Ambulance billing uses a unique system of origin and destination modifiers that identify where the patient was picked up and where the patient was transported. These modifiers are mandatory for most Medicare ambulance transportation claims and are created by combining two alphabetic characters.

The letter D represents a Diagnostic or Therapeutic Site, other than a physician’s office (P) or hospital (H) when those are used as origin codes. The letter D cannot be billed alone. It must always be paired with another origin or destination code to form a valid ambulance modifier, such as RD, DR, HD, or SD. CMS requires providers to report the first letter as the origin and the second letter as the destination.

Modifier

Modifier Name

Diagnostic or Therapeutic Site (Other Than Physician’s Office or Hospital When Used as Origin Codes)

Plain English Explanation

The letter D tells Medicare:

“The ambulance trip either started from or ended at a diagnostic or therapeutic facility that is not classified as a physician’s office or hospital.”

Purpose of Modifier D

Modifier D is used to:

Identify diagnostic or therapeutic facilities.
Accurately report ambulance trip origins and destinations.
Support proper Medicare ambulance reimbursement.
Distinguish diagnostic facilities from hospitals and physician offices.
Ensure compliance with CMS ambulance billing requirements.

Understanding Modifier D

Ambulance modifiers always consist of two letters.

The first letter indicates the pickup location (origin).

The second letter indicates the drop-off location (destination).

The letter D may appear in either position.

Examples:

Modifier	Meaning
RD	Residence → Diagnostic/Therapeutic Site
DR	Diagnostic/Therapeutic Site → Residence
HD	Hospital → Diagnostic/Therapeutic Site
DH	Diagnostic/Therapeutic Site → Hospital
SD	Scene of Accident → Diagnostic/Therapeutic Site
DN	Diagnostic/Therapeutic Site → Skilled Nursing Facility

What Is a Diagnostic or Therapeutic Site?

Examples include:

Independent imaging centers
Radiation therapy centers
Outpatient infusion centers
Cardiac catheterization laboratories not classified as hospitals
Wound care centers
Dialysis-related treatment sites (when another origin/destination code is not more appropriate)
Other approved diagnostic or therapeutic treatment facilities

When to Use Modifier D

Use Modifier D whenever:

The patient’s pickup or destination is a diagnostic or therapeutic facility.
The location is not classified as a physician’s office (P).
The location is not classified as a hospital (H).
Ambulance transportation meets Medicare coverage requirements.

When NOT to Use Modifier D

Do not use Modifier D when:

The location is a hospital (use H).
The location is a physician’s office or clinic (use P).
The location is the patient’s residence (use R).
The location is a skilled nursing facility (use N).
The location is a dialysis facility with its own designated origin/destination code (use G or J, when applicable).
Modifier D is reported by itself.

Medicare Rules

CMS requires an origin/destination modifier on virtually every covered ambulance transport claim.

Key rules include:

Modifier D is never reported alone.
The first character identifies the origin.
The second character identifies the destination.
Origin and destination must accurately reflect the patient’s transport.
Incorrect origin/destination modifiers may result in claim rejection or denial.

Commercial Insurance Rules

Many commercial insurers follow CMS ambulance origin/destination coding conventions.

However, individual payer requirements may vary regarding:

Ambulance medical necessity
Documentation requirements
Covered destinations
Reimbursement policies

Always verify payer-specific billing guidelines.

Documentation Requirements

Documentation should include:

Pickup location
Destination location
Ambulance run report
Medical necessity for transport
Date and time of transport
Mileage documentation
Patient assessment
Crew signatures
Receiving facility documentation, when applicable

Real Billing Examples

Example 1 – Residence to Imaging Center

A patient is transported from home to an independent MRI facility.

Billing

HCPCS A0428
Modifier RD

Example 2 – Imaging Center to Hospital

A patient develops complications during imaging and requires transport to a hospital.

Billing

HCPCS A0428 or appropriate ambulance code
Modifier DH

Example 3 – Hospital to Radiation Therapy Center

A patient requires ambulance transportation from a hospital to a freestanding radiation therapy center.

Billing

HCPCS A0428
Modifier HD

Example 4 – Incorrect Use

A provider submits:

A0428-D

This is incorrect because D cannot be reported by itself. A complete two-letter origin/destination modifier is required.

CMS-1500 Claim Example

Field	Example
HCPCS	A0428
Modifier	RD
Diagnosis Pointer	Appropriate ICD-10-CM diagnosis
Mileage	A0425 (if applicable)

Common Denial Reasons

Invalid origin/destination modifier.
Modifier D billed alone.
Incorrect pickup location.
Incorrect destination.
Missing ambulance documentation.
Medical necessity not established.
Missing mileage documentation.

How to Correct the Denial

Review the ambulance trip documentation.
Verify the pickup location.
Verify the destination.
Confirm the correct two-letter modifier combination.
Correct the claim and resubmit if appropriate.
Submit supporting documentation if requested.

Coding Tips

Never bill Modifier D by itself.
Always use a valid two-letter origin/destination combination.
Verify that the facility truly qualifies as a diagnostic or therapeutic site.
Review CMS origin/destination tables before claim submission.
Maintain complete ambulance run documentation.

Modifier D vs Modifier H

Modifier	Description
D	Diagnostic or therapeutic site
H	Hospital

Modifier D vs Modifier P

Modifier	Description
D	Diagnostic or therapeutic site
P	Physician’s office

Modifier D vs Modifier R

Modifier	Description
D	Diagnostic or therapeutic site
R	Residence

Frequently Asked Questions (FAQs)

Q1. Can Modifier D be billed by itself?

Answer: No. Modifier D must always be combined with another origin or destination code to create a valid two-letter ambulance modifier.

Q2. What does Modifier D represent?

Answer: It identifies a diagnostic or therapeutic site that is not classified as a physician’s office or hospital.

Q3. Is RD a valid ambulance modifier?

Answer: Yes. RD indicates transport from a residence to a diagnostic or therapeutic site.

Q4. Is DH a valid ambulance modifier?

Answer: Yes. DH indicates transport from a diagnostic or therapeutic site to a hospital.

AR Caller Tips

When following up on ambulance denials:

Verify the pickup and destination addresses.
Confirm the correct two-letter origin/destination modifier.
Review the ambulance run sheet.
Verify medical necessity.
Confirm mileage documentation.
Record the payer representative’s name, reference number, and appeal instructions.

Interview Questions

Question 1

What does Modifier D mean?

Answer: It represents a diagnostic or therapeutic site used in Medicare ambulance origin/destination coding.

Question 2

Can Modifier D be billed alone?

Answer: No. It must always be paired with another origin or destination code.

Question 3

What does RD mean?

Answer: Residence to Diagnostic or Therapeutic Site.

Practice Scenario

Scenario

A patient is transported by ambulance from home to an independent outpatient infusion center for medically necessary treatment.

Question

Which ambulance modifier should be reported?

Answer

The appropriate modifier is RD, indicating Residence → Diagnostic or Therapeutic Site.

Related Ambulance Modifiers

E – Residential, Domiciliary, or Custodial Facility
G – Hospital-Based ESRD Facility
H – Hospital
I – Transfer Site
J – Freestanding ESRD Facility
N – Skilled Nursing Facility
P – Physician’s Office
R – Residence
S – Scene of Accident or Acute Event
X – Intermediate Stop at Physician’s Office (Destination Only)

Common Billing Mistakes

Billing D as a standalone modifier.
Confusing diagnostic facilities with hospitals.
Confusing diagnostic facilities with physician offices.
Using the wrong origin or destination sequence.
Missing ambulance documentation.
Incorrect mileage reporting.

Key Takeaways

D represents a Diagnostic or Therapeutic Site.
It is not a standalone modifier.
It must always be combined with another ambulance origin/destination code.
The first letter identifies the origin; the second identifies the destination.
Accurate modifier selection and documentation are essential for Medicare ambulance claim payment.

References

CMS Medicare Claims Processing Manual, Chapter 15 – Ambulance Services.
CMS Origin and Destination Codes for Ambulance Service Claims.
Medicare Administrative Contractor (MAC) Ambulance Billing Guidance.
HCPCS Level II Code Book.

Conclusion

The D ambulance origin/destination code plays a critical role in identifying transports involving diagnostic or therapeutic facilities. Because Medicare requires ambulance claims to include valid two-letter origin/destination modifiers, understanding how to correctly pair D with other location codes is essential for accurate billing, proper reimbursement, and compliance with CMS ambulance billing policies.

Educational Disclaimer

This article was prepared with the assistance of artificial intelligence (AI) for educational and informational purposes. It is based on publicly available CMS guidance, Medicare ambulance billing principles, and general medical coding practices. It is not an official publication of CMS or the American Medical Association (AMA). Always consult the latest CMS Medicare Claims Processing Manual, HCPCS Level II codebook, Medicare Administrative Contractor (MAC) guidance, and payer-specific billing policies before coding, billing, or submitting claims.