Appropriate lumbar puncture (LP) needle length selection may be more difficult for less experienced practitioners or for patients who are of unusually large or small body habitus. The purpose of this study is to determine if there is a relationship between body height and weight, and the percutaneous depth to the spinal canal, which can more reliably assist in selecting an LP needle length.

This is a retrospective cohort study of patients who received an abdominal computed tomographic scan (for any reason) from July 1999 to December 2000. Lumbar puncture depth was measured on the computed tomographic scan and was used to derive a formula.

The final data pool consisted of 175 patients, aged 25 days to 80 years, with height of 48 to 181.5 cm, weight of 3.0 to 127.3 kg, and body mass index of 11.7 to 49.7 kg/m². Using this data set, the formula for predicting the required LP depth is (weight in kilograms, height in centimeters):

Using linear regression comparing the skin to mid–spinal canal depth measurements with the calculated LP depths, R² was 0.81 (P < .001). This formula selected a needle that was too short in 6% (less than that of 4 other previously published LP needle length selection methods) and a needle that was too long in 31%.

Compared with other formulas, this formula might be a more reliable predictor for estimating the required LP needle length, but this must be validated by further studies. It should be noted that none of the formulas were perfect.