I’ve generally used a U-40 insulin (PZI-Vet) with U-100 syringes. The advantages being that U-100 syringes are more widely available, and by using U-100 syringes with U-40 insulin I was able to make smaller dose adjustments. (Before Meow Meow went off insulin, she was getting 0.2 units (two-tenths of a unit) of insulin! NOT TWO UNITS – two-tenths of a unit! No way I could measure that amount using a standard match of syringe to insulin.
Many find converting insulin to a different syringe confusing – and if, for that reason, you want to match – GREAT! I recommend ALWAYS checking to make sure you KNOW which syringe and which insulin you’re using. I’ve seen disasterous results when people don’t, and aren’t working with what they think they are. Some insulins come in different strengths (like compounded PZI) and syringes obviously come in various markings – and there have even been cases where syringes were in the wrong box! ALSO – make sure you understand whether your syringe is marked only for whole units, or also half units! I’ve seen people advise others to count “lines” – well, whether you have lines only for whole units or whole and half units can DOUBLE YOUR DOSE!
Let’s assume you DO want to use a U-40 insulin (PZI-Vet, Vetsulin/Caninsulin, etc) with a U-100 syringe. Converting isn’t that difficult.
1 unit of U-100 insulin = 1 unit of U-40 insulin. Units are units.
The difference is the volume of liquid needed for that unit. There is 100 units in every 1 mL of liquid of U-100 insulin. There are 40 units per mL of liquid in U-40 insulin. Fewer units in U-40 insulin = less concentrated.
It’s like the difference between buying laundry detergent that’s concentrated – the “cap” which you fill with detergent per load of laundry is smaller the more concentrated the detergent is. So, if you have concentrated detergent, you use one cap from that bottle. If you have detergent that’s not concentrated, you use one cap from that bottle – but the cap is larger so you’re using more detergent. So if you lose the cap off your non-concentrated bottle of soap and have to use the smaller cap off your concentrated bottle of detergent, you may need to use 1.5 caps of soap per load.
Rather than give diabetics (or caregivers for diabetics) units marked with mL’s, they try to make it easier for them by giving them syringes marked with units. It is easy if you’re using syringes marked for the strength of insulin you’re using – but if you’re not you need to convert.
1 unit of U-100 insulin = 1 “whole unit mark” on a U-100 syringe.
1 unit of U-40 insulin = 1 “whole unit mark” on a U-40 syringe.
1 unit of U-40 insulin = 2.5 “whole unit marks” on a U-100 syringe.
There’s a nice printable conversion chart located here:
Conversion chart: http://www.felinediabetes.com/insulin-conversions.htm
Edited to add: Read more of my posts about feline diabetes