UB04 Field 37: Internal Control Number

Overview

^[1] explicitly states “FL 37 NOT USED” in its UB-04 instructions. CMS reinforces this in Chapter 25 of the Medicare Claims Processing Manual, noting “FL 37 – (Untitled) Not used. Data entered will be ignored.” ^[2] Similarly, CMS Transmittal R1915CP repeats that the field is not used. ^[3] Any information placed in Field 37 is stripped or ignored during electronic claims processing, making it a common source of confusion but not a functional data element.

When to Use

Under current billing standards, Field 37 should never be populated. It is neither required for inpatient nor outpatient claims, as confirmed by both Maryland Medicaid and Medicare guidelines. ^{[1] [2]} If a provider needs to include a control number for internal reconciliation, it should be placed in a non-claim document such as an internal log or a separate reference field in the practice management system. Using Field 37 for any purpose risks claim rejection or delays because the clearinghouse or payer may reject a claim containing unexpected or unrecognized data. Therefore, the only appropriate action is to leave the field completely blank in both paper and electronic submissions.

Step-by-Step Claim Example

Scenario: A hospital submits a UB-04 claim for a 3-day inpatient stay. Below is how Field 37 should appear at each stage:

1. Paper UB-04 (hardcopy): Box 37 is left empty. Do not print any numbers, dashes, or placeholder text. The box may appear visually blank.
2. Electronic submission (837I): The segment that maps to Field 37 (the CLM04 loop reference) must be omitted or sent with a value of “null.” Most billing software defaults to leaving this field empty; verify that no system-generated internal control number is automatically inserted.
3. Clearinghouse edit: The clearinghouse will strip any data in Field 37. If a value is present, it will be ignored by Medicare and most payers. ^[2] No rejection is generated for a filled Field 37, but the data will not appear on the finalized claim.
4. Payer adjudication: The payer’s system reads the claim and treats Field 37 as nonexistent. No audit trail or reference number from this field will be accessible.

Result: The claim processes correctly, relying on fields such as Patient Control Number (Field 3) or Treatment Authorization Code (Field 63) for identification. The internal control number is irrelevant to payment.

Common Mistakes & Audit Red Flags

• Mistake 1: Entering a proprietary ID. Some providers attempt to use Field 37 for internal tracking numbers. This is wasted effort because the data is ignored and cannot be referenced during post-payment audits. ^[3] Audit red flag: Payers may notice extraneous characters and flag the claim for manual review, delaying payment.
• Mistake 2: Confusing Field 37 with Patient Control Number (Field 3). Field 3 is the proper location for a unique patient or account number that the provider uses for reconciliation. Using Field 37 instead of Field 3 leads to lost tracking information.
• Mistake 3: Leaving old data in the box on resubmissions. When a corrected claim is produced, verify that Field 37 remains blank. If a previous submission had data, the resubmission must not carry over that information.
• Red flag: Data present in Field 37 on a high-volume claim. Payers may interpret unexpected data as an error and issue a “suspense” for manual review. Although the data is ultimately ignored, the extra handling can cause a 2‑ to 4‑week delay. ^[1]
• Best practice: Instruct billing staff that Field 37 is strictly off-limits. Update electronic health record templates to gray out or disable input for this field.

Related Codes/Fields

Field Number	Field Name	Relationship to Field 37
3	Patient Control Number	Primary field for provider’s internal unique ID. Field 37 is often mistakenly used for this purpose. Use Field 3 instead.
63	Treatment Authorization Code	Another control number field. Unlike Field 37, this is actively used for prior authorization numbers.
38	Responsible Party Name & Address	Not used for Medicare but may be required for other payers. Contrast with Field 37 which is universally unused.
49	Reserved for Assignment by NUBC	Another “not used” field similar to Field 37. Both should remain blank.
50	Payer Name	Active field. Field 37 holds no payer-identifying information.
56	NPI – Billing Provider	Required identifier; Field 37 is never an identifier.

These fields are defined in the same NUBC specifications

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References

[1] Maryland Medicaid — FL 37 — https://health.maryland.gov/mmcp/provider/Documents/ffs-billing/UB04-Hospital-Billing-Instructions%20%281%29.pdf

[2] CMS Chapter 25 — FL 37 — https://www.cms.gov/regulations-and-guidance/guidance/manuals/downloads/clm104c25.pdf

[3] CMS R1915CP — https://www.cms.gov/Regulations-and-Guidance/Guidance/Transmittals/downloads/R1915CP.pdf