Mixing Insulins

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If possible, avoid teaching patients to mix when first initiating insulin therapy, as the extra steps can overwhelm and confuse them. Emphasize use of a consistent technique. Commercial premixed insulin (70/30, 75/25, 50/50) is useful as interim therapy, or for individuals who cannot master accurate mixing.

It was identified a decade ago that mixing long-acting insulin with Regular insulin slows the effect of the Regular insulin through microcrystaJlization; most problematic are Ultralente and Lente, with NPH causing less of this effect. The precipitation starts in a few minutes and takes up to 24 hours to be complete. (Commercially prepared mixes are not subject to this issue as the manufacturers' formulations create stable ratios of 70/30, 50/50, etc.) It was thus recommended that insulin injections be administered immediately after mixing. Also, commercial mixes or the use of multiple syringes, each containing only one kind of insulin, were suggested for those who were having syringes prepared ahead of time by family or home health workers. This "contamination" effect of long-acting insulin on Regular insulin is now less of an issue because it does not occur with the rapid-acting analogs, lyspro and aspart. However, caution should still be taken when mixing. NPH should never be mixed with Lente or Ultralente because of precipitation. Also, glargine cannot be mixed with any other insulin because of the low pH of the glargine buffer (pH 4) necessary to keep it soluble for injection. As new insulins come to market, it is important to consult with the manufacturer for correct mixing practices.

Technique for Mixing Insulin

Guidelines for mixing insulins are similar to those for a single-drawn dose, with a few exceptions.

1. Insert air into each vial before drawing any insulin. The amount inserted per vial should equal the insulin dose to be withdrawn. Insert air into the long-acting (cloudy) insulin vial first.

2. After the remaining air is put into the short-acting (clear) insulin, draw up the prescribed dose and remove the needle. The short-acting insulin should always be drawn up first to prevent possible contamination of the vial with long-acting insulin, which could alter its kinetics of action.

3. Expel any remaining air, and inspect that the correct dose was taken.

4. Draw up the long-acting insulin, being careful not to overdraw.

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