CHICAGO – For adolescents, body weight and injection technique should guide the choice of needle length during immunizations, according to a poster study presented at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy.

“We looked at the performance of a 5/8-inch vs. a 1-inch needle–using both pinching and flattening of the injection site–in relation to the recipient's body weight,” said Dr. Michael Koster.

Using a needle that is too short may result in reduced immunogenicity and more adverse reactions to vaccines that are intended for intramuscular injection, whereas using a needle that is too long may strike bone and injure underlying neurovascular structures, Dr. Koster said in an interview.

With use of the pinching technique on adolescents weighing 88-155 pounds, a 1-inch needle is appropriate.

For those weighing less than 88 pounds, the shorter needle is best, said Dr. Koster, who conducted the study at Schneider Children's Hospital at North Shore in Manhasset, N.Y.

With use of the muscle-flattening technique, the longer needle likewise is recommended when body weight is 88-155 pounds, and the 5/8-inch needle is appropriate for adolescents weighing less than 88 pounds, said Dr. Koster, who is now a pediatric infectious disease fellow at Hasbro Children's Hospital in Providence, R.I.

The investigation included 141 participants. They were aged 11-15 years, with 87% of them aged 12-13 years.

Of the total, 28% of the participants weighed less than 88 pounds, while about 20% weighed more than 132 pounds.

The investigators recorded the adolescents 'height, weight, and arm circumference, and conducted upper-arm ultrasonography during both muscle pinching and skin flattening of the participants' nondominant arms.

These ultrasound measurements were duplicated by a second investigator and averaged for analysis.

“When injecting someone [while using] the pinching technique, you increase muscle and subcutaneous tissue layers, in which case you'll want a little longer needle. When flattening, typically using the forefinger and thumb, you can go up to [a weight of] about 50 kilos [110 pounds] and be safe in terms of getting the vaccine into the muscle without striking bone,” Dr. Koster said at the meeting, which was sponsored by the American Society for Microbiology.

As a general rule, with the pinching technique, a 1-inch needle is appropriate most of the time, and the shorter needle is appropriate only for patients who weigh less than 88 pounds.

With the flattening technique, the shorter length is appropriate only three-quarters of the time and only on patients weighing less than 110 pounds.

“Independent of technique, it would be appropriate to use a 5/8-inch needle on subjects less than 88 pounds,” he said.

Although females had a larger subcutaneous layer than did males of the same body weight, the difference did not result in the use of a different needle length, Dr. Koster said.

He added that body weight was a better predictor of required needle size than was arm circumference.

The American Academy of Pediatrics Red Book makes needle length recommendations for adolescents only in terms of body weight and sex.

For example, both sizes of needles are recommended for girls and boys who weigh less than 132 pounds. The 1-inch needle is recommended for girls who weigh 132-198 pounds and boys who weigh 132-260 pounds.

The AAP recommends an even longer (1&half;-inch) needle for females over 198 pounds and males over 260 pounds. Only 2 of the 141 subjects were that heavy, and the data on these two still are being analyzed.

Data from previous studies have suggested that obese adolescents who are immunized with 1-inch needles develop lower hepatitis B virus (HBV) vaccine titer levels, compared with those vaccinated with 1&half;-inch needles, and this seemed to have been borne out by findings in a limited study of 24 obese subjects aged 14-24 years (J. Adol. Health 2006;38:101).

In that study, after randomization to 1-inch and 1&half;-inch needle groups, subjects (girls over 198 pounds and boys over 265 pounds) were given an HBV vaccination using a 0-, 1-, and 4-month schedule.

HBV surface antibody was obtained 2 months after the third vaccination, and data showed that the final titer levels in the 1&half;-inch needle group were statistically higher to a significant degree than those in the 1-inch needle group.

“I hope that adjustments are made in the 2009 Red Book to reflect our finding that optimal needle length is influenced by intramuscular injection technique,” Dr. Koster said.

He added that there is heightened need for these data because several vaccines–including Tdap (tetanus, diphtheria, and pertussis), meningococcal conjugate vaccine, and human papillomavirus vaccine–recently have been licensed and recommended for all adolescents in the United States.

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Independent of technique, a 5/8-inch needle is appropriate to use on subjects less than 88 pounds. DR. KOSTER

Teen Weight, Vaccination Technique Key to Needle Length