ICD-10-CM Fundamentals

Note: this course targets the FY2026 guideline cycle. See the Overview section's "Recent Guideline Changes (FY 2026)" for substantive revisions (HIV sequencing, BMI/Z68, SDoH codes, new diabetes-remission code, expanded R10 pain codes) since older FY2023-era material.

How to Look Up a Code

The ICD-10-CM manual has two parts:

1.Alphabetic Index — Look up conditions by name → gets you to a code

2.Tabular List — Verify the code, check for required characters, read notes

Always verify in the Tabular List. Never code from the Alphabetic Index alone.

Step-by-Step Process

Step 1: Identify the main term from the diagnosis
Step 2: Look up the main term in the Alphabetic Index
Step 3: Review any subterms (qualifiers, sites, type)
Step 4: Note the suggested code
Step 5: Turn to the Tabular List and verify
Step 6: Read all includes, excludes, and notes
Step 7: Assign additional characters as required
Step 8: Confirm code to highest level of specificity

Example: Coding Acute Appendicitis

Diagnosis: Acute appendicitis with perforation

Step 1: Main term = "Appendicitis"
Step 2: Alphabetic Index → Appendicitis → acute → with peritonitis
Step 3: Index suggests K35.2
Step 4: Tabular List: K35.2 = "Other and unspecified acute appendicitis"
        K35.20 = Acute appendicitis with perforation, without abscess
        K35.21 = Acute appendicitis with perforation and localized peritonitis
Step 5: Select K35.20 (perforation, no abscess documented)

Common Conditions and Codes

Diabetes Mellitus

Code	Description

|------|-------------|

E10._	Type 1 diabetes mellitus
E11._	Type 2 diabetes mellitus
E11.9	Type 2 DM without complications
E11.65	Type 2 DM with hyperglycemia
E11.649	Type 2 DM with hypoglycemia without coma
E11.40	Type 2 DM with diabetic neuropathy, unspecified
E11.311	Type 2 DM with unspecified diabetic retinopathy with macular edema
E11.22	Type 2 DM with diabetic chronic kidney disease, stage 3

Key rule: Diabetes codes require coding the type first. Always code the manifestation with a Use Additional Code note.

Hypertension

Code	Description

|------|-------------|

I10	Essential (primary) hypertension
I11.0	Hypertensive heart disease with heart failure
I11.9	Hypertensive heart disease without heart failure
I12.9	Hypertensive chronic kidney disease with stage 1-4 CKD
I13.10	Hypertensive heart and CKD without heart failure, stage 1-4

Key rule: ICD-10-CM assumes a causal relationship between hypertension and heart disease and between hypertension and CKD. Code with combination codes.

Respiratory

Code	Description

|------|-------------|

J06.9	Acute upper respiratory infection, unspecified
J18.9	Pneumonia, unspecified organism
J44.0	COPD with acute lower respiratory infection
J44.1	COPD with acute exacerbation
J45.20	Mild intermittent asthma, uncomplicated
J45.51	Severe persistent asthma with acute exacerbation

Mental Health

Code	Description

|------|-------------|

F32.9	Major depressive disorder, single episode, unspecified
F33.0	Major depressive disorder, recurrent, mild
F41.1	Generalized anxiety disorder
F10.20	Alcohol use disorder, moderate
F17.210	Nicotine dependence, cigarettes, uncomplicated

Z Codes — Factors Influencing Health

Z codes are used when a patient has no illness but needs healthcare, or for factors affecting health:

Z00.00   Encounter for general adult medical examination without abnormal findings
Z23      Encounter for immunization
Z51.11   Encounter for antineoplastic chemotherapy
Z79.4    Long-term (current) use of insulin
Z87.891  Personal history of nicotine dependence
Z82.49   Family history of ischemic heart disease
Z96.641  Presence of right artificial hip joint

Sequencing Rules

Inpatient (UHDDS Guidelines):

1.Principal diagnosis = condition after study chiefly responsible for admission

2.Sequence complication codes after the underlying condition

3.For DRG, sequence the code that drives the highest reimbursement (when equal clinical validity)

Outpatient:

1.Code the reason for the visit (chief complaint if diagnosis not confirmed)

2.Use "possible," "probable," "suspected" diagnoses only in inpatient settings

3.For outpatient: code to highest degree of certainty documented

Signs and Symptoms:

•Code signs/symptoms when no definitive diagnosis established

•Do NOT code symptoms when they are routinely associated with the confirmed diagnosis

- Example: Chest pain is symptom of MI — don't code both for inpatient

Practice Case 1

Documentation: 65-year-old male admitted for acute anterior ST elevation MI. He also has type 2 diabetes managed with insulin and stage 3 chronic kidney disease secondary to diabetic nephropathy. Has been a smoker for 30 years.

Codes:

I21.09   ST elevation (STEMI) myocardial infarction involving other coronary artery of anterior wall
E11.22   Type 2 diabetes mellitus with diabetic chronic kidney disease, stage 3
N18.3    Chronic kidney disease, stage 3 (moderate)
Z79.4    Long-term (current) use of insulin
F17.210  Nicotine dependence, cigarettes, uncomplicated

Rationale:

•STEMI is the principal diagnosis (reason for admission)

•Combination code E11.22 captures DM + CKD relationship

•N18.3 required as "use additional code" per E11.22 instructions

•Z79.4 required for insulin use per diabetes guidelines

•Tobacco use coded as an additional diagnosis affecting management

ICD-10-CM — Fundamentals

ICD-10-CM Fundamentals

How to Look Up a Code

Step-by-Step Process

Example: Coding Acute Appendicitis

Common Conditions and Codes

Diabetes Mellitus

Hypertension

Respiratory

Mental Health

Z Codes — Factors Influencing Health

Sequencing Rules

Practice Case 1