Auditing de-id leakage

De-identification is verified, not assumed. A model-driven redaction can miss a structured identifier (an SSN typo'd with spaces, an account number in a footer, a date in an odd format) — and a single residual identifier defeats the whole release. This skill is the adversarial second pass: scan the output of de-identification for anything that still looks like an identifier, score it, and block release on any leak. It is the verification half of OpenMed's leakage-first ethos — gate on leakage, not on F1.

When to use

• Right after deidentifying-clinical-text, before the de-identified text leaves a trust boundary (export, share, train, publish).
• When the user wants proof that "no PHI leaked," a release gate, or a CI check that fails the build if any identifier survives.
• As a belt-and-suspenders detector independent of the model that produced the redaction — a deterministic checker catches different failures than the NER.

Run this on the de-identified text, not the original. The original is expected to be full of identifiers.

Quick start

Two complementary passes — a deterministic structural scan plus a model second-pass diff:

import re
import openmed

# Synthetic — the de-identified OUTPUT we are auditing for residual leaks.
deid_text = "Patient [NAME] seen on [DATE]. Backup contact 415-555-0184; acct 4111111111111111."

def luhn_ok(digits: str) -> bool:
    nums = [int(d) for d in digits]
    nums[-2::-2] = [(2 * d - 9 if 2 * d > 9 else 2 * d) for d in nums[-2::-2]]
    return sum(nums) % 10 == 0

DETECTORS = {
    "SSN":   (r"\b\d{3}-\d{2}-\d{4}\b", "critical", None),
    "EMAIL": (r"\b[\w.+-]+@[\w-]+\.[\w.-]+\b", "high", None),
    "PHONE": (r"\b(?:\+?1[-.\s]?)?\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}\b", "high", None),
    "DATE":  (r"\b\d{1,2}[/-]\d{1,2}[/-]\d{2,4}\b", "medium", None),
    "MRN":   (r"\bMRN[:#\s]*\d{5,}\b", "high", None),
    "CARD":  (r"\b(?:\d[ -]?){13,19}\b", "critical", luhn_ok),  # checksum-gated
}

findings = []
for label, (pattern, severity, checksum) in DETECTORS.items():
    for m in re.finditer(pattern, deid_text, flags=re.IGNORECASE):
        token = m.group()
        if checksum and not checksum(re.sub(r"\D", "", token)):
            continue  # fails Luhn -> not a real card number, skip
        findings.append({"label": label, "severity": severity,
                         "start": m.start(), "end": m.end()})  # offsets, not text

# Second-pass model detector: re-run PII extraction on the de-id output.
residual = openmed.extract_pii(deid_text)          # PredictionResult
for ent in residual.entities:
    findings.append({"label": ent.label, "severity": "high",
                     "start": ent.start, "end": ent.end})

leaked = bool(findings)
print({"leak": leaked, "count": len(findings)})    # report carries NO plaintext
assert not leaked, "Release BLOCKED: residual identifiers detected."

Note what the report records: labels, severities, and offsets — never the leaked plaintext. Echoing the leaked identifier into a report or log re-creates the exact PHI exposure you are auditing for.

Workflow

1. Run deterministic format + checksum detectors on the de-identified text: SSN, email, phone, dates, MRN/account/ID patterns, and card numbers gated by the Luhn checksum so random 16-digit strings don't false-positive. These catch structured identifiers a model may skip.
2. Add an entropy heuristic for high-randomness tokens (long base36/base64 strings, hex blobs) that match no known format but look like keys, tokens, or record locators. Flag for review rather than auto-block; entropy is noisy.
3. Run a model second-pass: re-run openmed.extract_pii on the output and treat any returned entity as a residual leak. Because it's a different detector than the one that did the redaction, it catches different misses.
4. Score severity. critical (SSN, card, full DOB+name co-occurrence) > high (email, phone, MRN, names) > medium (partial dates) > low (entropy-only).
5. Block on any leak. The gate is binary for release: if findings is non-empty at high/critical, fail the export. Surface a no-PHI report (counts + offsets + severities) so a reviewer can locate and re-redact.

Hand-off to / from OpenMed

• From deidentifying-clinical-text: this skill consumes result.deidentified_text. Never audit result.original_text.
• OpenMed second-pass detector: from openmed import extract_pii — re-run it on the de-id output and diff. Equivalent MCP/REST surfaces detect PII spans for the same purpose. Any span returned on already-de-identified text is a leak.
• To reviewing-reidentification-risk: zero direct-identifier leaks is necessary but not sufficient — quasi-identifiers (age + ZIP + date) can still re-identify. Hand a clean-on-leakage dataset to QI risk scoring next.
• To evaluating-with-leakage-gates: wire this scan into the eval harness so a leakage regression fails CI, not just an F1 drop.

Edge cases & gotchas

• Never log the leaked value. Report offsets, labels, hashes — not the text. A leakage report full of plaintext SSNs is itself a breach.
• Checksum-gate card numbers. Apply Luhn before flagging 13–19 digit runs, or every order number and account id becomes a false "card leak."
• Surrogates are not leaks. If de-id used method="replace", the output contains fake names/emails by design. The model second-pass may flag them — diff against the known mapping/surrogate set so you don't block on synthetic data. True leaks are values present in the original text.
• Locale-aware dates and IDs. dd/mm/yyyy, yyyy.mm.dd, NHS/SIN/fiscal-code formats vary; tune detectors to the data's locale or you under-detect.
• Entropy is advisory. High-entropy ≠ identifier (could be a hash already). Route to human review, don't hard-block on entropy alone.
• Local-first. Run the whole scan on-device; do not ship the text to a cloud scanner to check whether it leaked.

Standards & references

• HIPAA Safe Harbor — the 18 identifier categories that must be absent: https://www.hhs.gov/hipaa/for-professionals/privacy/special-topics/de-identification/
• Luhn algorithm (ISO/IEC 7812-1) for payment-card checksum validation: https://www.iso.org/standard/70484.html
• NIST SP 800-188, De-Identification of Personal Information: https://csrc.nist.gov/pubs/sp/800/188/final