hi my name's amanda and i'm a pharmacist today i'll be talking about weight-based dosage calculations and if you find this video useful please press the like button subscribe to my channel and share it with others who may find it helpful too and if you'd like to support this channel with a donation press the heart thanks button to give me a super thanks thanks i really appreciate it so for some medications the amount of drug to be given to a patient depends on the patient's weight most medications for children are dose based on weight due to the extreme differences in patient size and this makes sense typically a small the smaller the patient's size the smaller the dose will be needed the greater the patient size the greater the dose will be needed there are also some adult medications that are dosed based on weight due to characteristics of a particular drug being given some characteristics of drugs that require weight-based dosages for adult medications include drug distribution this is how a drug is transferred to the tissues of the body for example if a drug is fat soluble and the greater the patient weight the higher the dose is typically needed for fat soluble drugs to reach appropriate concentrations in the body and another characteristic is the therapeutic index this is the ratio between the toxic dose and the effective dose with a narrow therapeutic index drug or an nti drug a small change in dose equals a large change in effect now we'll talk about how to solve weight-based dosage calculations so first weight-based dosages are typically specified in milligrams per kilogram or milligrams per kilogram per day so to solve weight-based dosage calculations first you want to determine the patient's weight in kilograms and our conversion equivalent for this is one pound equals 2.2 kilograms so basically what we're going to do on this we'll divide the pound weight by 2.2 that will give us the weight in kilograms next we'll determine the patient's dose this is done by multiplying their kilogram weight by the milligram per kilogram dose and then we'll determine the amount per dose if it's in milligrams per kilogram per day and to do this we'll divide the total daily dose by the number of doses per day and now we'll look at each of these steps in more detail so first we'll look at how to convert a patient's weight from pounds to kilograms so many weight-based dosages are specified in milligrams per kilogram or milligrams per kilogram per day of the medication so when a patient's weight is given in pounds it must be converted to kilograms and our weight conversion equivalent is one kilogram equals 2.2 pounds so in order to convert a patient's weight from pounds to kilograms and we can divide it by 2.2 you can memorize it just that and do it that way or if you just want to memorize the conversion equivalent you can also set up a set of fractions that cancel like units and multiply to get your answer so i have an example of that here if we have a 150 pound patient there's one kilogram per 2.2 pounds times 150 pounds over one so you have a pound on the top pounds on the bottom those cancel you're left with kilograms then you just do the math 1 times 150 is 150 then divided by 2.2 times 1. that it gives us 68.2 or as i said if you just take 150 divided by 2.2 68.2 kilograms so once the weight has been converted to kilograms the amount of drug to be given can be obtained as i said weight-based dosages are usually specified in milligrams per kilogram or milligrams per kilogram per day so to obtain the dose you can simply just multiply the weight in kilograms by the milligram per kilogram dose so if you have a 68.2 kilogram patient and a dose of 5 milligrams per kilogram we can just multiply 68.2 times five that will give us our milligram dose or if you want to set up the fractions that cancel like units you can do it that way too just so you can see how the units cancel and to it's a good double check on yourself so 68.2 kilograms over one times five milligrams per kilogram you have a kilogram on the top kilogram on the bottom those cancel you're left with milligrams so then you just do the math 68.2 times five divided by one that would equal 341 milligrams so if the dosage is specified in milligrams per kilogram per day and the directions are not qd that's once a day then the amount per dose may also need to be calculated so to calculate the amount per dose divide the total daily dose by the number of doses per day so we'll continue with our example we have five milligram per kilogram per day and that equaled 341 milligrams per day and we'll say that's given q6 hours q6h is q6 hours so q6h we have 24 hours in a day divided by six hours that would be four doses per day so if we take 341 and divide it by four that equals 85.25 milligrams so the patient would receive 85 milligrams q6h so 85 milligrams would be the amount per dose so sometimes the dosage is specified in milligrams per kilogram and the frequency is stated with the dose so the amount per dose is already determined so for example we have our 68.2 kilogram patient to be given 5 milligrams per kilogram q6 hours if you just do the math of 5 milligrams per one kilogram times 68.2 kilograms over one uh we cancel a kilogram on the top kilogram on the bottom we're left with milligrams five times 68.2 is 341 and that's 341 milligrams q6h because our our amount is already determined our dose is already determined the five milligrams per kilogram is every six hours so now we're going to look at some examples so example one a 176 pound patient is to be given lovenox 1.5 milligram per kilogram sqd what is the patient's total daily dose so the medication we're working with here is lovinox its generic name is anoxiparon and it's a low molecular weight heparin blood thinner to treat or prevent blood clots so we're giving it 1.5 milligrams per kilogram sqd sq stands for subcutaneously that means under the skin and qd is once every day so we're wanting the patient's total daily dose so since it's once a day we just figure out our milligram per kilogram dose and that will be the amount given for the day so one kilogram equals 2.2 pounds so there's one kilogram per 2.2 pounds times 176 pounds over one our pounds cancel one's on the top ones on the bottom so they cancel we're left with kilograms so 176 divided by 2.2 equals 80 kilograms and then we're given the dose of 1.5 milligram per kilogram times 80 kilograms over one kilogram on the top kilogram on the bottom those cancel we're left with milligrams that gives us a total of 120 milligrams so that is our total daily dose since it's administered once a day now example number two a 148 pound patient is to receive vancomycin 10 milligram per kilogram iv q12h what is the amount to be given for each dose so our medication here is vancomycin it's a glycopeptide antibiotic that's used to treat a variety of serious infections it's given iv and it's one that is dose-based according to weight and also other factors such as a patient's renal function so we're given vancomycin 10 milligram per kilogram iv q12h so that's every 12 hours and we're wanting to know what the total amount is to be given for each dose so we have one kilogram equals 2.2 pounds so we have to figure out the how the much the patient weighs in kilograms first so one kilogram per 2.2 pounds times 148 pounds over one so one times 148 148 divided by 2.2 is 67 kilograms so that's the patient's weight in kilograms and then our dose is 10 milligrams per kilogram times 67 kilograms we're left with milligrams a kilogram on the top kilogram on the bottom those cancel so 10 times 67 is 670 milligrams for each dose so since it says it's 10 milligram per kilogram every 12 hours that is the amount for each dose it's already we don't have to determine how many doses in the day example number three a child is prescribed cephalexin 75 milligrams per kilogram per day given qid the child weighs 30 pounds what amount of cephalexin will be given per dose so cephalexin this is an antibiotic that's a cephalosporin and so we have 75 milligram per kilogram per day qid qid is four times a day so first we have to convert the child's weight into kilograms so one kilogram per 2.2 pounds times 30 pounds 30 divided by 2.2 is 13.6 kilograms so now we have a dose of 75 milligrams per kilogram and that's going to be per day so 75 milligram per kilogram times 13.6 kilograms we're left with milligrams on the top kilogram on the top a kilogram on the bottom those cancel 75 times 13.6 is 1020 milligrams so that is the total dose per day so this is given qid so four times a day so we're going to divide that by 4 to get the amount per dose so 1020 divided by 4 equals 255 milligrams per dose okay now we'll look at our last example example number four and it has three parts so a child is to receive amoxicillin at a dose of 50 milligram per kilogram per day what is the total daily dose if the patient weighs 21 pounds then the dose of amoxicillin is to be given q12h how many milligrams should be given per dose and then part c if amoxicillin 400 milligram per 5 ml is dispensed how many milliliters should be given per dose so we'll look at part a first so a child is to receive amoxicillin at a dose of 50 milligram per kilogram per day what is the total daily dose if the patient weighs 21 pounds so amoxicillin that's an antibiotic in the penicillin class um we're dealing with it at a dose of 50 milligram per kilogram per day so we're looking for the total daily dose if the patient weighs 21 pounds so one kilogram equals 2.2 pounds so 21 pounds divided by 2.2 that equals 9.5 kilograms then we have a dose of 50 milligram per kilogram so 50 times 9.5 equals 477 milligrams so that is our total daily dose 477 milligrams part b the dose of amoxicillin is to be given q12h how many milligrams should be given per dose so q12h means every 12 hours so 24 hours divided by 12 hours so that would be two doses so total daily dose was 477 milligrams if we divide 477 by 2 that equals 239 milligrams per dose so now we'll look at part c if amoxicillin 400 milligram per 5 milliliters is dispensed how many milliliters should be given per dose so this one we know we need 239 milligrams per dose so the way we'll do this is we're going to set up equivalent fractions cross multiply and then divide so if there are 400 milligrams per 5 milliliters set that equal to 239 milligrams per x milliliters so we're solving for x so if we take 5 times 239 that equals 1195 then divide to solve for x divided by 400 equals 2.99 we'll round that to 3 so it'll be 3 milliliters would give us 239 milligrams and that would be the amount that the patient should take per dose okay so now we'll just look at a summary and some key points to remember so weight based dosages are typically specified in milligram per kilogram or milligram per kilogram per day to solve weight based dosage calculations first determine the patient's weight in kilograms remember one pound equals 2.2 kilograms and a shortcut to do this you can just divide the pound weight by 2.2 or if you want to set up the fractions that cancel like units you can do that as well just so you can see exactly what's going on next you determine the patient's dose so we'll we can do this by multiplying the kilogram weight by the milligram per kilogram dose and then determine the amount per dose if the milligram per kilogram if it's in a milligram per kilogram per day and to do this we'll divide the total daily dose by the number of doses per day thanks for watching please like and share this video with others who may find it helpful and please subscribe to see more of my pharmacy learning videos and if you would like to support this channel with a donation press the heart thanks button to give me a super thanks thanks i really appreciate it