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Koster MP, Stellato N, Kohn N, Rubin LG. Needle length for immunization of early adolescents as determined by ultrasound. Pediatrics 2009;124:667-72.

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Commentary 

The purpose of this interesting study was to determine the appropriate needle length for intramuscular injection of vaccines in adolescents ages 11 to 15. Given the increasing percentage of overweight and obese children, ensuring the delivery of the vaccine into the deltoid muscle is important. With ultrasonography, the investigators measured muscle depth at the deltoid injection site, when the area was bunched or flattened. In this population, 26% weighed <40 kg, and 20% were between 60 and 108 kg. The mean and median body mass index percentiles were 58th and 62nd, respectively, with 28% of children above the 85th percentile. The authors found that a 16-mm needle is appropriate for children weighing <60 kg, and a 25-mm needle is appropriate for those weighing 60 to 70 kg (and probably for those up to 90 kg), with either technique. A longer needle may be necessary for those over 90 kg, but that was not addressed in this study. To ensure appropriate delivery of vaccine, clinicians should remember that one size does not fit all.