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Background: EULAR recently published ‘Points to Consider’ in Therapeutic Drug Monitoring (TDM) of biological therapies in rheumatic diseases [1] . One of the eleven research questions related to clinicians’ preferences when utilizing the test. Little is known about why clinicians prospectively ask for a TDM test.
Objectives: To identify why clinicians ask for TDM and outline testing population demographics, drug usage and dosing frequency. Data was collected from a 5 year period following the introduction of a new TDM service.
Methods: Data was collected using unique patient identifier between 2018 and 2023 for patients attending hospitals in Greater Glasgow and Clyde NHS. Baseline demographics included disease, drug, brand, dose frequency of administration, duration since last dose and associated immunosuppression. Reasons for the test request were collected using a ‘drop down’ options box for clinicians.
Results: 8725 test requests were processed over the 5 year period. Table 1 outlines the number of test requests for Infliximab (IFX) and Adalimumab (ADA) drug levels across disease types.
Table 2 demonstrates the reason for doing the TDM test for a dalimumab and infliximab in rheumatic conditions, the dosing intervals and references the inflammatory bowel disease figures for comparison.
| Disease | IFX number of tests | Percentage of total tests | ADA number of tests | Percentage of total tests |
|---|---|---|---|---|
| Crohn’s | 2554 | 66.7 | 2971 | 60.7 |
| Ulcerative Colitis | 866 | 22.6 | 851 | 17.4 |
| Unspecified | 117 | 5.7 | 0 | 0 |
| JIA | 70 | 1.8 | 97 | 2 |
| PSA | 43 | 1.1 | 427 | 8.7 |
| RA | 42 | 1.1 | 246 | 5.0 |
| AS | 12 | 0.3 | 98 | 2 |
| Adalimumab | |||||
|---|---|---|---|---|---|
| Indication for testing/Disease | JIA | RA | PsA | AS | IBD |
| End of induction (week 12-14) | 0 | 4 | 2 | 0 | 426 |
| Primary non-response | 5 | 37 | 67 | 8 | 259 |
| Secondary non response | 26 | 134 | 297 | 72 | 824 |
| Post dose adjustment | 4 | 5 | 2 | 2 | 207 |
| Maintenance of treatment review | 56 | 59 | 49 | 15 | 2865 |
| Other | 6 | 7 | 10 | 1 | 327 |
| Totals | 97 | 246 | 427 | 98 | 4896 |
| Dosing frequency | |||||
| Weekly | 11 | 11 | 30 | 4 | 831 |
| Every 2 weeks | 85 | 221 | 378 | 89 | 3805 |
| Every 3 weeks | 1 | 6 | 12 | 3 | 169 |
| Every 4 weeks | 0 | 3 | 3 | 1 | 21 |
| Other | 0 | 5 | 4 | 1 | 70 |
| Totals | 97 | 246 | 427 | 98 | 4896 |
| Infliximab | |||||
| Indication for testing/Disease | JIA | RA | PsA | AS | IBD |
| End of induction (week 12-14) | 2 | 1 | 1 | 0 | 293 |
| Primary non-response | 0 | 1 | 8 | 1 | 59 |
| Secondary non response | 8 | 6 | 17 | 5 | 93 |
| Post dose adjustment | 2 | 0 | 2 | 0 | 229 |
| Maintenance of treatment review | 56 | 30 | 15 | 6 | 2264 |
| Other | 2 | 4 | 0 | 0 | 482 |
| Totals | 70 | 42 | 43 | 12 | 3420 |
| Dosing frequency | |||||
| Every 4 weeks | 35 | 5 | 4 | 2 | 445 |
| Every 6 weeks | 25 | 20 | 19 | 3 | 554 |
| Every 8 weeks | 4 | 15 | 15 | 2 | 1851 |
| Other | 6 | 2 | 5 | 5 | 570 |
| Totals | 70 | 42 | 43 | 12 | 3420 |
Within rheumatic conditions, clinicians test more frequently for TDM in PsA (n=427) as in RA (n=246). The main clinical question being asked of the test for patients on ADA was whether secondary- non response has developed (60.9%), while that of patients on IFX was to review their maintenance of treatment (64.1%). Only 167 TDM tests were requested for IFX by rheumatologists compared with 868 for ADA, reflecting current usage.
Adjuvant cDMARDs were used in similar percentages for patients on RA and JIA. In RA, cDMARDS were used in 74 % of IFX and 78% of ADA patients. In JIA, cDMARDs were used in 76% of IFX patients and 75% of ADA patients. A larger percentage of patients on IFX were given cDMARDs than those on ADA for PsA and AS. In PsA, cDMARDS were used in 72% of IFX patients and 27% of ADA patients. In AS, cDMARDS were used in 50% IFX patients and 11% ADA patients.
Conclusion: Rheumatologists use TDM primarily to clarify if secondary non-response has developed. They test most frequently in PsA, perhaps indicating greater uncertainty of disease activity when compared to RA. Use of TDM in rheumatology is dwarfed by the request volume for gastroenterology.
REFERENCES: [1] Krieckaert C, Hernández-Breijo B, Gehin JE , et al S Therapeutic drug monitoring of biopharmaceuticals in inflammatory rheumatic and musculoskeletal disease: a systematic literature review informing EULAR points to consider RMD Open 2022;8:e002216. DOI: 10.1136/rmdopen-2022-002216.
Acknowledgements: NIL.
Disclosure of Interests: Martin Perry Abbvie, Galapagos Advisory Board, Celltrion Advisory Board, Frenesius Kabi Advisory Board, Biogen Advisory Board, Jaansen Advisory Board, Chanel Kuah: None declared.