ICD-10-CM — Complete Coding Guide (FY2026)

What is ICD-10-CM?

ICD-10-CM (International Classification of Diseases, 10th Revision, Clinical Modification) is the US standard diagnostic coding system used for:

•Medical billing (claims to Medicare, Medicaid, private insurance)

•Statistical tracking and public health reporting

•Home health documentation (OASIS + PDGM)

•Inpatient and outpatient facility coding

Code structure: Alphanumeric, 3–7 characters

•Character 1: Letter (A–Z)

•Characters 2–3: Numbers (subcategory)

•Character 4+: Specificity (etiology, anatomic site, severity, laterality)

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ICD-10-CM Chapter Structure (A–Z)

Chapter	Categories	Subject

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

1	A00–B99	Infectious and Parasitic Diseases
2	C00–D49	Neoplasms
3	D50–D89	Blood Diseases
4	E00–E89	Endocrine, Nutritional, Metabolic (Diabetes here)
5	F01–F99	Mental & Behavioral
6	G00–G99	Nervous System
7	H00–H59	Eye and Adnexa
8	H60–H95	Ear and Mastoid
9	I00–I99	Circulatory System (HTN, Heart disease)
10	J00–J99	Respiratory System (COPD, Pneumonia)
11	K00–K95	Digestive System
12	L00–L99	Skin and Subcutaneous
13	M00–M99	Musculoskeletal (Arthritis, fractures)
14	N00–N99	Genitourinary
15	O00–O9A	Pregnancy, Childbirth
16	P00–P96	Perinatal
17	Q00–Q99	Congenital malformations
18	R00–R99	Symptoms, Signs (unspecified — avoid when specific known)
19	S00–T88	Injuries, Poisoning
20	V00–Y99	External causes (cause of injury)
21	Z00–Z99	Factors Influencing Health (Z codes — history, status)

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Recent Guideline Changes (FY 2026)

This course targets the FY2026 guideline cycle (effective October 1, 2025, updated again April 1, 2026). The most consequential changes since FY2023: a substantially revised HIV coding/sequencing guideline (Section I.C.1.a.2), a revised BMI (Z68) coding rule requiring an associated reportable diagnosis, expanded Social Determinants of Health Z-codes, a new diabetes-remission code (E11.A), and a significant expansion of abdominal/pelvic pain codes (R10 category, including new flank-pain and multi-site-pain codes). See this course's Coding Guidelines module for the full breakdown. Always verify against the current-year official guidelines (cms.gov / cdc.gov) rather than an older PDF or study guide.

Coding Conventions — Must Know

Alphabetic Index vs Tabular List

1.Always use Alphabetic Index FIRST — find the term, locate the code suggestion

2.Verify in Tabular List — never code from the index alone; tabular has instructional notes

Key Conventions

Etiology/Manifestation (E/M) Convention:

•When a disease causes another condition, code the underlying disease first, then the manifestation

•Manifestation codes are in [brackets] in the index

•Example: Diabetic retinopathy — code diabetes first (E11.3-), then retinopathy

Code first (CF): Instructional note at manifestation code — sequence this code after the underlying condition

Use additional code: Add a code to provide further detail (e.g. body mass index, smoking status, resistance to antibiotics)

Excludes 1: Codes should NEVER be used together (mutually exclusive)

Excludes 2: Condition may coexist — patient may have both conditions

NOS (Not Otherwise Specified): Use when documentation is insufficient to assign a more specific code. Equivalent to "unspecified."

NEC (Not Elsewhere Classified): Use when a specific code exists but not for this particular condition.

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High-Complexity Coding Areas

Diabetes Mellitus (Chapter 4)

Most common coding scenario — must get right every time.

Type 1 vs Type 2 vs Secondary:

•E10- : Type 1 Diabetes Mellitus (insulin-dependent, autoimmune)

•E11- : Type 2 Diabetes Mellitus (most common; may or may not use insulin)

•E13- : Other specified diabetes (secondary to another condition, drug-induced)

•O24- : Diabetes in pregnancy (separate category — O codes)

"Diabetes with" rule: ICD-10 Official Guidelines — if patient has diabetes AND a condition commonly associated with diabetes, assume a causal relationship unless documented otherwise (no need for physician to explicitly say "due to diabetes").

Commonly linked diabetic complications:

•E11.65 = Type 2 DM with hyperglycemia

•E11.40 = Type 2 DM with diabetic neuropathy, unspecified

•E11.311 = Type 2 DM with unspecified diabetic retinopathy with macular edema

•E11.621 = Type 2 DM with foot ulcer (also add L97.- for site and severity)

•E11.641 = Type 2 DM with hypoglycemia with coma

•E11.9 = Type 2 DM without complications

•E11.A = Type 2 DM without complications, in remission (new code, added FY2026 — did not exist in earlier fiscal years)

Hypertension (Chapter 9)

•I10: Essential (primary) hypertension — single code for most patients

•HTN + Heart disease = Hypertensive Heart Disease (I11-) — causal relationship assumed

- I11.0 = Hypertensive heart disease with heart failure (also code I50.- for type of HF)

- I11.9 = Hypertensive heart disease without heart failure

•HTN + CKD = Hypertensive CKD (I12-) — causal relationship assumed

- I12.9 + N18.3 = HTN with CKD Stage 3

•HTN + Heart + CKD = I13- (combination code for all three)

•Secondary hypertension (I15-): Due to renovascular, endocrine, etc. — code underlying cause first

Sepsis

Sepsis coding is complex — requires careful application of guidelines.

Sepsis: Life-threatening organ dysfunction due to dysregulated host response to infection.

Severe Sepsis: Sepsis + acute organ dysfunction

Septic Shock: Severe sepsis + vasopressor requirement despite adequate fluid resuscitation

Coding sepsis:

1.Primary/Admitted for sepsis: Code sepsis first + the organism

- A41.9 + causative organism (A41.01 for MRSA sepsis)

2.Sepsis develops during hospitalization: Code the condition causing hospitalization first, then sepsis as complication

3.Severe sepsis: Code sepsis + R65.20 (without septic shock) or R65.21 (with septic shock)

Note: Sepsis code NEVER goes as principal diagnosis when caused by postprocedural infection — code the complication code first.

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OASIS (Outcome and Assessment Information Set)

OASIS is required for Medicare/Medicaid home health patients to determine PDGM payment group.

Key OASIS data items for coding:

•M1021/M1023: Primary/secondary diagnoses — must reflect patient's current conditions

•M1033: Risk of hospitalization

•M1800-M1870: Functional status (bathing, dressing, grooming, mobility)

•M2020: Management of oral medications

•M2200: Therapy need

PDGM (Patient Driven Groupings Model):

Payment system since January 2020 for 30-day home health periods:

1.Timing: Early (first 30 days) or Late (subsequent)

2.Admission source: Community or Institutional (hospital/SNF)

3.Clinical grouping: 9 categories (MMTA, Behavioral Health, Complex Nursing, etc.)

4.Functional impairment: Low, Medium, High

5.Comorbidity adjustment: None, Low, High

These 5 factors determine one of 432 PDGM groups and the base payment rate.

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Coding Scenarios — Practice

Scenario 1: Patient admitted for right lower extremity cellulitis. MRSA confirmed.

•L03.115 (Cellulitis of right lower limb) + B95.62 (MRSA as cause)

Scenario 2: Home health patient, Type 2 DM with diabetic peripheral neuropathy and Stage 3 CKD.

•E11.40 (T2DM with neuropathy) + N18.3 (CKD Stage 3) + I12.9 (hypertensive CKD, if HTN present)

Scenario 3: Fall from ladder at home; fracture left distal radius.

•S52.501A (Unspecified fracture of lower end of left radius, initial encounter)

•W11.XXXA (Fall on/from ladder, initial encounter)

•Y93.89 (Activity, other specified)

Scenario 4: Patient with COPD exacerbation and pneumonia (organism not identified).

•J44.1 (COPD with acute exacerbation) + J18.9 (Pneumonia, unspecified)

•Note: When COPD and pneumonia coexist, both are coded. The reason for the visit determines sequencing.

ICD-10-CM — Overview