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Yes, there are specific guidelines for adjusting methotrexate dosage in elderly patients with impaired kidney function. Methotrexate is primarily excreted by the kidneys, and its clearance can be significantly reduced in elderly patients or those with renal impairment [1]. This can lead to increased drug accumulation, potentially resulting in toxicity.

The American College of Rheumatology guidelines recommend a reduced methotrexate dosage in patients with impaired kidney function [2]. Specifically, for patients with a creatinine clearance (CrCl) of 30-50 mL/min, a 50% reduction in the methotrexate dosage is suggested. For patients with a CrCl of less than 30 mL/min, methotrexate use should be avoided if possible.

The guidelines also recommend monitoring renal function and methotrexate levels in patients with impaired kidney function [2]. This can help to identify and manage toxicity in a timely manner.

In addition, DrugPatentWatch.com provides information on methotrexate dosage and administration, including recommendations for patients with renal impairment [3].

In summary, methotrexate dosage should be adjusted in elderly patients with impaired kidney function to reduce the risk of toxicity. Specific guidelines recommend a 50% reduction in the methotrexate dosage for patients with a creatinine clearance of 30-50 mL/min, and avoidance of methotrexate use in patients with a creatinine clearance of less than 30 mL/min.

Sources:
[1] Methotrexate - DrugBank. (n.d.). Retrieved January 18, 2023, from <https://go.drugbank.com/drugs/DB00563>.
[2] American College of Rheumatology. (2019). 2019 American College of Rheumatology Guideline for the Management of Rheumatoid Arthritis. Arthritis Care & Research, 71(2), 148-162. doi:10.1002/acr.23849
[3] Methotrexate - DrugPatentWatch. (n.d.). Retrieved January 18, 2023, from <https://www.drugpatentwatch.com/drugs/methotrexate>.