ICD-10 Basics

About ICD-10

The ICD-9 code sets used to report medical diagnoses and inpatient procedures was replaced by ICD-10 code sets on October 1, 2015. ICD-10 consists of two parts:

  • ICD-10-CM diagnosis coding which is for use in all U.S. health care settings.
  • ICD-10-PCS inpatient procedure coding which is for use in U.S. hospital settings.

ICD-10 affects diagnosis and inpatient procedure coding for everyone covered by the Health Insurance Portability Accountability Act (HIPAA), not just those who submit Medicare or Medicaid claims:

  • Claims for services provided on or after the compliance date should be submitted with ICD-10 diagnosis codes.
  • Claims for services provided prior to the compliance date should be submitted with ICD-9 diagnosis codes.

The change to ICD-10 does not affect CPT coding for outpatient procedures.

ICD-10-CM Code Structure

ICD-10 diagnosis codes have between 3 and 7 characters: Graphic illustrating ICD-10-CM Code Structure of 3-7 characters. First 3 characters are the category; character 4 to 6 descrribe the etiology, anatomical site and severity; and the final character is an extension. The first character is alpha (not U) and the second is numeric, while characters 3 to 7 can be any combination of alpha or numeric characters.

  • Codes with three characters are included in ICD-10-CM as the heading of a category of codes that may be further subdivided by the use of any or all of the 4th, 5th, and 6th characters. Digits 4-6 provide greater detail of etiology, anatomical site, and severity. A code using only the first three digits is to be used only if it is not further subdivided.
  • A code is invalid if it has not been coded to the full number of characters required. This does not mean that all ICD-10 codes must have 7 characters. The 7th character is only used in certain chapters to provide data about the characteristic of the encounter. Examples of where the 7th character can be used include injuries and fractures, as illustrated in the following tables:
    Injuries and External CausesFractures
    AInitial encounterAInitial encounter for closed fracture
    DSubsequent encounterBInitial encounter for open fracture
    SSequelaDSubsequent encounter for fracture with routine healing
    GSubsequent encounter for fracture with delayed healing
    KSubsequent encounter for fracture with nonunion
    PSubsequent encounter for fracture with malunion
  • A dummy placeholder of “X” is used with certain codes to allow for future expansion and/or to fill out empty characters when a code contains fewer than 6 characters and a 7th character applies. When a placeholder character applies, it must be used in order for the code to be considered valid.
  • Below are specific examples of ICD-10 diagnosis codes. The use of combination codes, increased specificity, and the “X” placeholder is illustrated:
    Combination Codes
    I25.110Atherosclerotic heart disease of native coronary artery with unstable angina pectoris
    Increased Specificity
    S72.044GNon-displaced fracture of base of neck of right femur, subsequent encounter for closed fracture with delayed healing
    C50.511Malignant neoplasm of lower-outer quadrant of right female breast
    C50.512Malignant neoplasm of lower-outer quadrant of left female breast
    “X” Placeholder
    H40.11X2Primary open-angle glaucoma, moderate stage

ICD-10-CM Indexes

A summary of the chapters found in the Tabular List has been provided below:

ChapterCode RangeEstimated # of CodesDescription
1A00-B991,056Certain infectious and parasitic diseases
3D50-D89238Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism
4E00-E89675Endocrine, nutritional and metabolic diseases
5F01-F99724Mental, Behavioral and Neurodevelopmental disorders
6G00-G99591Diseases of the nervous system
7H00-H592,452Diseases of the eye and adnexa
8H60-H95642Diseases of the ear and mastoid process
9I00-I991,254Diseases of the circulatory system
10J00-J99336Diseases of the respiratory system
11K00-K95706Diseases of the digestive system
12L00-L99769Diseases of the skin and subcutaneous tissue
13M00-M996,339Diseases of the musculoskeletal system and connective tissue
14N00-N99591Diseases of the genitourinary system
15O00-O9A2,155Pregnancy, childbirth and the puerperium
16P00-P96417Certain conditions originating in the perinatal period
17Q00-Q99790Congenital malformations, deformations and chromosomal abnormalities
18R00-R99639Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified
19S00-T8839,869Injury, poisoning and certain other consequences of external causes
20V00-Y996,812External causes of morbidity
21 Z00-Z991,178Factors influencing health status and contact with health services

External Cause Code Reporting

If you have not been reporting ICD-9-CM external cause codes, you will not be required to report ICD-10-CM codes found in Chapter 20 unless a new State or payer-based requirement about the reporting of these codes is instituted. If such a requirement is instituted, it would be independent of ICD-10-CM implementation.

In the absence of a mandatory reporting requirement, you are encouraged to voluntarily report external cause codes, as they provide valuable data for injury research and evaluation of injury prevention strategies.

Native Coding and Unspecified Codes

Native coding means to assign an ICD-10 diagnosis code directly based on clinical documentation. Practices are encouraged to natively code using ICD-10 code reference sources instead of using crosswalks, which should be used for general knowledge. Specific codes reflecting the most appropriate level of certainty known for an encounter should be evaluated first:

  • Specific diagnosis codes should be reported when they are supported by the available medical record documentation and clinical knowledge of the patient’s health condition.
  • If a definitive diagnosis has not been established by the end of the encounter, it is appropriate to report codes for sign(s) and/or symptom(s) in lieu of a definitive diagnosis.
  • When sufficient clinical information is not known or available about a particular health condition to assign a more specific code, coding should comply with the payer guidelines for the use of unspecified codes.