Overview
Field 73 on the UB‑04 claim form is officially designated as “Reserved for Assignment by the National Uniform Billing Committee (NUBC).” [1] The Center for Medicare & Medicaid Services (CMS) confirms that the field is “Reserved – Not used. Data entered will be ignored.” [2] In transmittal R1915CP, CMS reiterates: “FL 73 - Reserved Not used.” [3] The purpose of Field 73 is to hold a future data element that has not yet been defined by the NUBC. Until such time as the NUBC assigns a specific meaning, every payer—including Medicare and state Medicaid programs—treats this field as non‑functional. Any information placed in Box 73 will be ignored during processing, and no revenue code, payer identifier, or other billing data should be entered there. [2] Providers must therefore leave Field 73 completely blank to avoid confusion and potential claim rejections.
When to Use
Field 73 should never be used in current billing practice. All authoritative guidance states that it is reserved exclusively for future assignment by the NUBC and is currently inactive. [1] CMS explicitly notes that the field is “Not used” and that any “Data entered will be ignored.” [2] There are no circumstances where a provider should intentionally populate Box 73—not for an inpatient claim, not for an outpatient claim, and not for any other claim type. [3] Because the field serves no current purpose, its value is always blank. If a billing system automatically inserts a value into this field (for example, through a template error), the claim will still process, but the data will be discarded. [2] The correct action is to ensure that Field 73 remains empty on all UB‑04 forms and electronic equivalents.
Step-by-Step Claim Example
To illustrate proper handling, consider a Medicare inpatient claim for a 5‑day stay with a principal procedure. The biller completes the UB‑04 form following the CMS Chapter 25 guidelines. When reaching Box 73, the biller reviews the instruction: “FL 73 – Reserved Not used.” [2] The field remains blank; no characters, spaces, or zeros are entered. The claim is submitted electronically. The Medicare fiscal intermediary receives the 837I transaction. Because Field 73 has no assigned purpose, the clearinghouse software ignores any data that might have been inadvertently included. [3] The claim proceeds to adjudication without error. On the paper UB‑04, box 73 is simply empty. The biller verifies that other reserved fields (e.g., FL 68 for NUBC assignment, FL 75 for future use) are similarly left blank. [1] The claim pays correctly, and no audit flags are raised because Box 73 was properly omitted. In contrast, if the biller had mistakenly entered a condition code or provider identifier, that data would be silently dropped, but the claim would still process normally—albeit with an unnecessary risk of confusion in manual review. [2]
Common Mistakes & Audit Red Flags
A frequent error is populating Field 73 with information that belongs in another box, such as a plan ID or a provider number. Because the field is reserved, any data entered is ignored, but this practice can create audit red flags. [2] For example, a biller might mistakenly use Box 73 to record the National Provider Identifier (NPI) of the attending physician, which correctly belongs in FL 76. [1] Another common mistake is treating the field as an open remarks area. While FL 80 is dedicated to remarks, Box 73 has no such function. [3] Auditors reviewing a claim with data in Field 73 may question whether the provider understands current billing standards. Although the claim will not be denied solely because of data in an ignored field, internal audits and payer desk reviews may flag the claim for further investigation—particularly if the entered data appears to conflict with other fields. [2] To avoid these red flags, providers must configure billing software to leave Field 73 blank and train staff never to type anything in that box. Additionally, when using electronic claims, ensure the 837I or 837P loop segments that correspond to FL 73 are omitted or set to null. [3]
Related Codes/Fields
The following table lists other UB‑04 fields that share the reserved or future‑use status, as documented in the same sources.
| Field | Description | Source |
|---|---|---|
| FL 68 | Reserved for Assignment by NUBC | [1] |
| FL 73 | Future Use (this field) – Reserved, not used; data ignored | [2]; [3]; [1] |
| FL 75 | Reserved for Assignment by NUBC | [1] |
All three fields must be left blank on every UB‑04 claim until the NUBC assigns a specific data element. [2]
References
[1] Maryland Medicaid — FL 73 — https://health.maryland.gov/mmcp/provider/Documents/ffs-billing/UB04-Hospital-Billing-Instructions%20%281%29.pdf
[2] CMS Chapter 25 — FL 73 — 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
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Last Updated: 2026-06-03
Sources: CMS Pub. 100-04 Chapter 25, NUBC Official UB-04 Manual, Medicare Contractor Bulletins (Noridian, Palmetto, CGS)