Pen devices
When injecting using a pen device, the needle should be kept in the skin (with a lifted skin fold, if necessary) for at least 10 seconds after delivering the drug.
This helps to ensure complete expulsion of the injectable therapy through the needle and avoid dribble through equalisation of pressure inside the pen and in the fat layer injected into (Hicks et al, 2011).
Sequence for injecting
The optimal sequence for injection technique should be:
Make a lifted skin fold if necessary;
Insert the needle into the skin at a 90° angle;
Administer insulin;
Leave the needle in the skin for at least 10 seconds after the insulin has been injected;
Withdraw the needle from the skin;
Release the lifted skin fold if used;
Dispose of the used needle safely following local sharps disposal guidelines (Hicks et al, 2011).
Absorption of insulin
Insulin absorption can be affected by many factors, which can either speed up or slow down the rate of absorption (Chowdhury and Escudiet, 2003). A change in the rate will either strengthen or weaken the predicted action of the insulin.
Factors that can speed up absorption and so increase risk of hypoglycaemia are:
Warm/hot environment, increasing blood flow to the injection area;
Rubbing or massaging the area;
The injection being delivered into a deeper layer of skin.
Factors that may slow down insulin absorption and so cause a potential increase in blood glucose are:
A cold environment, reducing blood flow to the injection area;
Increased volumes of insulin, as the ability to absorb larger amounts is reduced;
More concentrated insulin, such as 500 units per ml instead of the usual 100 units per ml and which is only available in UK on a named patient basis;
Unhealthy injection sites, for example those that are bruised or scarred.
Sites for administering insulin
For insulin to work in a predictable way, it has to be injected into subcutaneous tissue (Guerci and Sauvanet, 2006) (Fig 2).
Complications of poor technique
Poor technique, including using the incorrect needle length, can lead to insulin not being absorbed in a predictable manner. This may cause immediate problems such as hypoglycaemia (a sudden drop in blood sugar because of accelerated insulin absorption if the insulin is injected into muscle) and/or hyperglycaemia (a rise in blood sugar because of slow insulin absorption or insulin running out too quickly).
Gibney et al (2010) demonstrated that it does not matter if insulin is injected into fat just under the dermis or just above the muscle, as long as it is injected into fat. In patients with very little fat, short needles may be useful or the area with very little fat or subcutaneous tissue should be avoided and a more suitable injection site found (Figs 3a and 3b).
Lipohypertrophy
A common problem resulting from poor injection technique is the development of lipohypertrophy (commonly referred to as lipos).
This is the accumulation of fat under the skin, partly caused by injecting too frequently in the same area. Lipohypertrophy can be unsightly and painful (Figs 4a and 4b); in some people the lesions can be hard or scar like. To detect lipohypertrophy, injecting sites should be both inspected and palpated, as some lesions can be more easily felt than seen. Healthy skin can be pinched tightly together, while areas of lipohypertrophy cannot.
In some cases, lipoatrophy can develop, which is the wasting of subcutaneous tissue. It is less common now due to the purification of human and analogue insulin but is once again being seen occasionally in clinical practice because impure insulin is being used in some developing countries.