Needle-to-bone safety margin of adrenaline auto-injectors in Japanese children: Impact of injection site variation.

BACKGROUND: The adequacy of the 12.7-mm needle length of pressure-activated adrenaline auto-injectors (AAIs) for intramuscular administration in Japanese children remains unclear, particularly regarding the injection site within the thigh. METHODS: A single-center cross-sectional ultrasonographic study was conducted in children weighing 10-20 kg. Fully compressed skin-to-muscle distance (fcSTMD) and skin-to-bone distance (fcSTBD) were measured at the distal one-third of the anterolateral thigh and, when feasible, at the midpoint between the greater trochanter and the lateral femoral epicondyle. Associations with weight, height, thigh circumference, and the Kaup index were assessed, followed by receiver operating characteristic analyses to identify predictors of fcSTBD less than 12.7 mm. RESULTS: Among 97 children analyzed, the maximum fcSTMD was 8.37 mm at the distal one-third of the thigh, indicating that a 12.7-mm needle would reach the muscle in all participants. However, fcSTBD at this site was less than 12.7 mm on one or both sides in 11 children (11.3%). At the midpoint, the maximum fcSTMD was 9.92 mm, and fcSTBD was less than 12.7 mm on one side only in two children (3.7%), both weighing under 12 kg. Thigh circumference and weight predicted fcSTBD less than 12.7 mm at the distal one-third, with an exploratory weight cutoff around 12 kg. CONCLUSION: For Japanese children weighing 10-20 kg, a 12.7-mm needle in AAIs is generally sufficient to reach the muscle. However, needle-to-bone safety varies by injection site, warranting caution in children weighing less than 15 kg. Precise mid-thigh injection and caregiver education are essential.