Currentmedical mutual - ohioPolicy AHS - G2115 - Metabolite Markers of Thiopurines Testing

Metabolite Markers of Thiopurines Testing

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Criteria for coverage of phenotypic TPMT testing and monitoring of thiopurine metabolites (6-TGN, 6-MMP/6-MMRP) for individuals receiving thiopurine therapy; governs when testing meets coverage criteria and when it does not for Medical Mutual - Ohio members.

Policy Summary

Payermedical mutual - ohio

PolicyMetabolite Markers of Thiopurines Testing

Policy CodePolicy AHS - G2115 - Metabolite Markers of Thiopurines Testing

Change TypeLiterature reviewno material changes

Effective DateFeb 1, 2026

Last ReviewSep 4, 2025

Next Review DateSep 4, 2025

Key ActionObtain one-time phenotypic TPMT testing prior to initiating azathioprine, 6-mercaptopurine, or 6-thioguanine and document a covered indication when ordering metabolite or pharmacogenetic testing.

SourceLink

POLICY UPDATE CHANGES

No material clinical or coverage changes in this revision.

1-timephenotypic TPMT analysis allowed

230–400optimal 6-TGN range

≤12 mg/m2/day6-TG dosage probably safe

0.3%negligible TPMT activity

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Currentmedical mutual - ohioPolicy AHS - G2115 - Metabolite Markers of Thiopurines Testing

Metabolite Markers of Thiopurines Testing

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Policy Summary

Payermedical mutual - ohio

PolicyMetabolite Markers of Thiopurines Testing

Policy CodePolicy AHS - G2115 - Metabolite Markers of Thiopurines Testing

Change TypeLiterature reviewno material changes

Effective DateFeb 1, 2026

Last ReviewSep 4, 2025

Next Review DateSep 4, 2025

SourceLink

POLICY UPDATE CHANGES

No material clinical or coverage changes in this revision.

1-timephenotypic TPMT analysis allowed

230–400optimal 6-TGN range

≤12 mg/m2/day6-TG dosage probably safe

0.3%negligible TPMT activity

3guideline groups cited

09/04/2025last review

Coverage Criteria for TPMT and Thiopurine Metabolite Testing

Medically Necessary / Covered Indications

Covered when ANY of the following are met

TPMT testing (phenotypic): One-time phenotypic analysis of TPMT meets coverage criteria when performed prior to initiating azathioprine, 6-mercaptopurine, or 6-thioguanine; or for individuals on thiopurine therapy with abnormal complete blood count (CBC) results that do not respond to dose reduction.

Referenced in policy III.1

Metabolite monitoring in IBD: For individuals with inflammatory bowel disease, monitoring of thiopurine metabolite levels meets coverage criteria to: (a) measure blood levels in individuals suspected of having toxic responses to AZA and/or 6-MP (e.g., hepatotoxicity or myelotoxicity); or (b) measure drug levels in individuals who have not responded to therapy (e.g., persistent fever, further weight loss, and bloody diarrhea).

Referenced in policy III.2

Metabolite monitoring in ALL: For individuals with acute lymphoblastic leukemia, monitoring of thiopurine metabolite levels meets coverage criteria for: (a) individuals showing signs of a lack of myelosuppression while on therapy; or (b) individuals who do not tolerate thiopurines.

Referenced in policy III.3

Guideline-based covered indications

Covered when clinical indications align with guideline-recommended uses:

Indications supported by professional guidelines: Testing is indicated for: (1) pre-initiation TPMT testing (and consideration of NUDT15) to identify patients at risk for severe dose-dependent myelosuppression; (2) evaluation of patients with excessive thiopurine toxicity to assess for genetic causes or measure metabolites; (3) assessment of patients with nonresponse or loss of response to guide dosing and adherence evaluation via metabolite measurement; (4) pediatric patients prior to thiopurine use to identify homozygous or extremely low TPMT activity and avoid thiopurines in those individuals.

Sourced from NCCN, NASPGHAN, CPIC, BSG, ECCO/ESPGHAN, AGA

Pediatric considerations

Additional pediatric-specific stance:

Pediatric TPMT guidance: TPMT testing is recommended before initiation of thiopurines in pediatric inflammatory bowel disease to identify individuals who are homozygous recessive or have extremely low TPMT activity; those with extremely low activity should avoid thiopurines. Routine monitoring (CBC with differential) remains required for all individuals on thiopurines regardless of TPMT testing results.

NASPGHAN consensus

AGA monitoring recommendations

Therapeutic drug monitoring stance in adults (AGA):

Reactive vs proactive monitoring: For adult patients treated with thiopurines, the AGA suggests reactive thiopurine metabolite monitoring to guide treatment changes in active IBD or when adverse effects are suspected; the AGA suggests against routine proactive metabolite monitoring in adults with quiescent IBD on thiopurines.

AGA 2017 guidance and updates

Phenotypic TPMT analysis and measurement of thiopurine metabolite markers (including 6-MMRP and 6-TGN) are excluded from coverage when requested for any situation that is not specifically listed in the policy's covered indications. Requests outside the enumerated indications in Section III do not meet coverage criteria and may be denied.

Professional society guidance (NASPGHAN and the AGA) states that routine and repetitive thiopurine metabolite testing has little or no role in patients who are clinically doing well on an acceptable thiopurine dose. Such repetitive proactive monitoring in asymptomatic, well-controlled patients is not supported as standard practice.

The most recent literature review and policy revision (09/04/2025) did not require any changes to the coverage criteria. Existing indications, exclusions, and prior-authorization practices remain unchanged following the review.

Any indication for TPMT testing (phenotypic) or thiopurine metabolite testing that is not explicitly specified among the policy's covered indications in Section III is considered not medically necessary under this policy.

Consistent with NASPGHAN and AGA guidance, the policy does not support routine or repetitive metabolite testing in well-controlled patients who lack clinical indications. Reactive metabolite monitoring is recommended for suspected toxicity or loss of response, but ongoing surveillance in quiescent patients without clinical concern is not supported.

Specific Covered Indications

Pre-therapy TPMT phenotypic testing prior to initiating azathioprine/6-MP/6-TG

One-time phenotypic TPMT analysis meets coverage criteria when the following are true:

Pre-therapy TPMT testing: One-time phenotypic analysis of TPMT is performed prior to initiation of azathioprine, 6-mercaptopurine, or 6-thioguanine.

Policy III.1(a)

TPMT phenotypic testing for abnormal CBC not responsive to dose reduction

Phenotypic TPMT analysis meets coverage criteria for individuals on thiopurines with persistent CBC abnormalities despite dose reduction when ALL of the following are met:

Abnormal CBC not responsive to dose reduction: Individual is receiving azathioprine, 6-mercaptopurine, or 6-thioguanine; has abnormal complete blood count (CBC) results that do not respond to dose reduction; and TPMT phenotypic testing is ordered to evaluate persistent myelotoxicity.

Policy III.1(b)

Non-covered Indications

Not Medically Necessary — Not covered

Not covered — the following do NOT meet coverage criteria:

TPMT testing not covered: Phenotypic TPMT analysis does not meet coverage criteria for all other situations not addressed in the covered indications.

Policy III.4

Metabolite analysis not covered: Analysis of metabolite markers of azathioprine and 6-mercaptopurine (including 6-MMRP and 6-TGN) does not meet coverage criteria for all other situations not addressed in the covered indications; routine and repetitive metabolite testing in well-controlled patients without clinical indication is not supported.

Policy III.5; AGA/NASPGHAN statements

Phenotypic TPMT analysis and thiopurine metabolite analysis are not covered for indications other than the specific, listed covered scenarios (pre-therapy TPMT testing, evaluation of unexplained abnormal CBC despite dose reduction, and reactive metabolite evaluation for suspected toxicity or nonresponse).

Routine proactive metabolite monitoring in quiescent or well-controlled patients is not supported by AGA and NASPGHAN guidance and therefore is not covered under this policy absent a defined clinical indication.

Procedure Codes and Coding Guidance

6-TGN therapeutic/toxicity thresholds

Optimal therapeutic rangeApproximately 230–400 pmol/8x10^8 RBC (reported optimal concentration by Dubinsky et al.).

Marrow toxicity correlationBone marrow toxicity correlated with 6-TGN levels >400 pmol/8x10^8 RBC.

Study-specific cut-offsMeta-analysis and cohort studies cite thresholds near 235–250 pmol/8x10^8 RBC associated with higher odds of remission.

Genotype-stratified cut-offsNUDT15/TPMT genotype-based cut-offs reported (e.g., CT genotype cut-off ~319.2 pmol/8x10^8 RBC; CC cut-off ~411.5 pmol/8x10^8 RBC) to improve prediction of leukopenia.

Clinical interpretation noteThresholds vary by study and patient population; use alongside TPMT/NUDT15 assessment and clinical context when guiding dosing.

Provider Responsibilities and Operational Notes

Prior Authorization

Coverage limited to specified clinical indications

Coverage is limited to the indications listed in Section III (pre-initiation TPMT testing; abnormal CBC not responsive to dose reduction; metabolite monitoring for suspected toxicity or lack of response in IBD and ALL). Testing for any other situations does not meet coverage criteria and may require prior authorization or be denied.

One-time phenotypic TPMT analysis meets coverage when performed prior to initiating AZA, 6‑MP, or 6‑TG or for abnormal CBC not responsive to dose reduction.
Metabolite monitoring meets coverage for suspected toxicity or lack of response in IBD and for lack of myelosuppression or intolerance in ALL.

Billing Rule

Include relevant procedure codes and specimen/source

Key Definitions

6-TGN

Definition6-thioguanine nucleotides (6-TGN): primary active metabolites of azathioprine/6-mercaptopurine associated with therapeutic efficacy and bone marrow toxicity.

Clinical useMeasured as part of therapeutic drug monitoring to assess adherence, optimize dosing, and evaluate nonresponse or myelotoxicity.

Measurement methodsQuantified in erythrocytes using HPLC or mass spectrometry (TDM techniques).

6-MMP / 6-MMRP

Definition6-MMP and 6-MMRP: 6-methyl-mercaptopurine and 6-methyl-mercaptopurine ribonucleotides; metabolites of thiopurines associated with hepatotoxicity.

Clinical relevanceElevated 6-MMP/6-MMRP levels have been linked to hepatotoxicity and may prompt evaluation of dose adjustment or alternative therapy.

Background and Clinical Context

Thiopurines (azathioprine, 6-mercaptopurine, 6-thioguanine) undergo variable metabolism across individuals, which drives differences in therapeutic effect and risk of toxicity (bone marrow suppression and hepatotoxicity). TPMT enzyme activity is a major determinant of thiopurine metabolism, and genetic variation (including TPMT and NUDT15) can identify patients at higher risk for severe myelosuppression. Measurement of thiopurine metabolites such as 6-TGN and 6-MMP can assist in assessing adherence, explaining lack of response, or evaluating suspected toxicity.

Member Eligibility Requirements

Coverage is limited to the clinical indications listed in Section III. Clinical documentation must demonstrate that the requested testing is for a covered indication (for example, one-time TPMT phenotypic testing prior to initiation of azathioprine/6-MP/6-TG, evaluation of abnormal CBC that does not respond to dose reduction, or reactive metabolite testing for suspected toxicity or lack of response).

There are no additional family-history or prior-test prerequisites specified for eligibility. Testing decisions should align with the clinical scenarios described in the covered indications and relevant guideline recommendations.

Providers ordering TPMT or thiopurine metabolite testing should ensure the medical record contains the clinical rationale linking the test to a covered indication and, when applicable, reference to guideline-based recommendations (e.g., pre-initiation testing, evaluation of toxicity, or assessment of nonresponse). Laboratory tests that are laboratory-developed should be validated and performed in CLIA-certified high-complexity laboratories as required by applicable regulations.

Policy Revision History

2025-09-04policy_reviewLatest

Reviewed and updated background, guidelines/recommendations, and evidence-based references; literature review did not necessitate modifications to coverage criteria.

Policy Summary

Payermedical mutual - ohio

PolicyMetabolite Markers of Thiopurines Testing

Policy CodePolicy AHS - G2115 - Metabolite Markers of Thiopurines Testing

Change TypeLiterature reviewno material changes

Effective DateFeb 1, 2026

Last ReviewSep 4, 2025

Next Review DateSep 4, 2025

SourceLink

On This Page

Thiopurine metabolite monitoring in IBD for suspected toxicity or lack of response

Thiopurine metabolite monitoring in individuals with IBD meets coverage criteria when ANY of the following apply:

Suspected toxicity: Metabolite testing is ordered to measure blood levels in individuals suspected of having toxic responses to azathioprine and/or 6-mercaptopurine (for example, hepatotoxicity or myelotoxicity).

Policy III.2(a); AGA/NASPGHAN/ECCO guidance

Lack of response: Metabolite testing is ordered to assess drug levels in individuals who have not responded to therapy (for example persistent fever, continued weight loss, or bloody diarrhea) to distinguish nonadherence or hypermetabolism from true nonresponse and guide dosing adjustments.

Policy III.2(b); ECCO/NICE guidance

Thiopurine metabolite monitoring in ALL for lack of myelosuppression or intolerance

Thiopurine metabolite monitoring in individuals with ALL meets coverage criteria when ANY of the following apply:

Lack of myelosuppression: Metabolite quantitation is ordered for individuals showing signs of lack of myelosuppression while on thiopurine therapy to determine whether nonadherence or hypermetabolism is the cause.

Policy III.3(a); NCCN

Intolerance to thiopurines: Metabolite quantitation is ordered for individuals who do not tolerate thiopurines to evaluate for causes of intolerance and guide alternative therapy or dosing.

Policy III.3(b); NCCN

Guideline summary of contexts for testing

Contexts in which TPMT/NUDT15 testing or metabolite measurement are recommended by professional societies:

Pre-initiation and toxicity evaluation: Pre-initiation TPMT testing (and consideration of NUDT15) to identify risk for severe myelosuppression; evaluation of patients with thiopurine-related toxicity (genetic testing or metabolite measurement); and assessment in cases of nonresponse to guide therapy and adherence assessment.

Recommendations summarized from NCCN, NASPGHAN, CPIC, BSG, ECCO/ESPGHAN, AGA

References support use of thiopurine metabolite testing and TPMT/NUDT15 pharmacogenomic information in management of IBD, ALL, and autoimmune hepatitis.

Key references supporting clinical use:

Guideline and evidence support: Multiple society guidelines and studies support the use of thiopurine metabolite testing and TPMT/NUDT15 pharmacogenetic information in management of inflammatory bowel disease (Crohn's disease, ulcerative colitis), pediatric and adult acute lymphoblastic leukemia, and autoimmune hepatitis.

See referenced literature and guideline list

When submitting requests or claims, include the appropriate procedure code for the test being performed (e.g., TPMT enzyme testing, TPMT/genotype, or metabolite quantitation) and the specimen/source per laboratory requirements.

Relevant CPT codes listed: 80299 (quantitation of therapeutic drug), 82657 (enzyme activity), 84433 (TPMT).
Procedure codes should be used as the payer reference when requesting authorization or submitting claims.

Prior Authorization

Prior authorization requirements unchanged after review

The policy's revision history notes that the literature review did not change coverage criteria; prior authorization requirements remain as previously specified in the policy.

09/04/2025 review updated background and references but did not modify coverage criteria.

Note

TPMT testing prior to initiation and ongoing standard monitoring

Providers should consider TPMT testing prior to initiating thiopurine therapy and continue standard monitoring such as CBC (and LFTs where clinically indicated); thiopurine metabolite testing is useful in defined scenarios (suspected toxicity, nonresponse, adherence assessment).

TPMT assay prior to initiation to identify patients at risk for severe myelosuppression.
Continue routine CBC monitoring regardless of TPMT results; use metabolite testing reactively for toxicity or nonresponse.

Step Therapy

No step therapy requirements specified

The document does not state any explicit step therapy requirements for thiopurine metabolite or TPMT testing.

Guidelines recommend TPMT/NUDT15 testing pre-initiation or when toxicity is encountered, but no payer step therapy pathway is specified.

Step Therapy

Absence of step therapy mandates

No step therapy requirements are specified in the provided policy excerpts; providers should follow the coverage indications and guideline recommendations instead.

Evidence and guideline references are provided, but they do not establish payer-mandated step therapy steps in this policy.

Documentation Required

Document clinical indication when ordering

Clinical documentation supporting one of the covered indications must accompany the request: examples include documentation of planned initiation of azathioprine/6‑MP/6‑TG, abnormal CBC not responsive to dose reduction, suspected thiopurine toxicity, or lack of myelosuppression/intolerance in ALL.

For pre-initiation TPMT: chart note indicating planned start of AZA, 6‑MP, or 6‑TG.
For abnormal CBC: documentation that abnormalities did not resolve with dose reduction.
For metabolite monitoring: clinical rationale such as suspected hepatotoxicity, myelotoxicity, nonresponse, or adherence concerns.

Documentation Required

Ensure LDT validation and CLIA high‑complexity lab performance

Laboratory-developed tests must be validated and performed in-house in CLIA-certified high-complexity laboratories; documentation of appropriate CLIA/CMS accreditation and validation should be maintained.

LDTs are regulated by CMS as high-complexity tests under CLIA '88 and must be validated by the performing lab.
FDA clearance is not required for clinical use of LDTs, but CLIA compliance is expected.

Documentation Required

Document guideline-concordant rationale when applicable

The policy cites multiple society guidelines (e.g., AGA, ACG, NCCN, CPIC); when ordering tests, document how the clinical indication aligns with relevant guideline recommendations where applicable.

Reference guideline context (e.g., NCCN for ALL, AGA/ACG for IBD, CPIC for pharmacogenetics) in the clinical rationale when applicable.
Use guideline-concordant indications (pre-initiation, toxicity evaluation, nonresponse) to support medical necessity.

Denial Risk

Risk of denial for non‑covered indications

Requests for phenotypic TPMT analysis or thiopurine metabolite analysis that do not meet the specific covered indications listed in Section III may be denied as not meeting coverage criteria.

Phenotypic TPMT analysis and metabolite marker analysis are explicitly stated as not meeting coverage criteria for other situations.
Ensure the requested test maps to a listed covered indication to avoid denial.

Denial Risk

Government coverage policy takes precedence

If a conflict exists between this policy and an applicable government policy (LCD, NCD, or state Medicaid), the government policy prevails; lack of compliance with government coverage requirements could affect determinations.

Check applicable Medicare/Medicaid coverage determinations when requesting testing for beneficiaries covered by those programs.
Government policy will be used to make determinations in case of conflict with this policy.

Prior Authorization

Revision did not change authorization requirements

The literature review conducted during the 09/04/2025 update did not require changes to coverage criteria, indicating that existing authorization and denial rules remain in effect.

Policy review updated background and references but maintained prior coverage determinations.

Note

No mandatory genetic counseling stated; refer to CPIC as needed

No explicit mandatory pre- or post-test genetic counseling requirement is specified in this policy; CPIC phenotype-based dosing recommendations are referenced for pharmacogenetic interpretation where appropriate.

CPIC guidance addresses phenotype-based dosing and management of indeterminate genotypes but the policy does not mandate counseling.
Providers may refer to CPIC recommendations when communicating genotype/phenotype results.

Note

Order TPMT/NUDT15 testing per guideline recommendations

Guidelines recommend testing for TPMT and NUDT15 prior to thiopurine initiation or when excessive toxicity occurs; providers should follow guideline-directed ordering pathways and specimen requirements as noted by specialty guidance.

NCCN, ACG, NASPGHAN, and CPIC recommend pre-initiation testing and consideration of both TPMT and NUDT15 in relevant populations.
Order the specific test type needed (genotype vs enzyme activity vs metabolite quantitation) per the clinical question and laboratory instructions.

Note

No explicit restriction on who may order testing

The policy does not restrict who may order TPMT, NUDT15, or metabolite testing; recommendations are directed to clinicians managing Crohn's disease, ulcerative colitis, pediatric IBD, and treating oncologists.

No explicit ordering provider limitation is stated in the policy text.
Recommendations are aimed at treating clinicians (gastroenterologists, pediatric gastroenterologists, oncologists) involved in care.

Role in TDMMeasured alongside 6-TGN to distinguish toxicity (hepatotoxicity vs myelotoxicity) and guide management in IBD and ALL contexts.

TPMT

DefinitionTPMT (thiopurine methyltransferase): enzyme whose activity affects thiopurine metabolism and risk of toxicity.

Population variabilityMost individuals have normal TPMT activity; ~11% are heterozygous with low activity and ~0.3% have negligible activity.

Clinical impactTPMT activity influences 6-TGN and 6-MMP levels and informs dosing to reduce risk of myelotoxicity.

NUDT15

DefinitionNUDT15 (Nudix hydrolase 15): genetic variant/enzyme that can greatly reduce tolerance to thiopurines and increase risk of leukopenia.

Population differencesPoor metabolizer phenotype can be as common as ~1 in 50 in some East Asian populations; variant frequency varies by ancestry.

Clinical implicationNUDT15 variants markedly increase risk of thiopurine-induced leukopenia; genotype-informed thresholds improve predictive sensitivity.

TPMT/NUDT15 testing

What it isGenetic testing for TPMT and NUDT15 alleles and/or measurement of TPMT erythrocyte activity to predict thiopurine tolerance and guide dosing.

When to useRecommended prior to initiating thiopurines or when excessive toxicity is encountered (per CPIC, NCCN, NASPGHAN and other guidelines).

LimitationsGenetic testing may miss rare or undiscovered no-function alleles; CPIC suggests measuring TPMT erythrocyte activity for indeterminate genotypes.

Thiopurine metabolite testing

PurposeQuantification of erythrocyte thiopurine metabolites (e.g., 6-TG/6-TGN and 6-MMP) to assess adherence, optimize dosing, evaluate nonresponse, or investigate toxicity (leucopenia, elevated transaminases).

MethodsMeasured using therapeutic drug monitoring techniques such as HPLC or mass spectrometry in erythrocyte specimens.

Clinical scenariosUsed in IBD and ALL for suspected toxicity, lack of response, suspected nonadherence, or to guide dosing adjustments per guideline recommendations.

CPIC guideline

Guideline nameCPIC: 'Thiopurine Dosing Based on TPMT and NUDT15 Genotypes' guideline.

Key recommendationsRecommends genotype-guided dosing and provides advice for indeterminate phenotypes (consider TPMT activity measurement; monitor closely if NUDT15 status unknown).

Resource linkCPIC guideline is cited as the authoritative source for TPMT/NUDT15-based dosing recommendations and updates.