Is a 4mm Insulin Pen Needle Best for All? An Independent Review of the Literature

Recent International advice on insulin delivery advocates the use of a 4mm insulin pen needle for all. However, the evidence base is weak and potentially biased. Here we present an independent systematic critical review. A systematic search of MEDLINE, EMBASE, and CINAHL was undertaken in November 2016. The search strategy combined three facets: diabetes mellitus, insulin pen needle, and adverse effects. Multiple terms were used to define each concept and only English-written articles published on or after 2006 were included in the screening process. Free-text searches were also performed. The search extracted 2,382 citations. Sixteen articles were included in the literature synthesis; 11 trials and five observational studies. Of the identified papers 12 (75%) were funded by insulin pen manufacturing companies. Critical appraisal scores were mostly low to very low, with serious risk of bias such as lack of blinding, no power calculation, or influence of study design particularly in the use of scoring measures. Shorter needles were in general reported as preferred, though a large proportion of patients also stated no preference or preferred the longer needle. One of the ten studies that measured pain reported less pain with a 6mm needle compared to a 5mm, and two further studies reported no difference in pain scores for 5mm compared to 8mm needles. One paper reported less pain using larger volumes and an 8mm needle than with a 6mm. Bleeding leakage and bruising scored similarly for all the needle lengths; though these data were weak. Shorter needles in lean patients were associated with a low risk of intramuscular injection. Much inference has been made from this biased research. With better study design the literature may suggest that patients prefer any needle length from 4 to 8mm. It may be that short needles should be advocated in lean people; however, as the majority of insulin users are obese a more patient centred approach would be to offer a selection of needle lengths.