Objective. - To measure deltoid fat pad thickness and determine the optimal needle length for deltoid intramuscular immunization in healthy adults. Design, Setting, and Participants. - Prospective study of 220 healthy health care workers (126 women, 94 men) at the Mayo Medical Center, Rochester, Minn. Main Outcome Measures. - Deltoid fat pad thickness determined by high-resolution ultrasound scanning, weight, height, and mid- deltoid arm circumference. Results. - We found a highly significant difference between women and men in deltoid fat pad thickness, with women having a thicker deltoid fat pad (11.7 vs 8.3 mm; P<.001). Women had a greater deltoid skin-fold thickness than men (34.7 vs 17.2 mm, P<.001) and an equal body mass index. According to the ultrasound findings, a standard 16- mm (5/8-in) needle would not have reached 5 mm into muscle in 17% (16/94) of men and 48.4% (61/126) of women in this study. Conclusions. - Among healthy adults of the age range we studied, the following needle lengths appear to be appropriate for true deltoid intramuscular immunization: For men across the weight ranges we studied (59-118 kg), use of a 25-mm (1-in) needle would result in at least 5 mm of muscle penetration in all subjects. For women who weighed less than 60 kg, a 16-mm (5/8-in) needle would be sufficient to achieve muscle penetration of 5 mm. For women between 60 and 90 kg, a 25-mm (1-in) needle would be sufficient, and women greater than 90 kg would require a 38-mm (1.5-in) needle to ensure intramuscular administration.
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