPT system defines the “what,” visit our glossary page on Current Procedural Terminology.
How ICD Codes Are Organized
ICD codes may seem like random strings of letters and numbers, but they follow a specific structure that makes them logical and searchable.
The most current version used in the United States is ICD-10-CM (International Classification of Diseases, 10th Revision, Clinical Modification). It’s built around chapters that categorize diseases by body system or type. For example:
- A00–B99: Infectious and parasitic diseases
- C00–D49: Neoplasms (tumors and cancers)
- E00–E89: Endocrine, nutritional, and metabolic diseases (like diabetes and thyroid disorders)
- I00–I99: Circulatory system diseases (like heart disease or stroke)
- J00–J99: Respiratory diseases (like asthma or pneumonia)
- M00–M99: Musculoskeletal diseases (like arthritis or back pain)
- Z00–Z99: Factors influencing health status and contact with health services (like routine exams or aftercare)
Each ICD code begins broadly, then becomes more specific as additional characters are added. For example:
- J06 – Acute upper respiratory infections
- J06.9 – Acute upper respiratory infection, unspecified
This hierarchy allows for detailed recordkeeping, ensuring precision in both diagnosis and billing.
ICD Codes in Everyday Care
You’ll encounter ICD codes in almost every area of healthcare, including:
- Primary care visits: For general illnesses, preventive screenings, or follow-up care
- Specialty care: For chronic or complex conditions like hypertension or arthritis
- Hospital and emergency care: For injuries, surgeries, and critical illnesses
- Laboratory and diagnostic testing: To justify tests and imaging procedures
- Physical and occupational therapy: To track recovery and medical necessity
- Mental and behavioral health: To classify emotional or psychological conditions
Each ICD code anchors your care to a clinical reason—helping providers document your progress and insurers determine coverage.
And while you may never have to memorize these codes, understanding their purpose helps you verify that your record accurately reflects your condition and your care.
Questions to Ask Yourself When You Review Your Medical Bill or Records
When your bill, EOB, or patient portal lists ICD codes, those numbers tell the why of your care. Reading them can help you confirm that your diagnosis and treatment align with what you actually experienced.
Here are five key signpost questions to guide you:
- “What diagnosis or symptom does each ICD code represent?” Each code links to a condition or reason for care—ask your provider for clarification if any seem unfamiliar.
- “Do these ICD codes match what I was told during my visit?” Reviewing ensures your diagnosis was recorded accurately, which matters for both your health record and insurance claim.
- “How do these ICD codes connect to the CPT codes on my bill?” ICD codes explain why care occurred; CPT codes describe what was done. Together, they form a complete picture of your visit.
- “Were any of these codes used for preventive or follow-up care?” Some ICD codes indicate preventive services (Z00–Z99), which may be covered differently by insurance.
- “Can I get a summary explaining each diagnosis code?” Asking for an itemized explanation improves your understanding and helps prevent documentation errors.
When you review both ICD and CPT codes, you gain a more accurate and confident view of your care—from diagnosis through treatment.
Summary
The International Classification of Diseases (ICD) is the global foundation of medical documentation—it defines the why behind every healthcare service. Each ICD code translates a diagnosis, symptom, or medical condition into a universal language that connects patients, providers, and insurers.
By understanding how ICD works alongside Current Procedural Terminology (CPT)—the what of care—you can better interpret your medical bills, verify accuracy, and manage your records with confidence.
To learn more about the ICD system and its global application, visit the World Health Organization (WHO)—the organization responsible for maintaining this essential framework.
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Disclaimer: This education was brought to you today by The Patient Better Project Inc., a 501(c)(3) organization dedicated to reshaping the way patients and caregivers navigate care. We are committed to empowering individuals with the knowledge and tools necessary to take control of their health journeys, ensuring that everyone can access the care they need with confidence and clarity.
The information provided here is for educational and entertainment purposes only. It is not intended as, nor should it be considered a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. If you think you may have a medical emergency, immediately call 911 or your local emergency number.