Dose Adjustment Calculator By Body Surface Area

How does this dose adjustment calculator by body surface area work?

This health tool determines the dosage adjustment required for a standard dose when administered to a patient with known weight and height.

This dose adjustment calculator requires the following information:

■ Standard dose – the recommended drug dosage as prescribed, input in either milligrams, micrograms or grams.

■ Height – patient height in either centimeters or inches. The default measurement is in centimeters because this one is used in the BSA formula.

■ Weight – patient weight in kilograms or pounds. The default is in kilograms as per formula requirements.

■ Adjustment dose measurement unit – asks for the preferred unit, either in milligrams, micrograms or grams.

Please note that the calculator transforms any measurements input by the user from the Imperial system to the Metric system if needed.

For the body surface area determination, the DuBois formula is used:

BSA = 0.007184 x (Height in cm)^0.725 x (Weight in kg)^0.425

The BSA value is then employed along the standard dose to retrieve the adjusted dose:

Adjusted dose = Standard dose x BSA / 1.73

Example of a dosage calculation

Let’s take the case of a patient medication with a standard dose of 25 mg. The patient’s data is the following:

■ Height: 175 cm;

■ Weight: 68 kilograms;

■ The adjusted dose is required in mcg.

BSA = 0.007184 x 175^0.725 x 68^0.425 = 1.8255

Adjusted dose = 25 mg x 1.8255 / 1.73

Adjusted dose (rounded) = 26.38 mg

Transformation from mg to mcg: 26.38 mg = 26,380 mcg.

Please note that the BSA adjustment is not appropriate for all medications and in the cases of all patients so do check any exceptions.

Dose adjustment factors

Drug dosage regimens in many conditions and with a lot of medicines requires customization according to patient variables. While the most used may be weight and height, there are other elements to factor in when establishing the recommended dose.

Some of those include patient age, gender, comorbidities, severity of the disease or medicine half time. Pharmacological effects as well as route of administration, metabolism and eliminations are also to be taken in consideration when establishing a frequency of administration.

When simultaneous administration of two or more medicines is required, the specialist needs to look at addition, synergism and antagonism effects.

References

1) Du Bois D, Du Bois EF. (1916) A formula to estimate the approximate surface area if height and weight be known. Archives of Internal Medicine 17 (6): 863–71

2) Beumer JH, Chu E, Salamone SJ. (2012) Body-Surface Area–Based Chemotherapy Dosing: Appropriate in the 21st Century? American Society of Clinical Oncology.

3) Pai MP. (2012) Drug dosing based on weight and body surface area: mathematical assumptions and limitations in obese adults. Pharmacotherapy; 32(9):856-68