PHI Redaction for FHIR Resources

Standard redaction profile for Protected Health Information (PHI) applied on all read paths in the FHIR MCP guardrail stack. This is a technical minimization preview, not a legal Safe Harbor determination.

When to Use This Skill

• You need to understand how PHI is stripped from FHIR resources
• You are building a pipeline that must redact clinical data before AI access
• You need to implement or audit de-identification logic for FHIR

Redaction Rules

All redaction is applied at read time, not at storage time.

Names (HumanName)

• Family name: Kept as-is
• Given names: Truncated to first initial + period (e.g., "John" -> "J.")
• Text: Removed

Identifiers

• Value: Masked to last 4 characters (e.g., "123-45-6789" -> "***6789")
• System and type: Kept for reference

Addresses

• Line and text: Removed
• City, state, country: Kept (for demographic analysis)

Telecom (Phone, Email)

• Value: Replaced with [Redacted]
• System and use: Kept

Birth Date

• Truncated to year only (e.g., "1985-03-15" -> "1985")

Photos

• Removed entirely from the resource

Text Narratives

• Replaced with: <div xmlns="http://www.w3.org/1999/xhtml">[Redacted]</div>
• Status set to empty

Notes and Comments

• Replaced with [Redacted]

Contained Resources

• All contained resources are redacted recursively

Implementation Pattern (Python)

import json

def apply_redaction(resource):
    """Deep-copy the resource and redact PHI fields."""
    redacted = json.loads(json.dumps(resource))
    _redact_fields(redacted)
    for contained in redacted.get('contained', []):
        if isinstance(contained, dict):
            _redact_fields(contained)
    return redacted

Key implementation notes:

• Always deep-copy before redacting (never modify the stored resource)
• Redact contained resources recursively
• Apply on ALL read paths: direct reads, search results, context envelopes, upstream proxy responses

De-identification Preview Coverage

This redaction profile minimizes these common identifier classes:

• Names (partial)
• Geographic data smaller than state (addresses stripped)
• Dates (birth date truncated to year)
• Telephone/fax numbers (redacted)
• Email addresses (redacted)
• Medical record numbers (masked to last 4)

Not covered by this profile (would need additional implementation):

• Social Security numbers (not typically in FHIR resources)
• IP addresses, device identifiers (not redacted from extension fields)
• Biometric identifiers
• Full-face photographs (removed if in photo field, not scanned in attachments)

Integration with MCP Tools

The redaction is applied automatically by the guardrail stack. MCP tools like fhir.read, fhir.search, fhir.lastn, and context.get all return redacted data. No additional action is needed by the agent.

The $deidentify operation provides an explicit conservative preview on demand. Expert review is still required before export or disclosure.