What is ICD-10?

What is ICD-10?ICD-10 refers to the tenth edition of the International Classification of Diseases, which is a medical coding system chiefly designed by the World Health Organization (WHO) to catalog health conditions by categories of similar diseases under which more specific conditions are listed, thus mapping nuanced diseases to broader morbidities.

Many countries now use national variations of ICD-10, each modified to align with their unique healthcare infrastructure.

The US version of ICD-10, created by the Centers for Medicare & Medicaid Services (CMS) and the National Center for Health Statistics (NCHS), consists of two medical code sets—ICD-10-CM and ICD-10-PCS.

ICD-10-PCS stands for the International Classification of Diseases, Tenth Revision, Procedure Coding System. As indicated by its name, ICD-10-PCS is a procedural classification system of medical codes. It is used in hospital settings to report inpatient procedures.

ICD-10-CM stands for the International Classification of Diseases, Tenth Revision, Clinical Modification. Used for medical claim reporting in all healthcare settings, ICD-10-CM is a standardized classification system of diagnosis codes that represent conditions and diseases, related health problems, abnormal findings, signs and symptoms, injuries, external causes of injuries and diseases, and social circumstances.

For a medical provider to receive reimbursement for medical services, ICD-10-CM codes are required to be submitted to the payer. While CPT® codes depict the services provided to the patient, ICD-10-CM codes depict the patient’s diagnoses that justify the services rendered as medically necessary.

The Origins of ICD-10 Coding

The roots of ICD-10 coding go back to the 1850s. The first edition, known as the International List of Causes of Death, was adopted by the International Statistical Institute in 1893.

WHO assumed oversight of the International Classification of Diseases (ICD) in 1948 with the main intention of tracking—and helping to eliminate—diseases within various populations. At the time, the Sixth Revision, which introduced causes of morbidity to the system, had just been published.

In 1957 and 1968, WHO released ICD-7 and ICD-8, respectively. Shortly after the release of ICD-9 in 1979, the US created its own version, known as the International Classification of Diseases, Ninth Revision, Clinical Modification—or, ICD-9-CM.

The development of ICD-9-CM was a tremendous boon. Not only did the new system expand the ability to capture enhanced morbidity data, but it also incorporated surgical procedures and other items necessary to categorize the needs of hospitals.

But ICD-9-CM, updated annually by CMS and the NCHS, was a limited system with a limited capacity for the addition of codes to keep pace with modern healthcare. And it was already a 3-volume set, with the first 2 volumes dedicated to diagnosis codes and the third volume containing inpatient procedural codes.

So, after decades in the making, CMS and NCHS adopted ICD-10 and adapted the classification to create a new version, the International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM), which no longer included the third volume of inpatient procedure codes.

What happened to the inpatient procedure codes? CMS determined the need for better organization and funded a project with 3M Health Information Systems in 1995 to develop the International Classification of Diseases, Tenth Revision, Procedure Coding System (ICD-10-PCS).

The Difference Between ICD-10-CM & ICD-10-PCS

Both ICD-10-CM and ICD-10-PCS came into effect for medical claims reporting on Oct.1, 2015. But the two code sets differ vastly. The primary distinctions are

  • ICD-10-CMdiagnosis code set used for all healthcare settings
  • ICD-10-PCSprocedure code set used only in hospital inpatient settings

The terms ICD-10-CM and ICD-10 are used interchangeably in the US. This linguistic trend underscores the distinction between CM and PCS, in that ICD-10-CM is ubiquitous across healthcare settings, used by every medical coder as the singular means to report diagnoses.

The PCS code set, on the other hand, is 1 of 2 procedural coding systems. But unlike CPT®, ICD-10-PCS is used strictly in hospitals.

ICD-10 Provides Greater Specificity

Though some circumstance still require ICD-9 to ICD-10 code conversion, those circumstances are becoming uncommon, and ICD-9 is gradually fading into coding history.

The ICD-10 codes we use today are more specific than ICD-9-CM codes and allow for detailed classifications of patients’ conditions, injuries, and diseases. Medical coders are now equipped to capture anatomic sites, etiologies, comorbidities and complications, as well as severity of illnesses.  

The magnitude of ICD-10 codes currently in effect—72,184 versus 13,000 diagnosis codes in ICD-9-CM—illustrates the increased granularity available to represent real-world clinical practice and medical technology advances.

And that’s to say nothing of the enhanced capacity to provide essential data of disease patterns and outbreaks of disease, and to help illuminate characteristics and circumstances of individuals so affected.

With greater specificity, providers and payers can use ICD-10 diagnosis codes to track information about patients’ conditions and the types and number of treatments patients receive. They can gather and analyze code utilization to

  • Measure the safety and efficacy of patient care
  • Determine the health status and risk factors of defined populations
  • Improve and monitor providers’ performances
  • Assess healthcare costs
  • Investigate and prevent coding and billing abuses

Structure of ICD-10 Codes

ICD-10-CM codes consist of 3 to 7 characters. Every code begins with an alpha character, which is indicative of the chapter to which the code is classified. The second and third characters are numbers. The fourth, fifth, sixth, and seventh characters can be numbers or letters.

Here are some examples of ICD-10 codes and the conditions they represent.

  • G10 (Huntington's disease)
  • K26.1 (Acute duodenal ulcer with perforation)
  • A37.81 (Whooping cough due to other Bordetella species with pneumonia)
  • I25.111 (Atherosclerotic heart disease of native coronary artery with angina pectoris with documented spasm)
  • M80.021G (Age-related osteoporosis with current pathological fracture, right humerus; subsequent encounter for fracture with delayed healing)

Notice that with each additional character, the ICD-10 code depicts greater diagnostic information. You must always code diagnoses to the highest level of specificity available in the ICD-10 code set.

For instance, you should not code N04 for a patient diagnosed with nephrotic syndrome with minor glomerular abnormality. The finding of minor glomerular abnormality calls for an additional digit and would be coded as N04.0.

This degree of coding detail, of course, is not something you need to memorize. The ICD-10 list of codes is organized to lead you to the most specific diagnosis code selection.

Using the ICD-10 Tabular List

Remember—an ICD-10 code always begins with a letter and is followed by 2 numbers. The first 3 characters refer to the code category. As such, they represent common traits, a disease or group of related diseases and conditions.

Once you find an ICD-10 code in the Alphabetic Index, you’ll need to review the code details and instructions in the Tabular List to confirm that it’s the right diagnosis code and to code it properly.

In the chapters listed above, notice the code ranges included in the titles. These sets of alphanumeric characters further define the chapter title by telling you the categories contained within it.

You may need to code for a patient with a history of retinopathy, for instance. In the Alphabetic Index, you see Retinopathy (background) H35.00 and find the code details, as expected, in the chapter dedicated to diseases of the eye.

But if your patient has diabetic retinopathy, the Index will offer several code options in the E08-E13 range, which will then direct you to Chapter 4 Endocrine, Nutritional and Metabolic Diseases (E00-E89).

Additionally, the code range within each chapter in the Tabular List is broken down into subchapters called blocks, similar to the following block summary from Chapter 4.

  • E00-E07 Disorders of thyroid gland
  • E08-E13 Diabetes mellitus
  • E15-E16 Other disorders of glucose regulation and pancreatic internal secretion
  • E20-E35 Disorders of other endocrine glands
  • E36 Intraoperative complications of endocrine system
  • E40-E46 Malnutrition
  • E50-E64 Other nutritional deficiencies
  • E65-E68 Overweight, obesity and other hyperalimentation
  • E70-E88 Metabolic disorders
  • E89 Postprocedural endocrine and metabolic complications and disorders, not elsewhere classified

Each block contains one or more categories, many of which are divided into subcategories in the ICD-10 Tabular List.

In the example below, E13 is a category. E13.0, E13.1, E13.2, and E13.3 are subcategories, as are E13.31 and E13.32.

E13 (Other specified diabetes mellitus)

  • E13.0 (Other specified diabetes mellitus with hyperosmolarity)
    • E13.00 (... without nonketotic hyperglycemic-hyperosmolar coma (NKHHC))
    • E13.01 (... with coma)
  • E13.1 (Other specified diabetes mellitus with ketoacidosis)
    • E13.10 (... without coma)
    • E13.11 (... with coma)
  • E13.2 (Other specified diabetes mellitus with kidney complications)
    • E13.21 (Other specified diabetes mellitus with diabetic nephropathy)
    • E13.22 (Other specified diabetes mellitus with diabetic chronic kidney disease)
    • E13.29 (Other specified diabetes mellitus with other diabetic kidney complication)
  • E13.3 (Other specified diabetes mellitus with ophthalmic complications)
    • E13.31 (Other specified diabetes mellitus with unspecified diabetic retinopathy)
      • E13.311 (... with macular edema)
      • E13.319 (... without macular edema)
    • E13.32 (Other specified diabetes mellitus with mild nonproliferative diabetic retinopathy)
      • E13.321 (Other specified diabetes mellitus with mild nonproliferative diabetic retinopathy with macular edema)
        • E13.3211 (... right eye)
        • E13.3212 (... left eye)
        • E13.3213 (... bilateral)
        • E13.3219 (... unspecified eye)

Note that ICD-10 coding employs a decimal point following the category and preceding the subcategory. Every character to the right of the decimal point adds specific information about the diagnosis.

Among the diagnosis codes listed above in the E13 category, none are the same. Each ICD-10 code represents a unique diagnosis. That’s why you must always assign subdivisions until you have coded to the highest level of specificity when reporting ICD-10 codes to payers, claims clearinghouses, or billing and collection agencies.

The 7th Character in ICD-10 Coding

Not all ICD-10 codes require a 7th character, but when they do, the 7th character serves a particular purpose—like signifying laterality or defining whether the code represents an initial encounter for the problem, a subsequent encounter, or a sequela arising from another condition.

In the E13 category, codes demonstrating laterality for E13.321 are shown with the 7th character added. Your ICD-10 code book, however, will merely provide instructions to add the 7th character, such as

One of the following 7th characters is to be assigned to codes in subcategory E13.32 to designate laterality of the disease:

1 - right eye
2 - left eye
3 - bilateral
9 - unspecified eye

Because the 7th character is required to correctly code E13.321, option “9 - unspecified eye” is included in the event that clinical documentation doesn’t indicate which eye is affected.

Coding the 7th Character with Placeholder “X”

Another scenario you’ll encounter when reporting diagnoses on medical claims is codes requiring a 7th character that don’t have 6 characters at their height of specificity. This is where placeholder “X” comes into play.

ICD-10-CM placeholder “X” was created to accommodate expansion of the code set with anticipation that characters with specific meanings will eventually occupy the designated spaces. In the interim, medical coders are required to use placeholder “X” to fill in the missing character(s), thereby preserving the diagnosis code classification structure.

For codes with less than 6 characters that require a 7th character, the placeholder “X” should be assigned for all absent characters. In other words, the 7th character must always be the 7th character. A code is invalid if you don’t add the “X” to hold the place of the 4th, 5th, or 6th character before adding the 7th.

Some code examples using ICD-10 placeholder “X” include

  • T18.1 (Foreign body in esophagus) becomes T18.1XXS (Foreign body in esophagus, sequela)
  • H40.10 (Unspecified open-angle glaucoma) becomes H40.10X3 (Unspecified open-angle glaucoma, severe stage)

Becoming a Professional Medical Coder

Medical coders read clinical documentation to extract diagnoses from patient records and then translate those diagnoses into ICD-10 codes. While many coders use ICD-10 lookup software to help them, referring to an ICD-10 code book is invaluable to build an understanding of the classification system.

But whether you use software or a book, coding a medical record correctly requires ICD-10 training to follow the rules governing correct ICD-10 code assignments and to apply conventions like not elsewhere classifiable (NEC) and not otherwise specified (NOS), Excludes1 and Excludes2, and what is meant by code also. And then there’s the many nuances for proper use of Z codes.

Additionally, because ICD-10 coding involves a high level of specificity, you’ll need to develop familiarity with medical terminology, human anatomy and physiology, pharmacology, disease processes, diagnostic methods, and treatment.

Knowledge in these areas will enable you to interpret medical documentation, locate reporting errors, query providers, and assign accurate ICD-10 codes, which is why studying to become a certified professional coder is vital to preparing for a successful career in medical coding.

The demand for medical coders has never been greater, and now is an ideal time to take your career—and pay scale—to the next level. In just 4 months, you can be quipped to enter the healthcare marketplace as a professional medical coder.

Learn More

If you’re looking for effective ICD-10 training and resources to help guide your practice or career through the evolving diagnosis coding landscape, AAPC is staffed with nationally renowned ICD-10 experts to help make sense of what can be complicated information. Whether you're a large facility or an independent practice, we have ICD-10 solutions that will fit your needs.

And be sure to stay tuned—because ICD-11 is in the works! We'll keep you updated on this and other important news with Healthcare Business Monthly magazine. Stay informed and keep your coding in tip-top shape!

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