General Coding Guidelines for ICD-10-CM

The general coding guidelines for ICD-10-CM come down to two rules: locate the diagnosis in the Alphabetic Index and verify it in the Tabular List, then report the code at the highest number of characters and level of specificity the record supports. The remaining guidelines cover combination codes, sequela, laterality, and when sign, symptom, or unspecified codes are acceptable.

ICD-10-CM Basics

Selecting accurate diagnosis codes using ICD-10-CM is challenging due to the availability of more than 68,000 codes. In this blog, we shared general coding guidelines for ICD-10-CM, which will help you select accurate diagnosis codes. First, you’ll need access to two lists in ICD-10 CM: the Alphabetical Index of diagnostic terms and the Tabular List of ICD-10 codes.

The Alphabetical Index of diagnostic terms lists thousands of ‘main terms’ alphabetically. Under each of those main terms, there is often a sub-list of more detailed terms; for instance, ‘Cataract’ has a sub-list of 84 terms. The Tabular List of ICD-10 codes runs alphanumerically from A00.0 to Z99.89. Its chapters group codes by body system or condition.

General Coding Guidelines for ICD-10-CM

Locating a code in the ICD-10-CM 

To select a code, find the diagnosis or reason for the visit in the Alphabetic Index. Then verify that code in the Tabular List.

Read and follow the instructional notes in both the Alphabetic Index and the Tabular List. Always use both resources when you locate and assign a code.

The Alphabetic Index does not always provide the complete code. Only the Tabular List shows the entire code, including laterality and any applicable 7th character.

A dash (-) at the end of an Alphabetic Index entry indicates that the code needs additional characters. Even without a dash, check the Tabular List to confirm the code needs no 7th character.

Level of Detail in Coding 

Report diagnosis codes at the highest number of characters available and the highest level of specificity in the medical record. ICD-10-CM diagnosis codes contain 3, 4, 5, 6, or 7 characters.

Three-character codes serve as category headings in ICD-10-CM. Fourth, fifth, or sixth characters can subdivide these categories to add greater detail.

Use a three-character code only when the category has no further subdivisions. A code is invalid if it lacks any required character, including the 7th character.

Multiple Coding for a Single Condition 

The etiology/manifestation convention requires two codes for a single condition that affects multiple body systems. Other single conditions also need more than one code. ‘Use additional code’ notes appear in the Tabular List for codes outside an etiology/manifestation pair. They flag cases where a secondary code helps fully describe the condition.

The sequencing rule is the same as the etiology/manifestation pair. ‘ Use additional code’ tells you to add a secondary code if known.

Combination Code 

A combination code is a single code that classifies two diagnoses: a diagnosis with an associated secondary process (manifestation) or an associated complication. To identify combination codes, refer to sub-term entries in the Alphabetic Index and read the inclusion and exclusion notes in the Tabular List.

Assign only the combination code when that code fully identifies the diagnostic conditions involved or when the Alphabetic Index directs. Do not use multiple codes when one combination code captures every element the provider documents in the diagnosis.

When the combination code lacks specificity in describing the manifestation or complication, add a secondary code.

Sequela (Late Effects) 

A sequela is a residual effect that remains after the acute phase of an illness or injury ends. No time limit applies to sequela codes. The residual may be apparent early, such as in cerebral infarction, or it may occur months or years later due to a previous injury.

Examples of sequela include scar formation resulting from a burn, deviated septum due to a nasal fracture, and infertility due to tubal occlusion from old tuberculosis. Coding sequela generally requires two codes. List the condition or nature of the sequela first, and the sequela code second.

Use of Sign/Symptom/Unspecified Codes 

Sign/symptom and ‘unspecified’ codes have acceptable, even necessary, uses. Report specific diagnosis codes when the medical record and clinical knowledge of the patient’s health condition support them. In some encounters, though, signs/symptoms or unspecified codes reflect the visit most accurately. Code each healthcare encounter to the level of certainty known at that visit.

Impending or Threatened Condition 

Code any condition described at the time of discharge as ‘impending’ or ‘threatened’ as follows: 

• When the condition did occur, code it as the confirmed diagnosis.
• If it did not occur, reference the Alphabetic Index to determine if the condition has a subentry term for ‘impending’ or ‘threatened’ and also reference primary term entries for “Impending” and ‘Threatened.’
• Where the Index lists the sub-terms, assign the given code.
• Without listed sub-terms, code the existing underlying condition(s), not the condition described as impending or threatened.

Remaining Coding Guidelines for ICD-10-CM

Here are some additional important guidelines to keep in mind when coding with ICD-10-CM:

• Use the appropriate codes from A00.0 through T88.9, Z00-Z99.8, and U00-U85 to identify diagnoses, symptoms, conditions, problems, complaints, or other reason(s) for the encounter/visit.
• Symptom and sign codes are acceptable when the provider has not established a related definitive diagnosis.
• Do not assign signs and symptoms that routinely accompany a disease process as additional codes, unless the classification instructs otherwise.
• Code additional signs and symptoms that do not routinely accompany a disease process when they are present.

Acute, Bilateral, and Borderline Conditions

• If the same condition is described as acute (sub-acute) and chronic, and separate subentries exist in the Alphabetic Index at the same indentation level, code both and sequence the acute (sub-acute) code first.
• Report each unique ICD-10-CM diagnosis code only once per encounter. This applies to bilateral conditions without distinct laterality codes, and to two different conditions that share the same ICD-10-CM code.
• Some ICD-10-CM codes indicate laterality, specifying whether the condition occurs on the left, right, or is bilateral. If the condition is bilateral and no bilateral code exists, assign separate codes for both the left and right sides. When the medical record does not identify the side, assign the code for the unspecified side.
• If the provider documents a ‘borderline’ diagnosis at discharge, code it as confirmed unless the classification provides a specific entry (e.g., borderline diabetes). When a borderline condition has its own index entry in ICD-10-CM, code it accordingly. Because borderline conditions are not uncertain diagnoses, the rule applies the same way in inpatient and outpatient settings.

Medical Billers and Coders (MBC) is a leading medical billing company that provides comprehensive medical billing and coding services to healthcare providers across various specialties. For provider education purposes, we have shared general coding guidelines for ICD-10-CM. 

If you need assistance with medical billing and coding services for your practice, contact us at 888-357-3226 or info@medicalbillersandcoders.com.

Want accurate ICD-10-CM coding that holds up to payer scrutiny? Request a Billing Quote, and our coding team will review your specialty, payer mix, and claim volume to build a custom plan for your practice.

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Alex Peter

A Medical Coding Subject Matter Expert with over 16 years of experience in ICD-10 and CPT coding, clinical documentation, and revenue cycle management. Shares actionable insights to improve billing accuracy and support compliance-driven healthcare practices.