UB04 Patient Sex Code U: Unknown

Overview

The UB-04 patient sex field (Form Locator 11) is a mandatory one-character alphanumeric field used to capture the patient’s sex as part of institutional claim submission. According to the Form CMS-1450 layout summary in the Medicare Claims Processing Manual, Field 11 is defined as “Patient Sex” with a size of “1 AN” (one alphanumeric character) and a buffer space of 2 ^[1]. The National Uniform Billing Committee (NUBC) establishes and maintains the approved set of codes for this field ^[1]. The MassHealth UB-04 Billing Guide references a “Code Sets for the UB-04 Claim Form” section, but the excerpt does not include the actual code definitions ^[2]. Therefore, while the field itself is well-documented in the layout, the explicit mapping of code “U” to “Unknown” must be verified against official NUBC publications or payer‑specific instructions.

When to Use

The NUBC code set (not fully reproduced in the provided sources) includes several values; code “U” is distinct from “M” (Male), “F” (Female), and other potential values. Because the CMS layout expects exactly one alphanumeric character, the value must be entered without any additional spaces or punctuation ^[1]. The MassHealth guide does not provide specific instructions on when to use “U,” but the general requirement to complete the patient identification fields accurately applies ^[2]. Use code “U” only as a last resort when all reasonable efforts to obtain the correct sex have been exhausted.

Step-by-Step Claim Example

Assume a hospital is submitting a UB-04 claim for a patient who was brought in unconscious with no identification and whose sex could not be determined.

1. Locate Field 11 on the paper UB-04 form. According to the CMS layout, this field appears after Field 10 (Patient Birthdate) and before Field 12 (Admission/Start of Care Date) ^[1].

2. Enter the code: Type a single uppercase letter “U” in the one‑character space. Do not include any leading zeros, spaces, or punctuation. The code should be clearly legible if using a typewriter or pre‑printed form.

3. Complete other identification fields: Populate Field 8 (Patient Name), Field 9 (Patient Address), and Field 10 (Patient Birthdate) with any available information. If the birthdate is also unknown, follow payer guidelines for that field.

4. Submit the claim: After completing the rest of the form per the provider’s institutional billing instructions, send the UB-04 to the appropriate payer—Medicare Administrative Contractor, managed care plan, or other insurer—as described in the CMS chapter ^[1].

5. Electronic equivalent: If submitting via the ASC X12 837 institutional transaction, use the same code in the patient sex data element. The CMS chapter notes that instructions for completion are the same for paper and electronic claims unless otherwise indicated ^[1].

This example assumes the code “U” is accepted. Providers should confirm that the receiving payer recognizes this code; some state Medicaid programs (like MassHealth) may have specific requirements not detailed in the provided excerpts ^[2].

Common Mistakes & Audit Red Flags

Using code “U” inappropriately can lead to claim rejections, payment delays, or compliance audits. The following issues are frequently observed:

• Using “U” when the sex is actually known – If the patient’s drivers license, insurance card, or medical record already indicates “Male” or “Female,” using “U” is incorrect. Payers may cross-check against other data sources (e.g., Medicare beneficiary records) and flag discrepancies.

• Leaving Field 11 blank – CMS layout requires a value in this field; a blank or space may be interpreted as invalid, or the claim may be returned for correction ^[1]. Using “U” is a permissible way to complete the field when data is missing.

• Entering lower‑case “u” – The field expects uppercase. While the layout definition allows “AN” (alphanumeric), Medicare systems may treat lowercase as an error.

• Using “U” for administrative convenience – Some providers default to “U” to avoid asking the patient. This practice is likely non‑compliant with payer medical necessity and data integrity standards. Auditors may review patterns of “U” usage to detect systemic data quality issues.

• Not confirming payer acceptance – Not all payers (including some managed care plans or state Medicaid agencies) may accept “U.” The MassHealth guide does not explicitly list field 11 code values, so providers should consult payer-specific code lists or the NUBC official publication ^[2]. Claims using unsupported codes could be denied.

• Failing to document the reason – If “U” is used, the medical record should include a note explaining why the patient’s sex was not obtainable. While the UB-04 itself has no separate field for this, auditors may request supporting documentation during a review.

Related Codes/Fields

The patient sex code interacts with other patient identification elements on the UB-04. The table below lists related fields and their roles.

Field/Code	Description	Relationship to Code U
Field 10 – Patient Birthdate	8-digit date (MMDDCCYY)	When sex is unknown, birthdate may also be unknown; both fields should be completed with available data.
Field 08 – Patient Name	Last, First, Middle (19 AN)	Sex code “U” may be used if patient name gives no indication of sex (e.g., “Jordan Smith”).
Field 09 – Patient Address	Street, City, State, ZIP, Country	Address does not imply sex; code U can be paired with any valid address.
Code M (Male)	NUBC code for male	Only one sex code may be used per claim; code U is an alternative when M/F are inapplicable.
Code F (Female)	NUBC code for female	Same as above.
Field 12 – Admission/Start of Care Date	6-digit date (MMDDYY)	No direct relationship, but uncertain admission circumstances may lead to unknown sex.
Field 14 – Priority (Type) of Admission	1 AN (e.g., 1 for Emergency)	Emergency admissions (e.g., unconscious patient) are a common scenario for code U.
Field 18–28 – Condition Codes	2 AN each	Condition codes may capture a reason for unknown patient identity (e.g., “05” for Patient Unknown).

Note: The provided sources do not list the actual NUBC code values for M, F, or U. The above table relies on standard billing knowledge; for exact code definitions, consult the current NUBC Data Specifications Manual. The CMS layout confirms the field structure but not the code meanings ^[1].

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References

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

[2] MassHealth UB-04 Guide — https://www.mass.gov/doc/ub-04-billing-guide-0/download