Enter Spectacle Rx
This calculator is designed to convert eyeglass prescriptions to contact lens prescriptions using the back vertex power equation. It allows inputs in both minus and plus cylinder formats, automatically converting plus cylinder prescriptions to minus cylinder before calculation. The tool defaults to a 12mm vertex distance but allows users to adjust this value to suit their purposes. The output is a vertexed prescription rounded to the nearest 0.25 diopter.
Back Vertex Power Equation:
BVPcontacts = BVPspectacles/ (1−(vertex distance in meters × BVPspectacles))
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To convert between radius of curvature in millimeters (mm) and diopters (D), you can use the following formula:
- D = (n – 1) / R, where n is the refractive index of the medium and R is the radius of curvature in mm.
This calculator uses a simplified formula that assumes n = 1.3375, the refractive index of the cornea.
- D = 337.5 / R
- R = 337.5 / D
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Enter patient weight to calculate the safe daily dosage of Plaquenil.
Cumulative Dosage
Plaquenil Dosage Calculator
The Plaquenil Dosage Calculator simplifies the process of determining the appropriate daily dosage of Plaquenil for patients based on their body weight. Users can input their patient’s weight in pounds or kilograms to receive an immediate calculation of their safe dosage in milligrams, using the standard guideline of 5.0 mg per kilogram of body weigh established by the 2016 American Academy of Ophthalmology Guidelines.
Cumulative Dosage Calculator
The Cumulative Dosage Calculator uses dosage and duration to determine the patient’s lifetime buildup of hydroxychloroquine. It also determines when the user will reach a cumulative dose of 1000 grams, provided the drug administration remains unchanged.
Recommended Screening Guidelines
- Baseline Examination: A baseline fundus examination should be performed to rule out preexisting maculopathy. This should occur within the first year of starting the drug.
- Annual Screening After 5 Years: After five years of use, annual screening should begin, unless there are specific risk factors that necessitate earlier and more frequent monitoring.
- Risk Factors: Additional risk factors for toxicity include daily dosage exceeding 5 mg/kg of real body weight, over 1000g of cumulative dose, renal or liver disease, concomitant retinal disease, tamoxifen use, and age over 60.
- Screening Tests: The recommended tests for annual screening include automated visual fields plus spectral-domain optical coherence tomography (SD-OCT). For patients with additional risk factors, additional testing such as fundus autofluorescence and multifocal electroretinography (mfERG) may be considered.
- Dosage Adjustments: If the calculated risk is high, the guidelines suggest adjusting the dosage or discontinuing therapy, in consultation with the prescribing physician.
- Patients of Asian Ancestry: A 2016 study showed that Asian patients are more likely to experience pericentral retinal damage. Consider wider screening options for these patients, such as 24-2 visual fields and widefield or off-axis OCT.
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The Plus/Minus Cylinder Converter allows users to easily switch between plus cylinder and minus cylinder formats.
To convert from minus to plus cylinder there are four steps:
- Combine sphere power and cylinder power.
- Example: -3.50-1.25=-4.75
- This corresponds to the new sphere power in plus cylinder format.
- Change the sign of the cylinder power to plus.
- Example: -1.25 D becomes +1.25 D
- Switch the axis by 90 degrees. Remember the axis must range from 1 to 180.
- Example: 180 becomes 90 degrees.
- Combine into the final output:
- Example: -4.75+1.25×090
To convert from plus to minus cylinder we can follow similar steps:
- Combine sphere power and cylinder power.
- Example: -4.75D+1.25D=-3.50
- This corresponds to the new sphere power in minus cylinder format.
- Example: -4.75D+1.25D=-3.50
- Change the sign of the cylinder power to minus.
- Example: +1.25 D becomes -1.25 D
- Switch the axis by 90 degrees.
- Example: 90 becomes 180 degrees.
- Combine into the final output: -3.50-1.25×180