U-40 insulin/U-100 syringe conversion; diluted insulins


Many people special order protamine zinc insulin (PZI) that is U-40.  Some people order U-50 insulin, and some people use insulin that has been diluted.  If you are using something other than a standard U-100 insulin, you must understand what the U means and how many units of insulin you are giving your pet.

The U refers to actual units of active insulin. Units are a standard measurement system for many drugs. 

A U-100 insulin has 100 units of active insulin in each mL of liquid. You can think of it as being 100 pieces of insulin in each mL of liquid. 

A U-40 insulin has 40 units of insulin in each milliliter (mL) of liquid. 

This means that the same volume (liquid amount) of a U-100 insulin has 2.5 times more insulin in it than a U-40 insulin.  Or, the U-100 insulin is 2.5 times stronger than the U-40 insulin.

Most commercially available insulins are U-100 and the syringes we purchase at the pharmacy are U-100 syringes. U-100 syringes are specially designed to provide the proper dose of U-100 insulin.  If you use a U-100 insulin and a U-100 syringe, you fill the syringe the way the vet showed you. 

If you use a U-40 insulin and the U-40 syringes that are provided with that insulin, you fill the syringe in the normal manner and don't need to do any conversions.

Some people who use special order U-40 PZI often prefer to use the U-100 syringes because they are readily available at the local pharmacy and they have a very thin needle.

But if you use a U-40 insulin and a U-100 syringe, you must correct for the difference in the strength of the insulin when you fill the syringe.

The following information explains how to fill a syringe using a U-40 insulin and a U-100 syringe. 

To get a specific number of units of U-40 insulin using a U-100 syringe, you must multiply by 2.5, and fill the syringe to that mark.  For example, to get 5 units of U-40 insulin using a U-100 syringe you must fill the syringe to the 12.5 unit mark on the barrel of the syringe.  The chart below can be used as a reference.  You can print this chart and use it for reference, or create a chart of your own....just be sure you understand the math.

If you do not understand this concept, please discuss it with your veterinarian.  It is very important that you understand how many actual units of insulin you are giving and why you must fill the syringe this way. If you tell your vet that you give 12.5 "units" when you really mean that you fill the syringe to the 12.5 unit mark, you are actually giving 5 units of insulin. For your pet's health and safety, both you and any vet you talk to must know how many units of insulin your pet is getting.

This concept can also be used when dealing with diluted insulins.  A U-100 insulin that has been diluted is no longer a U-100 insulin.  The dilution has made it weaker.  So, the mark that you fill the syringe to is not the actual number of units of insulin you are giving.  Be sure you understand how your pet's insulin was diluted and how many units of insulin you are giving.  If your pet's insulin was a U-100 insulin that was diluted to be half of its original strength, it is now a U-50 insulin. This means that when you fill the standard U-100 syringe to the 5 unit mark, you are really giving 2.5 units of insulin.  

If you are using a diluted insulin, the insulin bottle should be properly labeled to show that the insulin was diluted. Be sure to have the person who diluted your insulin write down exactly what they did.  Have them write down what the strength (U-#) of the original insulin was, and how much "original" insulin and how much diluent they used to make the new, diluted insulin.  Even if your vet did the dilution, you should have this information in your records at home.  There may be a time when you have an emergency and must take your pet to a different vet.  That emergency vet will need to know exactly how the insulin was diluted.

A dilution tip: some owners have reported problems with insulins that were diluted using saline solution.  Sometimes there are problems with saline-diluted insulins not working properly.  Also, the injections of saline-diluted insulins can cause discomfort for your pet.  The  proper diluent (the liquid that is used to make that specific type of insulin) can be obtained from the company that manufactured the insulin. It is usually free.

Conversion chart for using a U-40 insulin with a  U-100 syringe
To get this many
units of
U-40 insulin
Fill a
U-100 syringe
to this mark
  To get this many
units of
U-40 insulin
Fill a
U-100 syringe
to this mark
0.25 0.6   6.25 15.6
0.50 1.3   6.50 16.3
0.75 1.9   6.75 16.9
1.00 2.5   7.00 17.5
1.25 3.1   7.25 18.1
1.50 3.8   7.50 18.8
1.75 4.4   7.75 19.4
2.00 5.0   8.00 20.0
2.25 5.6   8.25 20.6
2.50 6.3   8.50 21.3
2.75 6.9   8.75 21.9
3.00 7.5   9.00 22.5
3.25 8.1   9.25 23.1
3.50 8.8   9.50 23.8
3.75 9.4   9.75 24.4
4.00 10.0   10.00 25.0
4.25 10.6   10.25 25.6
4.50 11.3   10.50 26.3
4.75 11.9   10.75 26.9
5.00 12.5   11.00 27.5
5.25 13.1   11.25 28.1
5.50 13.8   11.50 28.8
5.75 14.4   11.75 29.4
6.00 15.0   12.00 30.0


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Updated August 2001
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