UB-04 Field 67: Principal Diagnosis Code

Field ID: 67 Slug: field-67

Overview

Field Locator (FL) 67, the Principal Diagnosis Code, is a mandatory field on the UB-04 (CMS-1450) claim form used to report the condition established "after study" to be chiefly responsible for the patient's admission to the facility CMS Pub. 100-04 Ch. 25. This field sits in the diagnosis section of the form, immediately following FL 66 (Diagnosis/Procedure Code Qualifier), which identifies whether the codes are ICD-10-CM (indicated by a "0") or ICD-9-CM (indicated by a "9") NUBC UB-04 Manual.

The principal diagnosis is the primary driver for Diagnosis Related Group (DRG) assignment in inpatient settings and significantly influences reimbursement rates ICD-10-CM Official Guidelines. Unlike the Admitting Diagnosis (FL 69), which reflects the patient's condition at the time of entry, the principal diagnosis is determined after the full diagnostic workup and treatment are completed AAPC Facility Coding.

When to Use This Field

This field must be completed for every institutional claim, including inpatient, outpatient, and emergency department visits. It is the anchor for medical necessity.

• Inpatient Admissions: Use this field to report the definitive diagnosis that necessitated the hospital stay. For example, if a patient is admitted for "chest pain" but the workup confirms an Acute Myocardial Infarction (AMI), the AMI code (e.g., I21.09) is reported in FL 67, while the chest pain (R07.9) may be reported in FL 69 HMSA UB-04 Guidelines.
• Outpatient Encounters: In the outpatient setting, the principal diagnosis is the condition, problem, or other reason for the encounter shown in the medical record to be chiefly responsible for the services provided CMS Claims Processing Manual.
• Hospice Claims: For hospice billing, FL 67 must contain the terminal diagnosis. Medicare will return claims to the provider (RTP) if an "unacceptable" principal diagnosis, such as a symptom or an unspecified code when a more specific one is available, is used CMS Transmittal 12538.

Step-by-Step Claim Example

Patient Scenario: A 68-year-old male presents to the Emergency Department with severe abdominal pain and vomiting. He is admitted to the hospital. After a CT scan and surgical consultation, he is diagnosed with Acute Appendicitis with localized peritonitis.

1. FL 66 (Qualifier): Enter "0" to indicate ICD-10-CM.
2. FL 67 (Principal Diagnosis): Enter code K35.30 (Acute appendicitis with localized peritonitis, without perforation or abscess). Do not include the decimal point on the paper form CMS Pub. 100-04 Ch. 25.
3. FL 69 (Admitting Diagnosis): Enter code R10.9 (Abdominal pain, unspecified) as this was the reason for admission prior to the "after study" results.
4. Payer Response: The Medicare Administrative Contractor (MAC) processes the claim. Because K35.30 is a valid, specific principal diagnosis, it maps to a surgical DRG (e.g., DRG 343). The claim is paid at the full DRG rate. If the biller had mistakenly used R10.9 in FL 67, the claim might have been denied or downcoded for lack of medical necessity for an inpatient stay Novitas Solutions FISS Edits.

Common Mistakes & Audit Red Flags

• Using Signs and Symptoms: Reporting a symptom (e.g., R05.9 for cough) when a definitive diagnosis (e.g., J18.9 for pneumonia) was established is a major audit red flag. The Medicare Code Editor (MCE) will flag these as "unacceptable principal diagnoses" CMS I/OCE Specifications.
• Lack of Specificity: Using a 3-digit category code when 4, 5, 6, or 7 digits are available. Claims with truncated codes are automatically rejected as "invalid" HMSA Specificity Requirements.
• Sequencing Errors: Placing a secondary complication (like a hospital-acquired infection) in FL 67 instead of the condition that caused the admission. This can lead to DRG Creep audits by the Office of Inspector General (OIG) ICD-10-CM Official Guidelines Section II.
• External Cause Codes: Reporting an "E-code" (External Cause of Morbidity, e.g., V00-Y99) in FL 67. These codes are always supplemental and never principal Oklahoma.gov UB-04 Instructions.

Related Codes & Fields

• FL 66: Diagnosis/Procedure Code Qualifier
• FL 67 A-Q: Other Diagnosis Codes
• FL 69: Admitting Diagnosis Code
• FL 70: Patient Reason for Visit
• FL 74: Principal Procedure Code

References

1. CMS Pub. 100-04 Ch. 25
2. NUBC UB-04 Manual
3. ICD-10-CM Official Guidelines
4. AAPC Facility Coding
5. HMSA UB-04 Guidelines
6. CMS Transmittal 12538
7. Novitas Solutions FISS Edits
8. CMS I/OCE Specifications
9. Oklahoma.gov UB-04 Instructions