OASIS M1021 and M1023: Primary and Other Diagnoses
Who picks the diagnoses, how to sequence them, how to rate symptom control, why symptom codes cause returned claims, and how the primary diagnosis ties to the PDGM clinical group.
The Short Answer
M1021 records the chief reason for home care. M1023 records up to five other diagnoses the plan of care addresses or which affect the patient's response to treatment. Both are completed at SOC and ROC, coded in ICD-10-CM to the highest specificity, sequenced by seriousness and the services provided, and rated 0-4 for symptom control. On the claim, the principal diagnosis sets the PDGM clinical group.
Part of the OASIS-E2 Item Guide. Sources: the CMS OASIS-E2 Guidance Manual Section I, the FY 2027 ICD-10-CM Official Guidelines (effective October 1, 2026), and Chapter 10 of the Medicare Claims Processing Manual.
What M1021 and M1023 Capture
- Primary diagnosis (M1021, row a): the chief reason the patient receives home care and the diagnosis most related to the current plan of care.
- Other diagnoses (M1023, rows b-f): comorbid conditions present at the assessment which the plan of care actively addresses, or which could affect the patient's response to treatment and rehab prognosis, even if no home health service targets them.
Column 1 holds the diagnosis description. Column 2 holds the ICD-10-CM code and the symptom control rating. Both items are collected at SOC and ROC only.
Who Assigns the Diagnoses
The assessing clinician decides the primary and other diagnoses after completing the comprehensive assessment. Inputs include assessment findings, practitioner orders, the medication list, referral information, and input from the physician or allowed practitioner. The manual lets a coding specialist enter the ICD-10-CM codes in Column 2, provided the clinician has already set the diagnoses in Column 1. HIPAA requires adherence to the ICD-10-CM Official Guidelines.
Report current diagnoses only. Leave out resolved conditions and conditions with no potential to affect skilled care. The manual's example: cholecystitis after a cholecystectomy is resolved. Diagnoses change during a stay, so reassess them at each time point where the item is collected.
Sequencing Rules
- Order diagnoses to reflect the seriousness of each condition and to support the disciplines and services provided.
- Order M1023 by how much each condition affects the patient's health and need for home care. Symptom control does not set the order. The manual ranks Type 2 diabetes controlled with difficulty above a poorly controlled toenail fungal infection.
- Fill Column 1 from the top down and leave any blank rows at the bottom.
- Code to the highest level of specificity. No surgical or procedure codes.
- Follow ICD-10-CM multiple-coding rules, such as etiology and manifestation pairs.
- External cause codes (V, W, X, Y) are not allowed in M1021. They are allowed in M1023.
- When a Z code is reported, the code for the underlying condition often follows it, if the condition is active and affects home care.
Symptom Control Ratings
| Rating | Meaning |
|---|---|
| 0 | Asymptomatic, no treatment needed now |
| 1 | Symptoms well controlled with current therapy |
| 2 | Controlled with difficulty, affecting daily function, needs ongoing monitoring |
| 3 | Poorly controlled, needs frequent treatment adjustment and dose monitoring |
| 4 | Poorly controlled, with a history of rehospitalizations |
Base the rating on presenting signs and symptoms, the type and number of medications, how often treatment changes, and how often the patient contacts providers. Do not rate V, W, X, Y, or Z codes. A dash is not valid for M1021 or M1023.
Symptom Codes as a Primary Diagnosis
The ICD-10-CM guidelines allow sign and symptom codes when the provider has not confirmed a related definitive diagnosis. Under Section II of the guidelines, symptom codes from Chapter 18 should not be the principal diagnosis once a related definitive diagnosis exists. The guidelines extend these principal diagnosis definitions to home health agencies. Symptoms routinely tied to a coded disease should not be coded separately.
Under PDGM, a symptom code in the first position also risks a returned claim. In rulemaking, CMS reported about 41% of all ICD-10-CM codes are not assigned to a clinical group. Coders often call the vague codes in this group "questionable encounter" codes. Check the current CMS grouper tables before you put any vague or symptom code first.
Why the Primary Diagnosis Drives the PDGM Clinical Group
The Medicare Claims Processing Manual describes how the Home Health Grouper works. Medicare combines claim data with OASIS data. From the claim, the Grouper reads the principal diagnosis and up to 24 secondary diagnoses. From the OASIS, it reads the functional items. The principal diagnosis places the 30-day period in one of 12 clinical groups:
- Musculoskeletal rehabilitation, neuro/stroke rehabilitation, wounds, complex nursing interventions, and behavioral health
- Medication management, teaching, and assessment (MMTA): surgical aftercare, cardiac/circulatory, endocrine, gastrointestinal/genitourinary, infectious disease/neoplasms/blood-forming diseases, respiratory, and other
Secondary diagnoses set the comorbidity adjustment of none, low, or high. The claim carries more secondary codes than the five rows in M1023, so the comorbidity adjustment depends on complete claim coding.
Returned claims: when the principal diagnosis is not assigned to a clinical group, the Grouper returns code 05 and Medicare returns the claim to the provider for correction. Since April 1, 2023, a separate validity flag (return code 03) catches other diagnosis coding issues.
The OASIS M1021 and the claim's principal diagnosis answer the same question: why is this patient receiving home care? When the two disagree, resolve the difference before billing. See the PDGM overview and the home health ICD-10 coding guide for more.
M1028 Active Diagnoses
M1028 remains on OASIS-E2 at SOC and ROC. It asks about two conditions tied to functional outcomes and pressure injury risk: response 1 for peripheral vascular disease or peripheral arterial disease, response 2 for diabetes mellitus, and response 3 for neither. Mark both 1 and 2 for a diabetic patient with PVD or PAD.
- The diagnosis must be documented by a physician or other allowed practitioner. A patient's report alone does not count.
- The diagnosis must be active, meaning it relates to current function, treatment, nurse monitoring, or risk of death.
- A complication such as neuropathy does not imply diabetes without a documented diabetes diagnosis.
Because these diagnoses raise pressure injury risk, M1028 pairs with the skin items in the M1311 guide.
Common Errors
- Copying the hospital's principal diagnosis when the home care focus is different.
- Sequencing by symptom control rating instead of impact on care.
- Listing resolved conditions.
- Leading with a symptom code after a definitive diagnosis is documented.
- Placing an external cause code in M1021.
- Rating symptom control on a Z code.
- Coding M1028 diabetes from a patient's report with no practitioner documentation.
Where Documentation Goes Wrong
Diagnosis errors often start when the visit conversation reveals a focus missing from the referral. Lime's OASIS Scribe drafts OASIS responses from the visit conversation and flags inconsistencies, such as a primary diagnosis out of step with the services in the plan, so your QA and coding team corrects them. The assessing clinician still determines the diagnoses.
CMS Sources
- OASIS-E2 Guidance Manual (PDF): Section I, M1021, M1023, M1028.
- ICD-10-CM Official Guidelines, FY 2027: Sections I.B.4-5 and II.A.
- Medicare Claims Processing Manual, Chapter 10: Home Health Grouper and return codes.
- CMS PDGM page: the 12 clinical groups.
- CY 2026 Home Health PPS final rule: case-mix variables and unassigned codes.
Related Item Guides
FAQ
M1021 and M1023, Answered
Common questions on OASIS diagnosis coding under OASIS-E2.
M1021 Primary Diagnosis records the chief reason the patient receives home care, the diagnosis most related to the current plan of care, with its ICD-10-CM code and a symptom control rating from 0 to 4. It is completed at Start of Care and Resumption of Care.
List the other diagnoses in order of how much each one affects the patient's health and need for home care, not by symptom control rating. Fill the rows from the top down, leave blanks at the bottom, code to the highest specificity, and follow ICD-10-CM sequencing rules for etiology and manifestation pairs.
ICD-10-CM guidelines allow symptom codes only when the provider has not confirmed a related definitive diagnosis. Even then, many symptom codes are not assigned to a PDGM clinical group. If the claim's principal diagnosis has no clinical group, Medicare returns the claim to the provider for correction.
The PDGM grouper reads the principal diagnosis from the home health claim, not from the OASIS. Both should name the same chief reason for care, though. A mismatch between M1021 and the claim's principal diagnosis is a QA flag worth resolving before billing.
Yes. M1028 Active Diagnoses asks whether the patient has an active, practitioner-documented diagnosis of peripheral vascular disease or peripheral arterial disease (response 1), diabetes mellitus (response 2), or neither (response 3). It is completed at Start of Care and Resumption of Care.
Yes, with a condition. The OASIS-E2 manual lets an agency coding specialist enter the ICD-10-CM codes in Column 2, as long as the assessing clinician has determined the primary and other diagnoses in Column 1.
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