Background
The American Academy of Ophthalmology (AAO) revised its guidelines in 2016 to decrease the maximum dosage of Hydroxychloroquine (HCQ) to 5 mg/kg/day of real body weight to minimize retinal toxicity. However, some clinicians remain non-compliant and continue to prescribe 400mg daily. Additionally, current calculators do not account for renal function, drug interactions, and dosing regimens for intact versus split pills.

Methods
The AAO and Royal College of Ophthalmologists clinical practice guidelines, the HCQ product monographs, relevant literature, and available calculators for HCQ dosing were reviewed using PubMed, MEDLINE, and Google Scholar. Academic rheumatologists and pharmacists were consulted. Microsoft Excel was used to develop an online spreadsheet calculator.

Results
The online calculator is available at https://tinyurl.com/yxzhwg4u and allows prescribers to dose HCQ appropriately according to the most up-to-date guidelines. It allows for 100mg increments, and accounts for patients with tamoxifen use, retinal disease, decreased creatinine clearance, and considers the pediatric population. 

Discussion
We hope that this HCQ calculator simplifies dosing calculations and encourages prescribers to adhere to dose recommendations for long term users of HCQ. In addition, patients should be counselled on the risks of HCQ-related toxicity, the importance of scheduled dosing regimens, and the need for ophthalmic monitoring.  (The maximum recommended dose may not apply to very short term use such as with COVID-19)

An online calculator for hydroxychloroquine drug dosing to minimize retinal toxicity

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