Dose accuracy of new versus used Novopen 4 insulin pens

To our knowledge, no studies have been performed testing dose accuracy in both new and used patient insulin injection pens. We hypothesized that the dose accuracy of used (>1 year) insulin pens to be less accurate than that of new insulin pens and investigated whether possible differences influence the treatment. This study compared the dosing accuracy of 11 new and 11 used (>1 year) Novopen(®) 4 pens (Novo Nordisk, Bagsvaerd, Denmark).

Dosing accuracy differences between new and used pens were studied by weighing the volume of the dosage of 8 international units of insulin (IU) and 32 IU of 11 pens. Each measurement was repeated 15 times. Whether the pens complied with the International Organization for Standardization (ISO) limits of 10% for 8 IU and 5% for 32 IU was tested. The statistical analyses were performed using the Mann-Whitney rank sum test (within Sigmaplot version 12.0; Systat Software, Chicago, IL), and a P value of <0.05 was considered to be statistically significant.

For the 8 IU dose, the mean delivered dose was 8.04 IU in new pens and 7.91 IU in used insulin pens. For the 32 IU dose, the mean delivered dose was 31.90 in new pens and 31.68 IU in used insulin pens. The difference in the median values between the two groups was statistically significant (P<0.001). Three individual doses in the 32 IU dose exceeded the ISO range in the lower range. The difference in mean variation coefficient between the two groups was also statistically significant (P<0.001).

There was a significant difference between the accuracy of new versus used insulin pens. More studies with larger sample sizes are necessary to confirm our findings and further elucidate the relationship between age of insulin pens and dose accuracy.