BackgroundMethotrexate (MTX) is widely used as disease modifying treatment for psoriasis and psoriatic arthritis (PsA). Rheumatological and dermatological guidelines to prevent MTX-induced adverse events diverge in the number and frequency of blood tests. These differences are not based on evidence indicating a higher risk for patients with psoriasis compared to PsA or rheumatic arthritis (RA). This raises the question if multiple testing increases safety, or results in false positive signals potentially leading to early withdrawal of an effective treatment.ObjectiveCompare the effects of MTX monitoring strategies by rheumatologists and dermatologists regarding drug survival, reasons for withdrawal and safety.MethodsRetrospective follow-up of all patients diagnosed with psoriasis by dermatologists or PsA by rheumatologists. Included were consecutive patients who started methotrexate (MTX) between 2006 and 2012 and had a scheduled follow-up by dermatologist or rheumatologist. Exclusions were: drug not started after the first prescription or incomplete availability of lab data. Data were extracted from electronic records: start and stop dates and dosing of MTX; treatment with folic acid and dose; reasons for withdrawal of MTX; numbers of blood sampling and types of laboratory tests performed for MTX safety; number of abnormal tests; occurrence of any serious adverse event (SAE).Results190 Psoriasis and 196 PsA patients starting methotrexate (MTX) were included. Age and sex were comparable. PsA patients used higher initial and maximum doses of MTX and folic acid, but psoriasis patients had a higher frequency of abnormal laboratory results (0.14 vs 0.03 per treatment month, p<0.001), resulting in a striking difference in withdrawal of MTX for abnormal liver enzymes (15.8% vs 3.6%, p<0.001). In PsA patients MTX was withdrawn less frequently for ineffectiveness (15.8 vs 24.2%, p<0.05) leading to longer drug survival in the first course of treatment (37.4±30 vs 18.8±19.1 months). Repeated courses of MTX were more often prescribed by rheumatologists than by dermatologists. There were no significant differences in the occurrence of SAE (psoriasis 0.0041 vs PsA 0.0038 per treatment month) or death (1.6% vs 2.0%) between these groups. Hospital admissions related to infection were recorded in 6 (3.1%) PsA vs 4 (2.1%) psoriasis patients.ConclusionStrict monitoring by dermatologists resulted in more abnormal findings, which reduced drug survival of MTX. The limited monitoring strategy by rheumatologists was not associated with more SAEs. Further research in other populations is needed to confirm whether reduced intensity of monitoring can optimize the use of MTX with sufficient long-term safety.