ICD-10-CM Sequencing Rules for Beginners — Which Code Goes First?

ICD-10-CM sequencing rules determine the order in which diagnosis codes are reported. The order is not arbitrary — it affects reimbursement, data accuracy, and medical necessity. On the CPC exam, sequencing questions are among the most consistently tested topics because getting the right codes in the wrong order is just as wrong as selecting the wrong codes entirely.

Sequencing rules come from two sources: the instructional notes built into the Tabular List (Code first, Use additional code, Code also) and the guideline sections that govern diagnosis selection by care setting. This guide covers both sources, explains how they interact, and walks through the real-world patterns the CPC exam uses to test sequencing. For the foundation that sequencing builds on, review the ICD-10-CM coding guidelines and How ICD-10-CM Codes Are Built.

Why Sequencing Matters

The first-listed or principal diagnosis communicates the primary reason for the encounter or admission. Insurance companies use this code to determine medical necessity — whether the services provided were justified by the diagnosis. If the sequencing is wrong, the claim may be denied even though the codes themselves are correct.

Sequencing also affects health data and statistics. Public health agencies, quality reporting programs, and research databases rely on the primary diagnosis code to categorize encounters. A patient admitted primarily for pneumonia who also has diabetes produces different data depending on which condition is sequenced first.

For the CPC exam, sequencing determines whether your answer is correct or incorrect. A question may present four answer choices that all contain the same codes — but in different orders. Only one order is correct according to the guidelines and Tabular List instructions.

The Three Sequencing Instructions

The Tabular List contains three sequencing instructions that appear as notes under code categories, subcategories, or individual codes. These instructions are mandatory — they override any general sequencing convention.

Code First

A “Code first” note appears under a code that should be sequenced as a secondary code, not the first-listed or principal diagnosis. It tells you that another condition must be coded and sequenced before the code you are looking at.

Example: Under category G63 (Polyneuropathy in diseases classified elsewhere), you see “Code first underlying disease.” If a patient has diabetic polyneuropathy, you code the diabetes (E11.42 — the combination code for type 2 diabetes with diabetic polyneuropathy) first, then G63 if an additional code is needed (though in this case E11.42 already captures both).

Use Additional Code

A “Use additional code” note appears under a code that should be sequenced first. It tells you to add a secondary code to provide more information about the condition.

Example: Under category J96 (Respiratory failure), you see “Use additional code to identify the underlying condition.” The respiratory failure code is listed first, and the underlying condition code follows.

How Code First and Use Additional Code Work Together

These two instructions are paired. When one code has a “Code first” note, the code it references will have a corresponding “Use additional code” note. They are two sides of the same instruction — one tells you where to look from the secondary code, and the other tells you where to look from the primary code.

Instruction	Where It Appears	What It Means	Action
Code first	Under the secondary code	Another code must come before this one	Find and sequence the referenced code first
Use additional code	Under the primary code	An additional code should follow this one	Add the referenced secondary code after
Code also	Under either code	Both codes should be reported but order is flexible	Assign both; sequence based on circumstances

Code Also

A “Code also” note means that both conditions should be coded, but unlike Code first / Use additional code, there is no mandatory sequencing. Either code may be listed first depending on the reason for the encounter and the circumstances of care.

Example: A “Code also” note may appear under a code for a condition that commonly occurs with another condition. You report both codes, but the encounter circumstances determine which one is first-listed.

Etiology / Manifestation Sequencing

Etiology/manifestation coding is one of the most important sequencing patterns in ICD-10-CM. The etiology (underlying cause) is always sequenced first, and the manifestation (the resulting condition) is sequenced second. This order is mandatory and cannot be reversed.

How to Identify Etiology/Manifestation Pairs

The Tabular List signals etiology/manifestation pairs in several ways:

• The manifestation code will have a “Code first” note referencing the underlying condition
• The manifestation code title often includes the phrase “in diseases classified elsewhere”
• The Alphabetic Index lists the manifestation code in brackets [ ] after the etiology code — codes in brackets are always secondary codes
• Some manifestation codes are printed in italics in the Tabular List (though this formatting convention varies by publisher)

Example: Dementia in Parkinson’s disease uses two codes: (1) G20 (Parkinson’s disease) — the etiology, sequenced first; (2) F02.80 (Dementia in other diseases classified elsewhere, without behavioral disturbance) — the manifestation, sequenced second.

You cannot reverse this order, and you cannot use the manifestation code (F02.80) as a standalone first-listed code — it always requires the etiology code before it.

Common Etiology/Manifestation Pairs Tested on the CPC Exam

Etiology (First)	Manifestation (Second)	Clinical Scenario
E11.36 (Type 2 diabetes with diabetic cataract)	H28 (Cataract in diseases classified elsewhere)	Diabetic cataract
G20 (Parkinson’s disease)	F02.8- (Dementia in other diseases)	Dementia due to Parkinson’s
B20 (HIV disease)	Various manifestation codes	HIV with opportunistic infections
M32.- (Systemic lupus erythematosus)	Various organ manifestation codes	Lupus with organ involvement

Note that many conditions that historically required etiology/manifestation dual coding now have combination codes in ICD-10-CM. When a combination code exists that captures both the underlying condition and the manifestation, use the single combination code instead of two separate codes.

Setting-Specific Sequencing Rules

Beyond the Tabular List instructions, the guideline sections provide sequencing rules based on the care setting.

Inpatient Sequencing (Section II)

The principal diagnosis is sequenced first for inpatient admissions. Section II provides specific guidance for situations where the principal diagnosis is not straightforward:

• If two conditions equally qualify as principal diagnosis, either may be listed first
• If the admission is for a complication of surgery or medical care, the complication code is the principal diagnosis
• If an obstetric patient is admitted for a condition unrelated to pregnancy, the obstetric complication code is still the principal diagnosis with the non-obstetric condition as secondary
• Uncertain diagnoses are coded as confirmed and may be sequenced as principal diagnosis

Outpatient Sequencing (Section IV)

The first-listed diagnosis is sequenced first for outpatient encounters. Key outpatient sequencing rules:

• Code the reason for the encounter as the first-listed diagnosis
• For outpatient surgery, the reason for the surgery is first-listed; if the postoperative diagnosis differs, use the postoperative diagnosis
• Uncertain diagnoses are never coded — signs and symptoms are first-listed instead
• Z-codes are first-listed when the encounter is for a circumstance other than disease or injury (screening, vaccination, aftercare)

Special Sequencing Scenarios

Several clinical scenarios have specific sequencing rules that override the general conventions.

Sepsis Sequencing

Sepsis has mandatory sequencing regardless of the encounter setting: first, the systemic infection code (A40.- or A41.-); second, R65.20 (Severe sepsis without septic shock) or R65.21 (Severe sepsis with septic shock), if applicable; third, code(s) for associated organ dysfunction, if applicable.

See Sepsis Coding Guidelines for the complete sequencing rules.

Hypertension with Heart Disease and/or CKD

The “with” convention in ICD-10-CM assumes a causal link between hypertension and heart disease or chronic kidney disease. The combination codes in categories I11, I12, and I13 are used instead of coding these conditions separately: Hypertension + heart disease → I11.- (Use additional code for the type of heart failure); Hypertension + CKD → I12.- (Use additional code for the CKD stage); Hypertension + heart disease + CKD → I13.- (Use additional codes for heart failure type and CKD stage).

Diabetes with Complications

Diabetes coding uses combination codes that capture both the diabetes type and the complication. When a diabetic patient has multiple complications, assign as many diabetes combination codes as necessary — each from the appropriate category (E08–E13). The “with” convention assumes a causal relationship between diabetes and documented complications.

External Cause Code Sequencing

External cause codes (V00–Y99) are never first-listed. They are always sequenced after the injury or condition code they describe. The injury code is first, followed by the external cause code indicating how, where, and what activity the patient was engaged in when the injury occurred. See Injury Coding and 7th Characters for details.

How the CPC Exam Tests Sequencing

Pattern 1 — Same Codes, Different Order

The four answer choices contain the same two or three codes arranged in different sequences. Only one sequence follows the Tabular List instructions and guideline rules. You must know whether “Code first” means the code is primary or secondary (it means secondary — the referenced code goes first).

Pattern 2 — Combination Code vs Separate Codes

The answer choices include a combination code option and a two-code option. When a combination code captures the complete clinical picture, it is the correct answer. When it does not fully describe all documented conditions, the correct answer includes the combination code plus additional codes.

Pattern 3 — Setting Identification

The question presents a clinical scenario and asks for the first-listed or principal diagnosis. You must identify the setting (inpatient or outpatient) to apply the correct selection rules — particularly for uncertain diagnoses.

Pattern 4 — Etiology/Manifestation Order

The answer choices present an etiology/manifestation pair in different orders. The correct answer always sequences the etiology first and the manifestation second. If an answer choice lists the manifestation code first, it is wrong.

Common Sequencing Mistakes

Reversing Code first instructions. “Code first” appears under the secondary code, not the primary. Some coders misread it as “this code goes first” — it actually means “put another code first before this one.”

Coding external causes as first-listed. External cause codes (V, W, X, Y codes) are never first-listed or principal diagnosis. They are always supplementary codes that follow the injury code.

Ignoring mandatory sequencing for flexible guidelines. Code first / Use additional code instructions are mandatory. Code also instructions are flexible. Do not treat mandatory instructions as optional.

Missing combination codes. Before assigning two separate codes in a specific sequence, check whether a combination code exists that captures both conditions in a single code. If it does, use the combination code instead.

Frequently Asked Questions