Occurrence Code 11: What It Means & When to Use It in Medical Billing

Occurrence Code 11 is an event date code on the UB‑04 claim form that indicates the date the patient first became aware of their symptoms or illness. In other words, it captures the “onset of illness” date for the condition being treated (Microsoft Word – UB04directions 3.doc). On the UB‑04 form, occurrence codes go in Form Locators 31–34 along with their dates (Microsoft Word – UB04directions 3.doc). By convention, code 11 is defined as “Illness – Onset of Symptoms.” (For comparison, occurrence codes 01–05 cover accidents; 12 covers chronic dependency; and codes 35, 44, 45, 46 cover therapy start dates.) Using code 11 tells payers when the patient’s condition began, which can help establish medical necessity and coverage timelines.

According to CMS/NUBC guidelines, Occurrence Code 11 “indicates the date patient first became aware of symptoms/illness”. This is its official purpose: to record the initial onset date of the condition treated. For example, if a patient’s symptoms began on January 10, 2025, the UB‑04 claim would show Occurrence Code 11 with date 01/10/2025. This is distinct from the date of treatment or admission – it literally marks the start of the patient’s illness. (In some payer rules, a related medical event like a surgery date can substitute, as noted below.) On paper claims this appears in Box 31–34, and in electronic 837I claims it appears in the Occurrence Code loop (Loop 2300) with the associated date.

When and Why to Use Occurrence Code 11

Occurrence Code 11 is primarily used on institutional (UB‑04) claims for outpatient services, especially rehabilitation/therapy claims. For Medicare Part A outpatient and outpatient hospital claims, code 11 is used to indicate onset of symptoms when services like physical, occupational, or speech therapy are provided (Occurrence Codes – JE Part A – Noridian). (In fact, Medicare’s guidelines emphasize that only outpatient claims use code 11.) For example, a hospital outpatient rehab department billing Medicare would include code 11 on the claim to show when the patient’s condition began (Occurrence Codes – JE Part A – Noridian).

Importantly, if a beneficiary receives multiple therapy types (PT, OT, SLP) in the same rehab episode, only a single Occurrence Code 11 is needed. Medicare’s Noridian contractor notes that when PT/OT/SLP are all provided, you report one code 11 date – not one per therapy (Occurrence Codes – JE Part A – Noridian). This avoids duplicate onset entries. Other therapy-related occurrence codes (35, 44, 45, 46) are used for treatment-start dates, but code 11 remains one per claim to mark when symptoms first appeared.

In practice, Occurrence Code 11 is most relevant when treatment is linked to an acute event or identifiable illness onset. For example, if a patient fell on June 1 (causing a broken arm) and began therapy on June 15, code 11 would be 06/01/2025 (onset of injury). If the patient’s illness (e.g. pneumonia) began on March 10 and therapy started March 15, code 11 = 03/10/2025. In contrast, inpatient hospital claims generally do not use code 11 (Medicare Medicare’s outpatient-only rule implies this) – it’s reserved for outpatient and similar contexts.

Payer-Specific Variations

• Medicare (CMS): For Medicare Part A (hospital) outpatient claims, Occurrence Code 11 is required for outpatient therapy bills. Noridian (a Medicare Administrative Contractor) specifies “Outpatient claims only” and requires one code 11 for multiple PT/OT/SLP services (Occurrence Codes – JE Part A – Noridian). CMS rules (NUBC) define code 11 as “date patient first became aware of symptoms/illness”. On inpatient Medicare claims, code 11 is typically not used or not applicable.
• Medicaid and State Plans: Many Medicaid programs follow CMS/NUBC definitions, but there can be nuances. For instance, Indiana Medicaid explicitly states that “the date of onset or surgery must be indicated with occurrence code 11 on the claim” for rehab services (Therapy Services). In other words, Indiana allows code 11 to represent either the symptom onset date or a related surgery date (whichever triggered the rehab). This highlights that some payers broaden code 11’s meaning to include surgical onset. Other states or Medicaid plans may have similar rules – always check your payer’s billing guide. (If not, stick to the symptom onset date as defined by CMS.)
• Commercial Insurers: Private payers often adopt NUBC coding guidelines, but requirements vary. Some commercial plans may require code 11 on rehab claims much like Medicare, while others leave it optional. If the insurer’s billing manual doesn’t mention occurrence codes, it typically isn’t mandatory – but including the correct onset date can help the claim reviewer understand the case. When in doubt, consult payer-specific instructions or provider portals.

Below is a quick reference for Occurrence Code 11 usage by payer type:

How Occurrence Code 11 Affects Claims and Reimbursement

Occurrence Code 11 itself does not directly change the payment amount, but it supports correct claim processing. It provides context (the condition’s start) that payers use for review. If Occurrence Code 11 is required (e.g. Medicare outpatient rehab), omitting it can cause the claim to pend, be returned, or even deny. Many Medicare MACs have bulletin notes that outpatient therapy claims (rev code 42x, 43x, 44x, etc.) must include code 11 (onset date), otherwise edits may apply. For example, one MAC reminder specifies that for physical therapy (rev 42x) lines, “occurrence code 11 (onset of illness/injury) … must appear on the claim” along with the appropriate value codes.

Although code 11 doesn’t have a dollar-value itself, it can influence coverage decisions. For instance, if a therapy claim shows an onset date far before the claim date, the payer may question gaps in treatment or coverage. Conversely, a same-day onset and service date (e.g. surgery and immediate therapy) suggests continuity. Correct use of code 11 can also distinguish an injury (accident code) from a natural illness, which may shift payer liability (auto vs. health insurer). In secondary-payer situations, knowing the onset date helps determine if primary coverage should have paid (e.g. a work-related injury code vs. illness onset date).

Bottom line: Always report Occurrence Code 11 if the payer’s rules call for an onset date. Doing so ensures the claim meets “completeness” checks. Omitting a required code 11 may trigger a missing occurrence date edit. Conversely, avoid putting code 11 on claims where it isn’t needed (such as inpatient or non-therapy outpatient bills), as it could cause confusion. (Note: Occurrence codes 31–34 on UB-04 are intended for situational dates – don’t fill them just to use codes. Only supply those relevant to the case.)

Common Mistakes (and How to Avoid Them)

• Skipping Code 11 on Required Claims: Failing to include occurrence code 11 on an outpatient therapy claim will likely lead to a return or denial. Always check if the claim’s Type of Bill or revenue codes imply therapy services. If so, ask the clinician for the symptom onset date and enter it with code 11 (Occurrence Codes – JE Part A – Noridian).
• Using Code 11 on Wrong Claim Type: Remember Occurrence Code 11 is for institutional (UB-04) claims only – it does not belong on professional (CMS-1500) claims. Also, Medicare generally limits code 11 to outpatient claims. Do not report it on inpatient hospital claims; inpatient claims may use different occurrence or condition codes (like admission/discharge reasons).
• Entering the Wrong Date: The date with code 11 must be the date of symptom onset or illness, not the first therapy date. For example, don’t enter the first day of PT; instead, enter when the patient actually began feeling sick or injured. (Some payers allow surgery date if surgery was the inciting event – see payer policy.)
• Double-Reporting for Multiple Therapies: As noted, even if a patient has PT, OT, and SLP, do not put Occurrence Code 11 three times. Use it only once with the earliest onset date. (Then use codes 35, 44, 45, etc., for each therapy’s start date, if needed (Occurrence Codes – JE Part A – Noridian).)
• Confusing with Accident Codes: Occurrence Code 11 is not for accidents. If the patient’s issue was due to an auto accident, slip/fall, work injury, etc., use the appropriate accident code (01–05) with its date (Microsoft Word – UB04directions 3.doc). Only use 11 when there is no external accident causing the illness – for natural disease onset. Mixing these up can cause claim routing errors (e.g. wrongly routing to Workers’ Comp or auto liability).
• Date Format Errors: On paper forms, dates must be MM/DD/CCYY format. In 837s, use YYYYMMDD. Mistyping or swapping month/day leads to errors. Double-check that the date in code 11 matches the documentation (e.g. history notes or surgery report).
• Overlooking Payer Rules: Some mistakes come from assuming all payers treat code 11 the same. Always review the payer’s claim submission instructions. For instance, a commercial plan might ignore code 11 entirely, while a Medicaid plan might use it for surgical onset. When uncertain, clarify with the payer’s provider manual or help desk.

Real-World Examples

• Outpatient Physical Therapy Clinic: Jane Doe twisted her ankle on March 5, 2025. Her doctor referred her to PT starting March 15. On the UB‑04 for those PT visits, the biller lists Occurrence Code 11 = 03/05/2025 (onset of injury) and Occurrence Code 35 = 03/15/2025 (date PT began). This tells the insurer exactly when the injury occurred relative to treatment.
• Hospital Outpatient Rehab (PT/OT/SLP): Mr. Smith had a stroke with symptoms beginning January 10, 2025. He received speech and occupational therapy starting January 20. The hospital’s claim includes Occurrence Code 11 = 01/10/2025 (stroke symptom onset) and other therapy codes (e.g. 35, 44, 45 for treatment start dates). Because he had multiple therapy types, the coder only uses one code 11 for the single onset date, per CMS guidance (Occurrence Codes – JE Part A – Noridian).
• Cardiac Rehabilitation (Indiana Medicaid Example): A patient underwent coronary bypass surgery on February 1, 2025, then went to outpatient cardiac rehab. Indiana Medicaid rules say Occurrence 11 can be “onset or surgery.” The claim lists Occurrence Code 11 = 02/01/2025 (surgery date) and Occurrence Code 46 = 02/08/2025 (first cardiac rehab session) (Therapy Services). Value Code 53 (total rehab visits) is also used, but Occurrence 11 identifies the start of the condition (the surgery).
• No Clear Onset: If a patient’s condition developed gradually (e.g. arthritis) with no single onset date, sometimes the policy allows using the date the condition was first documented or diagnosed. In those cases, billers will typically enter the earliest reasonable date in code 11, or leave it blank if the payer does not require it. When in doubt, document the rationale (e.g. “chronic onset”) in claim notes or comments.

Each of these examples shows Occurrence Code 11 anchoring the claim to a key event. It helps payers verify whether the timing of services is appropriate. Including it correctly speeds up claim processing.

FAQs: Occurrence Code 11

• What exactly is Occurrence Code 11 used for?
  It marks the onset date of symptoms or illness for the condition being treated. Use it on UB‑04 claims (especially outpatient rehab) to show when the patient first got sick or injured.
• Which claims require Occurrence Code 11?
  Medicare outpatient hospital and rehab claims generally require it. Check if the claim involves therapy (e.g. revenue codes 042x-047x). If yes, include code 11. Inpatient hospital claims do not use it for Medicare. For other payers, review their instructions: e.g. some Medicaid plans require it on rehab claims, others do not.
• What date should I put with Occurrence Code 11?
  Give the date the patient first noticed symptoms or became ill. If there was a triggering event (e.g. surgery), some payers allow using that date. Always match this to the patient’s chart/notes. Do not use the first visit or first therapy date – those go with their own codes (35, 44, etc.).
• Can I report multiple Occurrence Code 11 entries if I have multiple conditions?
  Only one Occurrence Code 11 is allowed per claim (because you can only list one date for code 11). If a patient has two unrelated issues, they typically need separate claims or explanation. For therapy: even if a patient gets PT and SLP, you still use one Occurrence 11 date (the earliest onset) for that claim (Occurrence Codes – JE Part A – Noridian).
• Do I need Occurrence Code 11 on a CMS-1500 (professional) claim?
  No. Occurrence codes 01–49 and values belong on the institutional UB‑04 form (or electronic 837I). Professional (CMS-1500) claims have different fields. If you bill therapy as a professional service, instead the date of service and other details cover it (and sometimes an additional remark if requested).
• How is Occurrence Code 11 different from the accident codes (01–05)?
  Accident codes (01–05) are for traumatic injury events (auto accident, fall, etc.). Code 11 is for non-accident illness/injury. If the patient’s issue was from an accident, use the appropriate accident code and date, not 11. Only use 11 when there’s no third-party liability or accident – it’s purely “sickness onset.”
• What other codes often appear with 11 on therapy claims?
  Common companions include Occurrence Code 35 (Date PT started), 44 (OT start), 45 (SLP start), and 46 (cardiac rehab start) to mark therapy begin dates (Occurrence Codes – JE Part A – Noridian) (Therapy Services). Value Code 53 (Total visits) or 52 (repeat treatments) may also appear. For example, a claim might show: 11/01/2025 (code 11), 11/05/2025 (code 35), 11/05/2025 (code 44), 11/05/2025 (code 45), plus Value Code 53 = 10 sessions (Therapy Services).
• What if I don’t have an onset date?
  If truly unknown, check medical records: perhaps the first mention of symptoms. If absolutely nothing, some coders use the admit date or start-of-care date, but this can be risky. It’s best to document “unknown” in notes and explain that in supporting documentation. Some payers might allow leaving code 11 blank if not applicable, but verify the payer’s policy first.

Summary / Quick Checklist

• Definition: Occurrence Code 11 = “Onset of Symptoms/Illness” – date patient first knew of the condition.
• UB-04 location: Enter in Form Locators 31–34 with the date (Microsoft Word – UB04directions 3.doc).
• When to use: Primarily on outpatient institutional (UB‑04) claims, especially therapy/recovery services (Medicare rehab claims, etc.) (Occurrence Codes – JE Part A – Noridian). Not used on CMS-1500 or typically inpatient claims.
• What date to put: Patient’s symptom onset date (or surgery date if payer allows) – not the therapy start date. Double-check patient records.
• Multiple therapies: Use only one code 11 per claim, even if PT, OT, SLP are all billed (Occurrence Codes – JE Part A – Noridian).
• Companion codes: Often paired with Occurrence Codes 35, 44, 45, 46 (therapy start dates) and Value Codes (e.g. 53 = number of visits) on rehab claims (Therapy Services).
• Avoid mistakes: Don’t confuse with accident codes (01–05), don’t duplicate it, and don’t omit it when required. Check payer rules – for example, Medicare requires it on outpatient therapy.
• Check payer guidelines: Some Medicaid/commercial plans may have special uses (e.g. Indiana Medicaid lets code 11 be onset or surgery date (Therapy Services)). Always follow the latest manuals or bulletins.

By carefully documenting Occurrence Code 11 with the correct date, medical billers ensure claims are clear and compliant. This small data element can prevent processing delays and illustrate medical necessity. Keep these points in mind, and refer to official UB‑04 coding guides or payer manuals whenever questions arise.

Sources: Official NUBC/CMS definitions and Medicare guidance on Occurrence Codes (Occurrence Codes – JE Part A – Noridian) (Microsoft Word – UB04directions 3.doc), plus payer instructions (e.g. Indiana Medicaid policy) (Therapy Services). These ensure the above advice is accurate and up-to-date.

UB-04 Condition & Occurrence Codes Explained: FL 18–36 Guide (2025)