How to calculate and prepare dextrose infusions
Percentage solutions in drip bags... Never fun, right?
The most common one we probably encounter in practice is the glucose (or dextrose - because vetmed loves more than one term for the same thing, am I right?!) CRI.
This is a treatment you’ll use lots if you’re managing diabetic patients, dealing with hypoglycaemic emergencies, caring for septic patients, sick parvovirus puppies, or monitoring a DKA patient who’s still ketonaemic but whose glucose is trending downwards.
Our role when caring for all of these patients as a vet nurse or technician is vast - and of course, we’ll be doing way more than ‘just’ preparing CRIs for them. But, when you’re juggling a full ward and intensive patients, the last thing you want to do is spend more time on maths than you need to (or any time doing maths, if we’re honest!)
So in this post, I’m going to show you exactly how to calculate and prepare a dextrose infusion (and the cheat way that I wish I knew when I first started nursing) - so you can spend less time doing complicated calculations and more time giving your patients great care.
Ok. So how do we calculate a dextrose CRI?
The first thing you’ll need to do is determine the amount of glucose injection to add to your patient’s bag. To do this, you need to know a couple of important things:
The strength you want to make your CRI up to (as a general rule, it’s usually 5% for a moderately hypoglycaemic patient, or 2.5% for a mildly hypoglycaemic patient - but this will vary depending on the individual and your vet’s preference, so check first!)
The strength of your undiluted glucose injection (off the shelf). This is usually 50%, but always check, as it could vary!
The volume of fluid you’re going to make up. This might be a 500ml or a 1000ml bag of crystalloids, or a 20ml syringe running through a syringe driver… it doesn’t matter how much volume, you just need to know what your target number is!
Once you’ve got this, we can do our calculation and get preparing our patient’s CRI.
First, take your desired percentage, and divide this by the percentage of your undiluted glucose injection.
Then, take the result and multiply this number by the volume (in ml) you want to make up.
This will give you the total volume of drug you need to add!
Let’s look at an example:
Say you’re preparing a 5% dextrose CRI. Your off-the-shelf glucose comes in 50% bottles, and you’ve got a 500ml bag of Lactated Ringer’s solution (LRS… or Hartmanns, or CSL, they’re all the same thing. You choose your term because there are a few for the same fluid!)
Following the steps above, our maths looks a little bit like this:
5% / 50% = 0.1
0.1 x 500 = 50
To make our 500ml bag into a 5% dextrose CRI, we’ll need to add 50ml of 50% glucose injection.
Let’s do another, but this time we’ll make a 2.5% dextrose CRI in a 20ml syringe for a 1kg parvo puppy.
2.5% / 50% = 0.05
0.05 x 20 = 1
To make our 20ml syringe into a 2.5% dextrose CRI, we’ll need to add 1ml of 50% glucose injection.
A quick warning about this, though…
For the overall percentage of our freshly-made CRI to be accurate, we must remove the same volume of fluid from the bag before adding our drug - the whole point is that we’re replacing, not adding. Otherwise, our percentage won't be accurate!
For example, take the 500ml bag we’ve just made up. If we hadn’t removed 50ml of LRS before adding the 50ml of glucose injection, we would have ended up with 550ml in total, not 500ml! 50ml of glucose in 550ml of fluid actually makes our concentration 4.5%, so we’d be slightly underdosing our patient!
But I promised you a cheat way to do this - with no maths. And that’s exactly what you’re going to get.
Once you know the principle of how we calculate and prepare these CRIs, you can skip the long calculations and use this rule to make preparing them super quick, and super easy.
To make a 2.5% CRI using 50% glucose injection, replace 0.05ml of fluid for every 1ml in the bag
To make a 5% CRI using 50% glucose injection, replace 0.1ml of fluid for every 1ml in the bag!
But remember, it’s replace, not add - so whatever you’re adding, make sure you remove that same volume from the bag first!
So there you have it! The quick, easy way to calculate and administer dextrose CRIs to your patients - without the long calculations or the headaches! If you found this useful, I’ve got a couple of resources I know you’ll love - designed to help you give even better care to your patients, and use more skills in the process. One is my deep dive into all things diabetes - 90 minutes to deepen your diabetic knowledge and feel REALLY confident nursing your diabetic patients.
And the other? A bumper session on calculations - so that you can work through medical maths with me, with examples, AND a bonus downloadable guide and workbook to help you really ‘get’ those calculations - without feeling like they hurt your brain.
Find them both in the Academy On-Demand Webinar Library here - or click the buttons below to head straight to the sessions!
