Inadequate resuspension of isophane insulin used in pen injectors is common, according to a study reported this week from Germany. The researchers recommend that patients tip the pen at least 20 times before injecting to ensure adequate mixing of the insulin.
Isophane insulin in cartridges needs adequate mixing for resuspension, by rolling or tipping the pen, before use.
The researchers collected pens from 109 patients and measured the isophane insulin content of their partly used cartridges by an optical system. Patients were also asked to complete questionnaires about their insulin technique. After further education about resuspension, a sample of patients was re-evaluated some months later.
Only 35 per cent of the original 109 cartidges had an insulin content within 20 per cent of the nominal value from control cartridges. Only 9 per cent of the 109 patients tipped and rolled their cartridges more than 10 times. There was no relationship between inadequate suspension and frequency of hypoglycaemic episodes. In the later analysis, after education about resuspension technique, suspension errors decreased in 35 of 44 patients (80 per cent) and, in these patients, the frequency of hypoglycaemic episodes per month decreased compared with the previous period.
The researchers say that, in the laboratory, a minimum of 20 cycles of tipping was needed for complete resuspension of the insulin (Lancet 1999;354:1604).
Patient information leaflets in the UK advise patients on the need for resuspending isophane insulin in cartridges before use. A spokeswoman for Eli Lilly, one of the manufacturers of cartridge insulin, said that the company was grateful to the authors for bringing the matter to the attention of health care professionals. The company agreed that patients should be educated in this matter. "Furthermore, if a patient regularly experiences variation in glycaemic control co-inciding with starting a new cartridge, it may be advisable to check how thoroughly the patient mixes the insulin," Lilly said.